Aircrafters’ Dave Saylor (left), with author Bob Fritz, discuss the differences in basic composite materials.

In Part 1 of our newest hands-on series, we’ll open your eyes to the wonders of modern composite materials. BY BOB FRITZ

W

hat do Burt Rutan, the police and a trickedout Toyota have in common? An off-axis question no doubt, and the answer is not the one Benjamin Braddock received in The Graduate, (“I want to say one word to you. Just one word. Plastics.”). No, the linkage here is composite materials, which are everywhere you look. Burt makes magic airplanes with them—has done so since the 1960s and continues to even today—the police wear Kevlar vests, and the guy that customized his Toyota loves the look of a carbon-fiber panel or two on his car. Sure, you’re familiar with composite’s use in skateboards, bicycles and bulletproof vests, and you’ve seen it in the body of Corvettes since 1953. But did you know that it was in use 5000 years ago? 36

KITPLANES May 2007

Peel-ply is used to help absorb excess resin in fiberglass or carbon-fiber cloth and helps create a clean (but comparatively rough) surface for secondary bonding.

Bricks and Mortar OK, I’ll back up a bit. What’s a composite? Answer: It is two or more materials that are bonded into a single structure where the originating materials retain their identity and contribute to the creation of new material. Simply put? Combine the materials such that neither dissolves and the sum is greater than the parts. By that definition bellybutton lint mixed into Elmer’s Glue qualifies, though I doubt it enjoys wide usage. More realistically, composites didn’t www.kitplanes.com

Let’s start with the materials: What are they, why use them, and how do you select which to use? Actually, this decision is made for you by the kit manufacturer, but let’s take a look at some of the elements that went into their thinking. For aircraft use, we start with what is popularly known as fiberglass. There are two basic types: E-glass and S-glass. The most common is E-glass. It’s strong, water resistant, easily formed and easy to repair. It’s cheap to manufacture and was initially optimized for its electrical properties, hence the E. It’s now found its way into surfboards and boats. The materials science guys wanted to keep their jobs, so they tweaked the

Fiberglass cloth comes in a variety of weights and weaves. The variations accommodate not only differing strength considerations but the ways in which the finished composite structure will be strong—it’s possible to fine-tune composites to an amazing degree.

change much from the mud and straw used to make bricks 5000 years ago in Sumeria until the Romans came up with concrete (cement and gravel). The next leap, although one that has yet to be adopted, was Pykrete. That was a dandy material composed of water and sawdust frozen into blocks. It came within a year or two of being the material from which torpedo-proof ships would be built for WW-II in the North Atlantic. You’ve probably come across terms such as E-glass, S-glass, Kevlar, carbon fiber, ceramic and a host of options that boggle the beginner and make the entrepreneur salivate. These materials have come to common use in homebuilt aircraft because they give the designer significant freedom in choosing shapes—both for aesthetics and aerodynamics—but also because they are comparatively easier to use than traditional materials. We say comparatively because while composite construction is most often used in large chunks—reducing the total number of pieces in any given airframe design—the medium requires its own set of skills and specialized understanding. Photos: Marc Cook and Bob Fritz

Looking Ahead In this series we’ll look at those options as they relate to amateur-built aircraft and take some of the mystery out of them. We’ll define the terms, explain the options, and offer some hints, help and advice. Moreover, for this first installment, we have worked through a small composites project to give you a general lay of the land in photographs. As we move forward in this series, we’ll connect the dots a bit more thoroughly, and offer you the benefit of experience and craftsmanship from several sources, including Dave Saylor, who owns Aircrafters in Watsonville, California, a thriving builder-assist shop that also does inspections and repairs on Experimentals.

Carbon fiber, mysteriously black, is dramatically stronger than fiberglass but considerably more expensive.

recipe a bit. What they got was S-glass. It’s a tiny bit lighter, but has 20% more tensile strength, hence the S for strength. Great stuff! However, as with all things scientific, there’s always a better version just around the corner. In this case, the wonder material was carbon fiber. The fibers themselves are stiffer than steel and are about 25% the weight of the grey stuff. But the matrix material in which the fibers are embedded lowers the modulus (engineer speak for stiffness) to being equal to or lower than that of steel while not losing the KITPLANES May 2007

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BYS Composites, Part 1 continued weight advantage. The result is a finished part that weighs a quarter as much as steel while being in the same strength range as steel. Compare CF to aluminum, and it still has the advantage. That CF part is almost twice as stiff as aluminum at about half the weight. This stuff has “airplane” written all over it…until you see the cost. The next ratchet on that wheel is Kevlar. Manufactured by DuPont, Kevlar has five times the strength of steel on a per-weight basis, 60% of the weight of fiberglass, is fire resistant, electrically insulating, sound damping and, like Superman, can even stop a speeding bullet. But, alas, each of these products is more expensive than the previous, with Kevlar being realistic only if you want a bulletproof airplane to carry you to your Swiss bank.

Saylor begins our sample project at a table dedicated to resin/epoxy preparation.

Underpinning It All: Technique There are trade-offs to these wonder materials. All of them have to be carefully handled in the construction of the finished product to make sure that air is not trapped between layers, and none of them is especially abrasion resistant. They will all cause your cutting tools to dull quickly, carbon fiber is not as good in impact resistance as the others, none is nearly as heat-resistant as metal, and even the wonder material Kevlar—also called Aramid—falls behind the others when in compression. A quick explanation is in order here: Tension means pulling on the individual fiber; compression is as if we had one end of a fiber being pushed directly towards the opposite end; shear is what one does to a fiber with scissors. Keep in mind that a cloth made of fibers has to be seen as individual fibers, each loaded individually in tension, compression or shear. That said, the strength-to-weight ratio, ease of fabrication and the ability to mold these materials into compound curves make them the first choice for the airframes of the latest go-fast aircraft. So, what goes into these wonder materials? In modern terms, the two compo38

KITPLANES May 2007

Using a specialized pump, Saylor is able to dispense the precise proportions of resin and hardener for the job.

nents are the binder, also known as the matrix, and the reinforcement. It’s the reinforcement (the fibers) that gives the strength, while the binder holds the fibers in position and transmits the load among them. Let’s simplify that a bit. As you can see from the photos, the reinforcement isn’t very impressive; wet noodle comes to mind. But tensile strength is where composites shine. Generally speaking, resistance to breaking when

Mixing takes place in a wax-free cup using wood tongue depressors to emulsify the resin and hardener. The purpose is to have both components mixed thoroughly in a way that introduces as few air bubbles as possible.

being pulled is known as tensile strength and it is key to these materials. “But we’re not pulling on them in an airplane, we’re bending them,” you say? Ah, but we are pulling. Try this: Take four similar boards, each a halfinch thick, stack them loosely on a pair of sawhorses and then stand on them. www.kitplanes.com

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Thickening agents, such as flox, cabosil or microballoons can provide a strong bond that doesn’t run out between layers.

They bend quite a bit. Now drive a few nails through them and stand on them. They’re a lot stiffer, right? That’s because they cannot slide against one another anymore; the bottom two boards are being stretched (tension loading) and the top two boards are being squeezed (compression loaded). The nails combine the strength of the boards just as using a solid, 2-inch-thick board is stronger than a group of four half-inch boards. Now substitute a stack of fibers and epoxy for the boards and nails and you see the mechanism of tension.

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Aligned with the Load To take advantage of the tensile strength of the material we have to align the fibers with the load. An airplane sees stresses in many directions, so we have to lay the fibers in many directions, too. There are a couple of ways of doing that. You can build the fuselage like they do at the super-high-tech shops with the entire fuselage on a rotisserie and a machine that lays down a tape that winds around the circumference on its way to the tail, and then continues the same filament back to the nose. It’s done that way on cylindrical shapes because it KITPLANES May 2007

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BYS Composites, Part 1 continued gives a fantastic strength-to-weight ratio on pressurized containers. It’s a couple of million bucks for the equipment, so if your airplane fuselage isn’t a cylinder or pressurized, it’s not an option for the do-it-yourselfer—or even an extremely well-funded kit manufacturer, were there such a thing. The practical way for homebuilders is hand layup. It can, if done well, be just as strong as a filament wound structure. It takes a bit longer, but it requires only a few basic tools and the right format for the fibers, and that format is usually a cloth. Before we get started on this discussion, let’s define some terms. • Weight: That’s in ounces per square yard of dry cloth. • Thickness: This is primarily a function of weight and fiber type, but also depends on the weave. • Filament: A single fiber sometimes known as a monofilament. • Strand or End: A group of monofilaments. • Roving or Tow: A group of monofil-

Our practice project is a hardpoint—in this case a #10 screw embedded in a foam sandwich—but the techniques will carry through many types of composite construction. Here, the foam is being wetted with plain epoxy.

The next step is to slather on a thin layer of microballoon mixture.

The two pieces of foam can be placed together. The critical concern here is to watch for sufficient “push out” so you know that the surfaces have sufficient micro/epoxy for a good bond. 40

KITPLANES May 2007

Now the screw stud is placed in the hole and surrounded with a thick flox/epoxy mixture.

The flox can be smoothed with a brush dipped in the plain epoxy.

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Next we create a micro/epoxy fillet where the two foam pieces join. Fiberglass cloth doesn’t like to go around sharp corners, so this transition is important.

After smoothing the fillet with a brush dipped in epoxy, it’s time to lay down the first layer of cloth.

aments all parallel to one another. These are usually designated by the number of filaments, e.g. 3K or 12K. • Yarn: A group of monofilaments twisted to hold the fibers together. Twisting lowers the strength of the fibers a bit. • Denier: A weight in grams of 9000 meters of yarn. The lower the number, the finer the yarn. • Warp or Ends: These are the yarns that run in the direction of the roll of material. • Fill or Picks: These yarns are shorter because they go across the roll. • Count: This refers to the number of warp yarns (ends) and fill yarns (picks) per inch. For example, a 24 x 22 fabric has 24 ends in every inch of fill direction and 22 picks in every inch of warp direction. • Drape-ability: This is a bit subjective, but it describes how easily the cloth conforms to a non-flat shape. Aluminum foil, for instance, has poor drapeability in that it has to be folded to wrap around a sphere. • Weave: This describes how the warp and fill yarns are interlaced to make what we see as cloth. It determines the drapeability and the strongest direction of the cloth if the cloth is woven with a bias to the warp or fill direction. To complicate the issue further, there are three classifications of reinforcement: particulates, discontinuous fiber and continuous fiber. An example of a particulate composite would be concrete, with the gravel being the particulate. If the key to the strength of composites is the tensile strength of the fibers, then longer strands are better because there are more places for the epoxy to grab on, loading the individual fiber to its maximum. That’s why they put long rather than short steel bars in concrete. Discontinuous-type construction uses short fibers and is, therefore, stron-

After the cloth is positioned, it can be wetted out with a brush. Work from the center out, and try to avoid using too much epoxy. Give the cloth time to wet out.

KITPLANES May 2007

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BYS Composites, Part 1 continued ger than particulate. But because the fibers are short, their strength is not fully employed. Instead, the load goes the length of a fiber and is then transmitted through the epoxy to the next fiber. However, it can be sprayed to make the layup, as is done in boat hulls. Spraying also means the fibers are randomly oriented; therefore, the strength is not directionally dependent. The last group, continuous fiber, is what we want for maximum strength. It’s our very long tug-of-war rope where enough guys can get their hands on it to really use its strength.

A clipped brush is effective for stippling out unwanted air bubbles.

Fabric, Meet Your Binder That analogy of guys grabbing a rope can be extended to the other half of this material, the binder. There’s no point in having a strong rope held by weak people, and the more hands on the rope the better. If we compare two teams, one called Vinylester Resin and the other Epoxy Resin, and look at how they grab the rope, it becomes obvious that the ER team is stronger. The VR molecules are long fellows that wrap around the fiber, while the ER molecules connect not only to the fiber in many places, but also to each other. The advantage is that the ER molecules put more “hands” on the fiber. It’s as if one team had extra players pulling on the belts of the guys holding the rope. The ER hands are a lot stronger as well. In fact, they’re about four times as strong as the VR guys. Not only that, ER will bond dissimilar or already cured materials where VR sometimes has difficulty. The real icing on the cake is that ER also has better resistance to microcracking. What’s the downside to ER? It’s a bit tougher to work with in that to achieve as many 3D bonds as possible, you have to mix the constituent components very accurately. If not, the excess material is just mixed in without finding a partner and weakens the finished product. VR, on the other hand, is a long chain that can be made longer, so mixing is a bit more forgiving. Did I mention that the 42

KITPLANES May 2007

Peel-ply can be used to absorb excess resin. Apply it like another layer of fiberglass.

One trick to doing good fiberglass work (true with carbon and Kevlar, too) is to know when to stop fussing. This project’s peel-ply is just about perfectly wetted out. Stop now, have a cup of coffee and let it cure.

cost is about half that of ER? Consequently, the kit makers use ER where it’s needed for strength. VR is the binder of choice for Glasair, Glastar and the Van’s series, which uses it wrapped around a metal frame. Lancair, with no metal, opted for ER. That’s what makes so many great airplanes possible: options. The bottom line is that if we lay down really long fibers of glass, and bind them with a resin, we get an affordable airplane with phenomenal strength, light

weight and, because the cloth is, well, cloth, it can be easily formed into compound curves that make love to the air rather than just shoving it out of the way. Right now you’re probably feeling like someone just dumped a 1000-piece jigsaw puzzle on the table. Not to worry. In future installments we’ll show you the box and point out all the edge pieces. You’ll find that handling the materials is actually much easier than you think. In fact, this will be fun.  www.kitplanes.com

The humble but noble tongue depressor. Shape it into a custom tool and let the epoxy coat it for extreme use.

In Part 2, we’ll discuss what you’ll need in the way of tools, supplies and a workbench.

D

BY BOB FRITZ

o you find operating a ballpoint pen a difficult task? Is a layer cake with canned frosting a tough assembly? No? Then working with composites is within your grasp. Last month we reviewed the basic materials and found that one end of the fiber spectrum is made up of boat hulls built of chopped E-glass held together with polyester resin; the other is multi-million-dollar aircraft made of single-strand Kevlar and carbon fiber laid down by multi-million-dollar machines. We homebuilders are in the middle of those extremes, doing layups of cloth made of S-glass or E-glass, and using either epoxy resin (ER) when needed for strength and vinylester resin (VR) the rest of the time—or when the kit manufacturers choose to use each of those materials. Typically, the choice is dictated easily by simply choosing the lowest cost material that meets the mission profile. An excellent example is Van’s RV aircraft, which use vinylester resin on the wingtips and epoxy resin on the cowling. You can spot the difference easily enough; the VR parts are white, and the ER parts are a transparent greenish color. But with that choice comes a new set of decisions: What tools are needed? Ever get really ticked off when you see your screwdrivers being used to dig weeds? “^@#$%, use the right tool for that!” Uh, huh. Use the right tool. So it is with composites, and though a lot of these items look like refugees from a Chinese kitchen, 38

KITPLANES June 2007

Not likely to be mistaken for paint rollers, these are used in much the same way to smooth out the fiberglass.

No, it’s not a pizza cutter. It’s a rotary cutter (aka circular razor blade).

The scissors are pricey, so labeling them will make them less likely to walk away.

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The table and cloth rack are essential. The hat is optional.

should cover it with a smooth, plastic sheet. Don’t use wood; the splinters will embed into the cloth. And forget metal; it will dull the cutters. Although Melamine looks nice, it too will cut and release contaminants. Plastic…that’s the ticket. The trick to cutting the fabric is to understand that it has a coating that encourages the movement of the fibers over themselves; the stuff is slippery. It can be a curse when cutting, but it’s what makes it conform to complex shapes.

The Blade they are no more so than a screwdriver is a weed digger. It works, but the right tool works better. So what’s different about working with fiber? Well, you have two basic materials to work with: cloth and resin. Unlike working with aluminum, you also have a timing issue. You can’t stop in mid operation and take it up tomorrow without expecting some back up and restart. There are also health issues that are unknown in metal work. And while the rivet bangers like a big workbench as much as anyone, for the glass guys it’s not an option.

assists the epoxy in wetting the fiber; water removes that coating. You’ve also noticed the black diagonal lines on the table. You might have heard your mother refer to a dress as having been cut on the bias. The term simply means cutting at an angle instead of straight across the material. It’s a way of keeping the cloth from de-weaving. The marks on the table are at 45° to the cloth and serve as a guide. The second reason for a large, flat workspace is that cut material tends to de-weave itself, so the less handling the better. More on this later. As for the surface of the table, you

Now that you have a table surface, you’ll want a cutting wheel. No, this is not a refugee from a pizza joint; it’s a circular razor blade. These come in several sizes, but the 45mm diameter is the best allaround tool for cutting long, straight lines and curved corners. Keep it on the worktable and don’t use it for any material that’s been soaked with resin. The solvents are hard on the handle, and though there are aluminum-handle versions available, any contaminants remaining on the blade will ruin it for further use. There are all-aluminum models that can be stored in a can of solvent, but that requires keeping a liquid in the cutting area or doing a careful job of clean-

Getting Started Let’s begin with how to handle the materials. The first thing you’ll need is a large, flat, smooth surface devoted to cutting cloth. If you try working this stuff on your regular workbench you’re asking for trouble for a couple of reasons: One, the material needs to be kept clean. That means no oils, water or general dirt should come in contact with the glass. Oil and dirt seem obvious as contaminants, but water? Yes, water. There’s a coating on the fibers that

Once you’re done cutting, you have to be careful moving the strips so that they don’t de-weave.

Photos: Bob Fritz and Courtesy the Manufacturers

KITPLANES June 2007

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Composites, Part 2 continued ing and drying prior to cutting on the table. Better to simply keep your cutting wheel clean and dry, advises Dave Saylor of Aircrafters. Now that you see the tool, let’s go back to the table and explain why we want this tool and that table. Once you’ve marked your cut with a Sharpie, you need to cut it out. But because the glass cloth is a loose weave, lifting it off the table to get at it with scissors distorts the fabric. Remember that term “drapeability” from the first installment of this series? It’s helpful when you want the fabric to conform to a compound curve, but in this operation it’s a hindrance. The solution is to cut the fabric without lifting it off the table. Using a cutting wheel against a flat table does just that, and having a backing of firm plastic ensures that the wheel cuts through the glass without dulling the blade. Transporting the pieces to the worksite is not a grab-n-go operation; you’ll get to your destination with loose threads all over the shop as the fabric deweaves itself. Instead, have a large board on which you can place the cut pieces and carry them to the worksite. A large sheet of paper will do if you have a helper to grab the corners. This is especially critical when doing a multi-piece layup such as the one around the leading edge of the windshield on the RV-10. (Hint:

We’re demonstrating that operation in the next issue.) Scissors seem like an obvious requirement. But don’t plan on appropriating them from the sewing kit; there’s a difference in the hardness and the edge profile on glass scissors that makes them work better. Good ones will cost about $30, so that should tell you something. You’ll find these are useful to cut the wetted glass that overlaps into areas it shouldn’t. If you’re going to add a bit of Kevlar to the project, then plan on buying special scissors for that as well. They look just like all the other scissors, so painting the handgrips a nice bright color would be a good way of differentiating your $60 purchase. The blades have tiny diamonds adhered to the edges. One use of these scissors on some material that’s been wetted with resin and the neighbors will learn a whole new set of words not to be repeated in polite company when you discover that resin has coated the sharp corners of the diamonds and now they won’t cut hard butter.

Dabbling in Fluids Now that you have the material to the worksite, you’re ready to start playing with the liquids. That means latex gloves and lots of them. Ultra-cheap ones tear easily, and you’ll find that they are more expensive because of it. But you don’t need surgery-qualified ones either. Dave

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Using the rotary cutter is surprisingly easy. Just don’t trim your fingernails with it.

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You want air over the wing, not in it.

Saylor recommends powdered gloves. “They’re easier to put on, and the powder is on the inside so it’s not a problem for the material,” he says. Rollers are used to squeeze out the bubbles and force the epoxy into the yarns on large, relatively flat surfaces. At first glance it would seem tempting to think these are great for use in curved areas, but in working on the RV-10 windshield I found it easy to push too hard and displace the layer of cloth. The flexibility of the paint brush reduced that tendency. However, you will need several sizes for large and small areas, and you don’t get these puppies at the local hardware store; they are not overpriced paint rollers. For instance, there are various lengths and diameters, but also curved ones, finned ones, and some that look like a pizza wheel gone insane. We’ll describe the uses of each as we

progress with this series. At first glance, peel ply may seem to be a material more than a tool. However, its function is the same as the paint brush or roller. It’s a 2- to 3-inch-wide strip of Dacron that laminates onto the layup as the last layer and is used to squeeze out excess resin. It cures along with the rest of the material, but it doesn’t adhere to the resin. Saylor was adamant about its use: “More epoxy does not make a stronger component,” he said. “Using peel ply correctly will let you squeeze out the excess resin and give you a lighter, smoother part.” An added benefit is that when you peel it off you have a surface ready for further work without sanding. Handy stuff. Among the essential tools is the humble 1-inch-wide paint brush. Buy the cheapest ones you can, and buy them by the box because you’ll throw them away after one use. The obvious uses are slathering on the epoxy and pushing the cloth into position; the not so obvious use is that when you cut them to about 3/8-inch, they also are perfect for smoothing out the bubbles under the peel ply. That white-ish dot (see photo) by the handle of the brush is an air bub-

ble. Resin has a hard time sticking to air, so white is not right. Some of my favorite tools are not only the least costly, but they also get better with use. They’re ordinary tongue depressors filed and shaped to conform to most any job. They get coated with the epoxy and when it dries, they’re harder and smoother than the original. Create a few of these in your shop and keep some in your shop coat. But wait until the epoxy has hardened or you’re in for a surprise when you grab one. Here’s a disaster scenario: The epoxy is setting, and you see the need for a bit more cloth. There’s only one problem. You suddenly find that the tool you want grew legs and walked off. In 30 minutes that epoxy will be jelling, and you are about to panic.

Your Workshop The answer to this problem is not two or three of everything; that’s an unnecessary cost and means you have to keep track of more tools. You wouldn’t make dinner on your workbench; you’d go to the workshop specifically suited to the task, the kitchen. Therefore, your composites workshop needs one thing: a workbench sized so that the parts and all the right tools you need are close at hand. There’s the key: close at hand. This will solve a lot of the problems. Build yourself a mobile work station devoted to composite work. It only needs to be about 2x3 feet, on wheels, and with enough room to carry all the chemicals, tools, materials, gloves, measuring and clean-up supplies in one place. Next month we’ll start to discuss the specifics of how to use all of this in the fabrication of an actual aircraft. We’ll be following various stages of construction of a Lancair IV-P owned by Bill George (that’s him behind the cutting table), and whatever Dave Saylor of Aircrafters in Watsonville, California, happens to be working on. To learn more about Aircrafters, I’d recommend a visit to his web site: www.aircraftersllc.com.  A short brush is useful for getting the air bubbles out of the laminate.

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Dave Saylor and his crew at work at Aircrafters.

In Part 3, we look at how to go about basic preparations.

Y

BY BOB FRITZ

ou walk past an airplane that has wings so smooth and free of ripples that it’s hard to see where the air ends and the paint surface begins. It seems to say, “Go ahead, punk, make my day” to every bug and bird in the vicinity, just daring them to try something that might stick to it. And you wonder how many gazillion hours were spent sanding that puppy. Well, there’s good news and there’s bad news. The good news is that it’s all in the preparation. The bad news is that it’s all in the preparation. But unlike painting aluminum where no amount of paint will make a bad surface look good, composite construction allows you to sneak up on perfection. Now that I’ve got you discouraged, go back out to the tiedowns and look at the percentage of really nice composite 38

KITPLANES July 2007

airplanes compared to the bad looking composite airplanes. Don’t see any real dogs, do you? Hmmm…must not be that hard.

Preparation Begins OK, let’s get to it. What’s it take to get off to a good start in this venture? Well, start with the right tools—as we have been saying all along—a dedicated cutting table, a portable composites bench, a lot of rubber gloves and the patience of a turtle with an arthritic hip…at least at the beginning. If you want to ensure success, start small with a guaranteed win. What’s the worst that can happen? Somebody might challenge you to something more difficult. With that in mind, let’s follow Dave Saylor of Aircrafters in the glass layup around the front of an RV-10’s windscreen. It’s a task almost everyone has to do, be it on an all (well, mostly) aluminum RV or www.kitplanes.com

Here are the three pimary fillers, left to right: millfiber, microballons and flox.

A fit like this makes for a winner.

mega-plastic Lancair IV. The first thing to do is get out the blue painter’s tape and tape off the bottom 3 inches or so of the inside of the windscreen. We’re going to be applying glass to the outside of this area, so painting the inside flat black means you don’t look at the inside of the layup from the pilot’s seat when we’re done. You’ll also find that, unlike the tan tape, the bright blue color makes finding torn bits of it really easy. Next, we need to fill in the gaps and irregularities on the area to which we

microscope you’ll see tiny glass spheres, hence the “balloons.” If you’re familiar with Bondo as used on cars, you’ve already used micro; it consists of micro mixed into polyester resin. We want something a bit stronger, so we’re using micro mixed with either vinylester resin or epoxy resin. Millfiber: That’s short for milled fiber. It’s just fiberglass that’s been chopped into tiny pieces. Flox (are you ready for this?) is really short-length strands of cotton. Each of these has its own unique use based primarily on strength, with millfiber the strongest and micro the weakest. Therefore, we would use micro for non-load-bearing filler around a windshield. To mix the micro into the resin, we have to first mix the resin. It’s critical that you follow the directions of your

intend to apply glass. Don’t get too wrapped up in achieving super smoothness here; there will be multiple layers of glass over this zone, so the small discontinuities that result can be addressed later.

Filler Material Here Over the course of this series we’re going to be working with three similar materials: millfiber, microballoons and flox. Micro, or Q-cell, as it’s sometimes called, looks like powdered sugar and flows almost like water. Under a

Even this tab can be buried and lost under properly applied glass.

Micro mixed to the consistency of creamy peanut butter is great for filling sharp corners.

Photos: Bob Fritz

kit manufacturer for this. As was discussed in a previous issue, kit aircraft use either vinylester resin or epoxy resin. VR is a three-part mix consisting of the base resin, a promoter and a catalyst. Buying promoted VR makes the mixing KITPLANES July 2007

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Composites, Part 3 continued

A couple hundred grand for an airplane, and it all comes down to a little wood block to ensure that you get the right amount of epoxy each time.

The by-volume mixing pump is an indispensable dispenser. So to speak...

easier, but it does have a shorter shelf life of three to six months. Un-promoted VR, however, has an almost indefinite shelf life. Care must be taken when mixing un-promoted VR, as combining promoter and catalyst results in an exothermic reaction—that’s a six-bit word meaning it gets hot, dangerously hot. The procedure is to mix the promoter into the base resin, then add the catalyst. It’s not a difficult job, just another step that must be done correctly. Epoxy resin is a two-part system and, while this is a bit simpler, it does tend to cause skin sensitization so gloves are necessary. And once sensitized, you could be stuck with an airplane to which you are allergic! Gloves should be worn with both types of resin simply to keep the sticky mess off your skin. Now that the resin is mixed, we can add the micro to different consistencies based on their use. Here, we’ll mix about five parts micro to one part resin by volume. Micro is of such a low density that mixing it thoroughly can be difficult, but you have about an hour before it sets up, so keep at it until it’s a uniform color. The resulting mix can then be easily 40

KITPLANES July 2007

applied with a wood tongue depressor. Two safety notes here: First, dry micro is extremely lightweight; a sneeze can blow it all over the shop and you

don’t want to inhale it. Second, never use it between layers of fiberglass; it has poor structural strength and you’ll be building in a weak point. Yes, here it’s the base layer, but this application is not load bearing, that is, it’s not holding anything in place other than itself. On the subject of mixing, there are two schools of thought: Measure the materials by weighing them on a scale, or measure them out with a pump. Both methods are perfectly valid, and your kit manufacturer will most likely recommend one or the other. However, be

Work the first layer gently, and don’t worry about loose threads. They can be fixed later.

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aware that both have advantages and disadvantages: A pump is fast because you simply adjust it to the ratio required, and then lower and lift the handle to get the perfect amount of two components. The downside is that a pump can clog, so you’ll have to periodically check that it’s clear. Scales are clog-proof, but are slower and require active attention to the numbers (Let’s see, was it 3:2 or 2:3?). Here’s a neat tip: Once the ratios are adjusted, you can set the quantity dispensed by placing a wood block under the pump handle. It will limit the down-

in interleaved layers like a deck of cards that have been shuffled but not aligned. Shorter pieces are easier to position. Each layer went on dry and was gently slathered with resin. Gently is the operative word here; you can and will undoubtedly pull some threads out of the weave. Not to worry, though. Gentle use of the brush in a poking motion will smooth out the wrinkles, and you’ll be able to sand the loose threads smooth

when everything is nice and hard. Take a close look at the photo, though, and notice that in addition to loose threads and wrinkles, there are whiteish spots. That’s air. You want air under the wings, not under the glass. Get the air out by continuing to gently smooth the layer. This is a step-and-repeat operation until you get all the layers down. It’s also an opportunity to have a friend

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stroke so that when you release the lever, the amount you get in that non-waxed cup will always be the same. Label the sides of the block with the quantities you use most often and then simply flip the block to the required quantity.

Laying It On As was described last month, cut your glass strips to length and transport them carefully to the work site. Using the first of a boxful of brushes, Saylor applied a liberal coating of the resin directly to the masked area. I was then surprised to see him lay the strips onto the resin base without first soaking them in resin. His reasoning was that it is easier to position dry strips on a non-horizontal surface. (Hmmm, seems to work well.) Here’s where the big advantage of composites shows up. Those nasty looking little aluminum tabs that position the windshield simply become lost under the layers of cloth. Van’s specifies seven layers in this area and, while you could do it with seven strips, Saylor used 14 strips laid down KITPLANES July 2007

41

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Composites, Part 3 continued help. Promise them some flight time when you’re done, promise them dinner, promise to help on their airplane, but do this with a buddy. It’s much easier and more than twice as fast. We’ve been fairly liberal in slapping on the resin, and now we’re going to get rid of some it by adding a layer of peel ply. This is a 3- to 4-inch-wide strip of Dacron cloth that we simply apply over the entire layup. Previously we noted that the strength of the assembly comes primarily from the fiber not the resin. In fact, more resin won’t make the assembly stronger, it’ll just be heavier. Peel ply won’t stick to the resin, so we can use a stubby brush to squeeze out the excess resin. Again, this is a good time to go after the bubbles. Don’t go overboard, though; you have to leave some resin in. Just smooth it out and give a rough shape to the entire layup. An option with large, flat surfaces is to put a layer of thin plastic over the peel ply and drag a squeegee across the surface to squeeze out the excess. Leave the plastic, if you use it, and the peel ply in place for 24 hours and go clean up the work area. I hope you put a

liner in the trash can. A friend of mine, who shall remain nameless, did not, and at the end of the week had to throw away the can. There was a solid mass of brushes and remnants stuck to the sides, much to the amusement of observers.

Ready, Set, Peel Now we get to why it’s called peel ply; you peel it off when the resin is fully set. Don’t forget to do this! I’ve heard of one poor soul who did not realize that it has only slightly better adhesion than air, and wondered why parts kept falling off every time he washed his plane. One last tip: If you want to get a perfect fit between two existing parts, such as the plastic that wraps around the aluminum surfaces of the tail, put some release tape (super-thin packaging tape works well) on the aluminum and a bit of resin on the mating part. Slip the two parts together and hold them in place until the resin has set. When you separate the parts you can remove the release tape and install the part, and you’ll find that the gap is gone. That’s all there is to it. Go give it try on something simple, and next time we’ll take up a larger project. 

A layer of filler was used between the fairing and the wing leading edge. After completion, this wedge will be filled on the outside, never to be seen.

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If the mold you’re working on is small, it can move all over the table if not secured. A bit of thin plywood makes a great base.

“I

Ordinary packing tape is great for holding the mold to its base. Just remember to stay outside the boundary of the finished part.

In Part 4, we make parts from molds.

t ain’t so much the things we don’t know that get us into trouble, it’s the things we know that just ain’t so.”—Artemus Ward With that admonition in mind, and with an understanding that there’s more than one way to do most anything in composites, let’s dive back in. Last time we laid down gently curving fiberglass strips on the leading edge of an RV-10’s windshield. This month we’re going to get a bit more advanced. We’ll make some parts from curved molds and then go to the opposite extreme, showing you a technique for making flat-side parts. As you’ll see, the previous advice about workspace preparation and proper tooling really comes into play. There’s that word: play. With the right tools, it will be just that. First, let’s do some mold preparation. We’re going to make some bits for a Falco. It’s called an all-wood, plansbuilt aircraft, but let’s face it, nothing is all-anything, so unless you’re an over-the-top expert in wood, you’re still going to have to do some work in composites. In this case, owner Mark Wainwright, a first-time builder, went to AirCrafters in Watsonville, California, for some assistance, and he was kind enough to allow us to document the process.

BY BOB FRITZ

Those two shiny spots are valleys on each side of a ridge. Fill them with micro so that the shape doesn’t transfer to this side of the finished part.

Note that this is not auto polish; it’s specifically for mold release.

Where to Begin Let’s start with some fairings for the control rods on the ailerons. This is a small part, so it needs to be held in position, and a nice way to do that is to use packaging tape to hold it onto a thin piece of plywood. It’s obvious that the mold in the photo isn’t the same as the 38

KITPLANES August 2007

one we just taped down, but it’s a good example of the next step: keeping discontinuities off of the visible surface of the finished part. Look closely and you’ll notice two shiny areas near the www.kitplanes.com

Micro and epoxy. You won’t need much, and it should be pretty stiff.

Here you can see the mold peeking out at the 6 o’clock position. Note that the threads of the fabric are also along that line, which is incorrect.

Smooth the micro on to the discontinuity and then don’t worry about it. The epoxy will fill in the small holes.

As you can see, the valleys were not wide, but they would have been seen in the finished part.

apex. Those are valleys with a large ridge in between. The first step toward fi xing this is to apply three coats of moldrelease wax, letting each dry for at least 15 minutes before polishing it off. Now whip up some micro with a bit of epoxy to about the consistency of cold peanut butter, and smear it onto the offending area, smoothing it with a wood depressor. There’s no need to get fancy. Let’s step off to the side for a moment and think this through. This is a male mold, so when you see the mold, you’re seeing the part. In this case, when the glass pops off the mold, the surface that does not Photos: Bob Fritz

touch the mold is the important one. But if I leave the discontinuity on the mold, the glass will follow the curve, generating an area to be fi xed later. So I’ll fill the valley with something that will stick to the glass but not to the mold. When the part comes off the mold, so does the micro. The result is that the discontinuity is still on your finished part, but only on the side not seen. At this point, the astute reader will be thoughtfully musing: “Meathead! Why don’t you just fi x the mold?” The question was put to Dave Saylor, owner of AirCrafters. “The surface of the mold is finished as necessary,” he said. “It can be nice and smooth or not. Smooth surfaces wax and release easier. High quantity production molds are usually smooth for ease of finishing and releasing. When the mold is hard to release, it gets dinged and scratched every time it’s used, and has to be repaired in order to maintain the surface finish. It’s like a woodsman sharpening his ax—the more time he spends sharpening, the less time he spends swinging.” Now, if this were a female mold, that is, a mold with a pocket to be filled, and had a similar discontinuity, we could do the same thing as long as the surface to be seen is on the side not touching the mold. Just keep in mind that the micro is going to stick to the glass, release from

Now we’ve rotated the fabric 45° so that the thread lines don’t cross the mold at right angles.

the mold and have the profile of the mold’s discontinuity. We also asked Saylor how to make a female mold. “A male mold is not the same as a plug,” he explained. “A plug is used to make a female mold. For example, to build a cowl, first you might build a plug that simulates the cowl, then ‘splash’ a female mold over the plug. The plug will not usually look exactly like the cowl, but rather it will look sort of blank or plain. Details are added later.”

Enter the Fabric Having waxed the mold and filled the discontinuities, let’s take a look at the fabric. The threads are at 90° to one another (warp and weave, remember?). To get the strength to be balanced we want to place the fabric onto the mold with as many threads as possible KITPLANES August 2007

39

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Composites, Part 4 continued crossing the long axis of the mold. As much as you might like to use that rotary knife (assuming your teenager hasn’t snagged it for cutting pizza), be aware that it works only on very smooth surfaces. If there are any grooves or bumps on the table surface, you won’t get a 100% cut and the piece of material will simply unravel. Scissors are the ticket, but make sure to use the ones you bought for cutting glass, and if you get any wet epoxy on them, clean them immediately. In fact, go clean them anyway; they work best when you use the metal rather than an epoxy coating as the cutting edge. Push and gently pull the fabric until it wraps down and stretches over the mold. This is an example of what we spoke of in the first article: drapeability. The process is much like last month in that you want to wet the fabric without displacing or over-stretching it. It’s important that you see the fabric change from shiny, silver-white, to dark, wet and translucent. If it’s not, add some epoxy.

These molds are about 10 inches long, 2.5 inches high and only 3/8 inch thick. It’s amazing how nicely the flat fabric will conform to that extreme shape.

Pushing the material to conform to the surface of the mold is easier than it seems. But you can take it only so far without it springing back. At some point it’s best to continue the forming with a very wet brush.

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Here comes our peel-ply, whose purpose is to chase out the excess epoxy and leave a surface finish with what a painter would call “tooth,” or roughness that allows the next coating to adhere. Just keep in mind that peel-ply is not a structural component; you’re going to remove it (we hope), so the fact that it won’t conform to the shape is immaterial. A patchwork of separate pieces is fine. Just make sure to cover the entire surface. On this mold you might try draping long, narrow strips first down the longest edges, followed by going diagonally over the glass. They stay in place better than 1-inch squares. Then just stipple it down with the brush, forcing the excess epoxy out through the peel-ply. Don’t go nuts trying to remove it from the peel-ply; you can let it dry there. Before we move on to the flat-side mold, let’s take a time-out to look at an interesting comparison. These two parts are identical except that one weighs

Like we showed last month, wet the fabric with the epoxy, being careful not to displace the fabric.

Don’t worry that the peel-ply is too stiff to go on in one piece; a patchwork works just fine.

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Here you see the start of the peel-ply and also the orientation of the fabric.

Harold Bunyi’s six years at the Lancair factory shows when he emphasizes scraping the table to get the old epoxy off. He uses an ordinary wood chisel for the job.

Peel-ply is all finished. Because they’re on a board, we can move the parts to a warm spot in the shop and have our bench clear for another project.

almost twice as much as the other. “Why is that?” you ask. The thick one weighs 49.5 grams, and the thin one weighs exactly 25 grams. The only difference is that the lightweight one had peel-ply treatment, and the heavy one did not. Now we all know someone who was

completely fanatical about the weight of his project during the building process and, after a couple of flights, started carrying an extra toolbox and a couple of quarts of oil in the plane. Or there’s the guy who spends 300 hours cutting out lightening holes and weighing the

Other than the glossy sheen, these pieces don’t look hugely different, but they are. The one on the left weighs double what the right one weighs.

Here you can see why there is such a weight differential even though they came off of the same mold.

KITPLANES August 2007

41

Composites, Part 4 continued scrap to make sure he has a really trim airplane. Only thing is, he had a couple of six-packs while doing it! The point is that if you squeeze out the excess epoxy, you’ll have room for the oil or the sixpack. But there is also the option of addressing the system weight. Both you and the airplane will fly better.

Working with Flat-Side Molds Anyway, speaking of making things flatter, let’s look at a different technique that works for flat-side molds. The operative word here is flat, so the first thing to do is to get the workbench clear of hardened epoxy drips and other blips. How flat? Follow through on this and it’ll become apparent. In this technique, we’re going to eliminate the excess epoxy even before we get to the mold. Start by measuring the flat area of the mold. Well, OK, it’s not flat. It curves, but not simultaneously in multiple planes, so simply measure one side, over the top and down the other side. Add an inch, and that’s the length of the fabric. For the width, measure the length of one side, plus one end, and then add an inch. This happens to be a component that’s going to take a lot of load, so we’ll need to cut two pieces of fabric, plus two sheets of plastic wrap 1 inch larger than the fabric. When cutting the fabric, remember to cut on a diagonal relative to the threads. If you can find a diagonal on the plastic wrap, let me know. Next we’ll lay down one sheet of the plastic wrap on our smooth workbench, and center a single sheet of fabric on it. Although it’s tempting to pour the epoxy, you should brush it on, smoothing and spreading it over the entire surface. Now look for the thin threads in the fabric and orient the second cloth with the thin thread at 90° to those of the first cloth, then center the second cloth over the first. If the cloth isn’t square, you’ll go nuts trying to do this by rotating the second cloth—ain’t gonna happen, Bunky. Simply flip the second cloth over, and there you go. You’re probably getting fuzzy-headed about now, 42

KITPLANES August 2007

Here’s how the sandwich begins. Not the eating kind, though you’ll use plastic wrap and a pizza wheel.

The sides are flat, and, ignoring the saddle at which Mark Wainwright is pointing, the curve over the top means a rectangular shape of material will cover it all.

but hang in there. This takes longer to explain than to do, and it’s obvious once you go through it the first time. As before, spread a bit of epoxy onto the second cloth; it won’t take much because the cloth below is already wet. Lay the second plastic wrap in place and, using a roller but not pressing excessively hard, squeeze the epoxy into the fabric and out beyond the plastic wrap. You could also put a thin layer of epoxy on top of this sandwich and then, using a squeegee, clean that epoxy off. That last coat of epoxy is a lubricant to keep from pulling the wrap.

Neat and Trim Next, you finally get to use the circular razor. Trim all around, cutting through the bottom layer of plastic wrap. Right now this sandwich looks like something you might put on your lower back for sore muscles. Word of advice, though— don’t. Instead, carry it over to the mold and set it aside while you give the mold

The sandwich is opened. It’s two layers of cloth placed 90° to one another between two sheets of plastic wrap.

Look closely and you’ll see that the fabric has thick threads and, at 90°, thin ones.

a thin coat of epoxy. Now peel off one layer of the plastic wrap, and drape the exposed cloth over the mold. You can center it, and work it around a bit, but eventually you’ll want to peel off that second plastic wrap. From here on you only need to fold the ends around and then, using your brush, push the whole thing into shape over the mold. Add some peel-ply, work out the excess epoxy (there won’t be much because of the sandwich method), and let the whole thing sit for a couple of days in a warm area. How long to cure depends partly on how you mixed your epoxy, but more so on the temperature. If your shop is around 50° F, as it was when we did this, you’ll need to let it sit for 24 hours before taking a small part from a simple mold, and three days for a full cure. As I said, it’s easier than you first thought and takes longer to describe than to do.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com. www.kitplanes.com

To create space for the epoxy-flox Harold Bunyi of AirCrafters created a tool to compress the paper honeycomb.

Part 5: working with carbon fiber.

L

ast month, we learned how to construct flat and curved parts from molds. This time, let’s see what it takes to build a gas tank out of carbon fiber. In this case, it’s a 325-gallon tank. Six pounds per gallon (OK, 5.85) multiplied by 325 gallons…that’s heavier than the 1320 pounds allowed for an entire Light Sport Aircraft! While this seems like a real “out there” project, consider that the construction techniques are the same as for a 32-gallon tank. Once you know the basics, it’s frequently just a matter of scaling up or down the projects that look intimidating. First off, the materials here were about $3500 for several sheets of two-ply/side carbon fiber with a 3/8-inch honeycomb core—sometimes called a two-core-two, meaning two plies of material then the core and then two more plies. Now aren’t you glad I said it could be scaled down? It sort of reminds me of the guy who said, “I’ve an unlimited budget and I’ve already exceeded it.” The advantage to us is that it’s a lot easier to see what’s going on with a project of this size. The first step is, of course, to have a set of drawings that 36

KITPLANES September 2007

BY BOB FRITZ

Take a good look at the green flange. It’s the secret to putting a glass fillet on all interior corners of a closed box.

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can be transferred onto the CF using a compass, straight edge, ordinary pencils and a lot of blue painter’s tape. Don’t use a grease pencil; it’ll contaminate the surface for later gluing. If it’s a large (spelled “expensive”) project, a mock-up in cheaper, easier-to-manage materials is a good idea.

The Value of Carbon Fiber You’ll remember in an earlier issue that we spoke of taking a stack of four 1-inch-thick boards and standing on them. They bent fairly easily until we drove a couple of nails through them, and then they didn’t sag so much. Those nails forced the bottom board to take a much larger radius turn and, hence, placed almost the entire tension load on that board. Inversely, the top board was being compressed. Thinking through this stack, you’ll realize that the second board was getting compressed less than the top board, and the third board was getting less tension than the bottom board. Now imagine pulling those two middle boards out of the stack while keeping the gap between the top and bottom boards. You’d still have the top board being compressed and the bottom board taking the entire tension load. This is exactly what CF honeycomb is doing. The paper in between keeps the top and bottom separated so that the bottom takes the entire tension load. This is completely reversible, of course. Top and bottom are just identifiers of

Jesus “Chuy” Gomez is using two sticks to aid in packing the epoxy-flox completely into the gap. You can see that he has periodically scraped the excess off the outer surfaces. It’s OK to pack those scrapings back into the gap.

Photos: Bob Fritz

Newtonian gravitational enhancement modules (bricks) are used to hold the top down.

the layers; flex the CF in the other direction and it works the same way. From just this example you can see why all modern composite structures are built like this, using some form of a core to separate the load-bearing fiberglass or carbon-fiber layers. Not only is it a strong way to build, it can be made surprisingly light.

Making the Cut We now have the pattern laid out, but how do you cut this stuff? The core is

just paper, so anything that cuts the CF is going to slice through the core like it was, well...paper. A toothed blade will do it, but a grit-edge blade does it better, especially if the part is too big to go on the bandsaw. Sanding the edges is also a bit different. Orbital or reciprocating sanders tend to lift the CF away from the paper and fuzz the edges. A hand-size belt sander is the ticket, but position it so that it doesn’t lift the CF. When it comes to needing a tight fit,

Basketball on a ping-pong table? No, laying out the pieces with painter’s tape. KITPLANES September 2007

37

Composites, Part 5 continued consider a plastic bag; it can be hard to tear until you create a tiny notch. That notch is known as a stress riser and serves to direct all the force into a small area, making the tearing pretty easy. That’s why your teeth, though they don’t cut the material, are more effective than your soft, round fingers at tearing open a bag of potato chips. It seems pretty obvious that to eliminate stress risers we have to spread out the load. In this case, that means getting the adhesive epoxy-flox to both sides of as many surfaces as possible. To do this, simply crush the paper core down about 3/ inch, exposing the interior sides of 8 the two CF sheets. This operation is surprising the first time you see it simply because going at the CF with a hammer seems anti-tech. As I said, don’t be intimidated by the technology; part of it may be rocket science, but not the part we’re dealing with. Once the interior sides of the CF are exposed, we have to add the epoxy-flox, but to ensure adhesion it’s best to paint the surfaces with straight epoxy. The procedure is referred to as wetting out, and is a simple job of slopping the epoxy into the groove. Again, this is to prevent stress risers in the form of gaps between the CF and the epoxy-flox. As a quick review, recall that flox is short cotton fiber and is quite strong when mixed with epoxy. Micro, on the other hand, is microscopic glass balloons that are not nearly as strong. They’re great when a lightweight material is needed as a filler in a corner or gap.

The cut is ready for sanding.

38

KITPLANES September 2007

Get in the Groove Once the material is wetted out, we can pack the groove with epoxy-flox. It should be a thick consistency, as we’ll be installing the part on-edge and don’t want the epoxy-flox to drip and run. It’s important to fill the groove completely, creating a mound that protrudes above the edge of the CF about half the distance between the CF layers. That is, if the gap between the layers of CF is 3/8 inch, the epoxy-flox should be about 3/16 inch above the edge. This general rule will tell you how much epoxy-flox to mix. The excess will squeeze out and be removed, so don’t go overboard. You’ll notice that glass, not CF, is used to radius-out the joints between the flat surfaces. This is because the advantage

Miss your line? Not to worry, just re-cut it. If the variation is less than about double the width of the blade you’ll find it difficult to keep the blade cutting on the line. Go for a belt sander instead.

Dual interactive self-referential layout modules with eco-friendly digital baseline (two guys with a stick). Lesson: Don’t be intimidated by techno-speak.

When the carbon fiber costs $3500, a mockup of the fuel tank is a wise precaution.

Doing the layout with an ordinary pencil works fine, but the carbon in the pencil lead all but disappears on the carbon fiber material. Follow up with blue painter’s tape, and then mark the tape to indicate the cutting edge.

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Note the orientation of the 1-inch-wide belt-sander Gomez is using. This is to avoid creating fuzz at the edge.

Cutting a straight line is aided by having a guide fence. Just remember to cut to the correct side of the tape

The 2x2-inch boards over the top are to hold the walls in place while the epoxy dries. It’s important that the surfaces are clean when placing the walls.

This vertical wall (the one with the hole) is an internal reinforcement as well as an anti-slosh barrier. Note how well the contour fits.

Isn’t that a clean, tidy junction? Hard to believe it’s so strong.

of CF is negated by not being able to vacuum-bag the junction. Once the wall is in place and the epoxy-flox has had time to harden, we only have to add a tape of fiberglass to the juncture. We covered that operation in a previous issue, and the procedure is the same here: Radius the corner with epoxy-micro, wet the surface, lay out the tape, smooth out any bubbles, and let it dry. This is getting easy, isn’t it? The top is the tricky part. Once you’ve laid out the pattern of the walls onto the bottom side of the top and placed a few triangles, you can paint on some epoxy and hope all the parts drop into place.

The Inside Story “So how could the killer have done it?” asked Dr. Watson. “All the doors and windows are locked from the inside.”

A bandsaw will cut CF honeycomb, but it also tends to tear the fibers, creating a fuzzy edge that has to be sanded. KITPLANES September 2007

39

Composites, Part 5 continued

Use of the tool is simple: Hit it with a hammer. The paper honeycomb is simply compressed out of the way.

The construction of the tool is equally simple: a piece of aluminum of a thickness to match the spacing of the CF and a pair of plates riveted to it to limit the depth to a distance equal to the distance between the layers of the CF.

It’s messy, but completely filling the groove is essential. Wetting out the groove ensures that the epoxy-flox comes in contact with all surfaces.

Just as was done prior to packing the groove full of epoxy-flox, it’s necessary to wet out the surface. That small, lightcolored triangle is held down with hot glue and is for positioning the bottom edge of the wall.

Scrape off the excess as best as you can.

Adding a layer of glass to radius-out the corner makes it even stronger.

40

KITPLANES September 2007

The wall (left) is in place and checked with roofing squares. When the epoxyflox has hardened, the triangles can be scraped off.

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#1 In Customer Service: We’re done with the interior walls. Putting it all together takes some planning but not a Ph.D.

The same incredulity applies here: How do you reinforce all of the interior junctions without being inside a closed box? You start by putting release tape on the last surface to be installed. Then build the walls into it, glassing all of the junctions. Then remove that taped last surface, and you find you’ve created glass flanges. Put the opposite wall in place, glass the corners, and let it harden. Finally, apply a coat of epoxy to the flange and that so-called last surface, and put it together. Elementary, my dear Watson. By now you’re wondering what a 325-gallon tank fits into. Well, here’s a non-clue: It’s one of four, and they’re all different shapes. But it’s a secret project. We’ll tell you all about it later when the owner gives the go-ahead. Next month we’ll tackle a smaller project, a set of fiberglass wheel chocks.

If you want to build with me I’ll be using the materials available in the Composite Materials Practice Kit from Aircraft Spruce, part number 01-15000. (There is also a p/n 01-15100 that’s the same kit without the book. I recommend you get the book because your education should never stop and, besides, it’s worth the extra money just to see photos of a very much younger Burt Rutan.) For this project, you’ll also need a circular razor, a couple of feet of rope, and a plastic or disposable surface about 24x36 inches. The first set I built got appropriated by my wife for her Cessna, so I’m making another. Come by and watch them come together.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.

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41

Stripping off the peel-ply. Don’t expect a flash of light, though.

You’ve rounded the corners to about a quarter-inch radius, now vacuum off the dust. Brushing it off just doesn’t remove it all, and it’ll mix into the epoxy.

Cutting the foam is easy even with a dull blade.

In Part 6, enough theory! Let’s build some wheel chocks.

I

recently sat in on Dave Saylor’s Fiberglass for RV10 Builders class (see the sidebar, Page 40) in which he had everyone build a set of wheel chocks. It was such a good experience I thought I would shamelessly pirate it for this article. I was also fascinated to see the different interpretations of seemingly straightforward instructions. More on that later. What we’ll do here is a dandy little application of what’s come before in this series. The nice part is that the result is not only useful, but you’ll be the envy of your neighbors and invited to give talks at the local Toastmasters. OK, that’s a bit over the top, but you will have a useful set of wheel chocks that weigh about a tenth of what those old wood wedges you’re lugging around weigh. I’d suggest you build at least two sets because your buddy will want one. 38

KITPLANES October 2007

BY BOB FRITZ

Materials List Glass cloth: 24 x 7 inches. Don’t forget to cut this on the bias. Flox: 1 cup. Microballoons: 1 cup. Epoxy: 2 cups. Mix per the manufacturer’s instructions…but not all at once! Stir sticks: a dozen or so. Rubber gloves: at least six pairs. Cups: Paper, not plastic, and definitely not wax coated. A dozen or so will do. Foam for mold: 2-inch square by 24 inches long. It should be small cell; the type that flower arrangers like to use is perfect. We’re going to split it into two www.kitplanes.com

Warped, isn’t it? That’s the bad half of the good/bad characteristic of drapeability.

Cut on the bias with the circular cutter, but be careful. It also helps to have a friend or a couple of clamps holding the straightedge, as the cloth is slippery and the straightedge moves.

A bit of gentle pulling, though, will move it right back into a nice rectangle.

Paint on a layer of epoxy-micro to seal the pores of the foam. One glove? The camera is on a timer, and I don’t want to risk epoxying the shutter button. A bare hand will know when it’s contaminated.

triangles so if you can buy a triangular piece, all the better. Rope: 18 inches long and any diameter between 1/8 and 3/8 inch is fine. Any material is OK, but plastic is best. Clear plastic-wrap: One 24-inchwide roll will do. Heavy gauge is best. Kitchen wrap clings to itself too easily and tears apart instead of coming off the hardened glass. Roller: Should be 3 to 4 inches wide, any diameter. Even a piece of broom handle will work, but you like tools so here’s an excuse. Circular razor, aka pizza cutter. Photos: Bob Fritz

1-inch brushes: a dozen of the cheap disposable kind. 60-weight sandpaper: One sheet is enough, really rough grit. One old, large, flat-blade screwdriver or chisel for digging out foam. Peel-ply: One piece about 3 inches wide by 24 inches long. Needle or pushpin for piercing bubbles. Bandsaw or cut-off wheel to cut the hard fiberglass. Step drill for use on hard fiberglass. Drill motor for that step drill. Safety glasses.

36-inch straightedge.

Other Requirements This project will take a couple of hours on day one, an hour on day two, 30 minutes on day three. You’ll also need warm weather or a warm workshop. Below 65° F, and the epoxy won’t harden. Use a small workbench with a plastic, or at least disposable, top. The epoxy will get stuck to it, ruining a wood surface.

Building Day 1 Split the foam square into two triangles about 24 inches long. Sand the three KITPLANES October 2007

39

Composites, Part 6 continued edges to about a 1/4 -inch radius and vacuum off the dust. You are wearing the safety glasses, aren’t you? Mix epoxy and add micro until it has the consistency of mayonnaise; the brand is not critical. Paint this mixture onto foam and set foam aside. This fills the pores of the foam. Lay out 30 inches of clear wrap. Lay out fiberglass cloth onto wrap. Work out the distortions so that the piece is square and straight. Lay second fiberglass on first and

straighten it. Mix about one cup of epoxy. Pour over cloth. Spread with brush or squeegee until completely wet. The cloth will be bright until soaked with epoxy. Lay the second sheet of plastic wrap over the wetted cloth. Use the roller to work out bubbles and excess epoxy. You need only a light pressure; don’t distort the glass. Use the straightedge and the circular razor to cut through the plastic and glass. You want to have a sandwich that will wrap around the foam, overlapping

RV-10 Builders Workshop

Aluminum seems “safe” for most of us compared to composite construction. After all, if metal shop is taught in high school (at least it was when I was there) it can’t be too tough, but composites? The rub of modern kit aircraft is that even the most ardent aluminum lover is going to have to pull on the rubber gloves and grab a stir stick, and it’s more than a little intimidating when the part to be worked with is so big that four people are needed to fit it to the fuselage. So it is with the roof/door/window section of an RV-10, one of the most popular projects on the market. Yet, after spending two full days at the RV-10 composites class at AirCrafters in Watsonville, California, the task of taking a saw to the single most expensive component in the kit was not so intimidating. At lunch several of the 10 participants, none of whom had prior experience in kit aircraft construction, commented that the class had really been worthwhile. “I’d have spent two weeks staring at it saying, ‘There’s no way I’m going to cut that much off,’” explained one builder. After the cabin, the item of most interest was the alignment of the wheel fairings. The cowling and doors were discussed, and a simple project of making a set of glass wheel chocks gave everyone some experience in using the materials. —B.F.

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KITPLANES October 2007

Epoxy-flox is poured onto the cloth. Some advocate brushing only, but this seems to work just fine.

at least one face, and be about 1/2 inch longer than the foam. Use the handle of the brush to push/ lift the corner of the upper plastic. Gently pull away the upper plastic and dispose of it. Place foam stick on the cloth 21/2 inches from one edge of the cloth. (Read the next step to understand this positioning.) Lift plastic so that it wraps the cloth over one side of the foam mold and halfway down the second side. Brush the surface of the plastic at 45° to smooth out air bubbles. This angle minimizes distortions in the cloth. Use the pin/needle to pierce any bubbles and continue smoothing out the air. Use the handle of the brush to push and lift the corner of the plastic. Lay plastic back down to original position leaving the glass cloth in place. Lift the other plastic/glass over and around the foam. Roll the foam/epoxy over if needed to get all the cloth in contact with foam/ epoxy. Be careful to not roll the plastic wrap in between the layers of glass. Brush the surface of the plastic at 45° to smooth out air bubbles. Don’t forget to use the pin if needed. Unwrap the plastic back down to original position, leaving glass cloth exposed on two sides of the chock. Lay the strip of peel-ply onto one side of the foam mold. Use a brush to smooth out the peel-ply. www.kitplanes.com

Not much point to using a rubber squeegee. It would become rigid when the epoxy dries and couldn’t be cleaned as easily as a steel blade. Also examine the bottom-right corner: You’ll see that there are two layers of glass. It’s referred to as two-bid for two layers of bi-directional cloth. Bi-directional means that the cloth has approximately the same number of threads in both the X and Y directions.

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Use the needle to pierce any bubbles and then continue smoothing. Rotate the part so that the peel-ply is down; remove the remaining plastic wrap and dispose of it. Place it on a flat sheet of new plastic wrap and let it dry 24 hours.

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Building Day 2 Pull off peel-ply. It will be a firm pull. Note that this side is now the “down” side. Use the saw to cut off both ends of the stick so that they’re square. Draw a cutline at the midpoint of the length of the stick. Place a mark on each side of the cutline about 1/2 inch away from the cutline. This is where you’ll drill holes for the rope. Cut the stick in half. No, not lengthwise! Use the screwdriver to dig out 1 inch of the foam from the end of both chocks where you marked the rope hole. That’s why I said it should be an old screwdriver. The foam is soft stuff, and we’ll fill this in with epoxy later. Dig out 1/4 inch of the foam from the other end of both chocks. Drill one hole, barely large enough for the rope, where you made the mark on each chock.

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41

Composites, Part 6 continued

The end of the brush works well for lifting the corner of the top layer of plastic sheet.

Don’t forget to peel back the clear plastic before wrapping the glass over itself. You have to have a friend help lay down the top layer of plastic. If you think kitchen wrap is frustrating, try this alone!

Bubbles, always bubbles. It’s surprising, though, how effective a simple pin is in getting rid of them.

Roll out the bubbles. It’ll take about 10 minutes of gentle work the first time you try this.

One sheet of peel-ply on the side where the glass wraps over itself, and we’re almost done with this phase.

A bit of brushing helps the peel-ply soak up the excess epoxy. Going all the way around the chock with peel-ply is possible, but it would leave the top two sides rough, and we’re looking for some aesthetics here.

Back to the circular razor to cut the sandwich to rectangular pattern just a bit longer than the length and about 3.5 times the “circumference” of the foam.

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KITPLANES October 2007

If you have to transport the stick, wrap it in heavyweight plastic. Lightweight plastic will stick to the epoxy, and there go the aesthetics. Guess how I found that out.

www.kitplanes.com

I had to transport the stick so the ends were distorted, and that required cutting off a bit more than planned. No big deal. Then find the middle and cut the stick in half.

These Unibit drills are the hot ticket for drilling through just about any thin material. You want a hole just a bit larger than the rope. It should be in the two ends from which you dug out that inch of foam.

Dig about half an inch of the foam out of one end of each chock and an inch out of the other two ends.

Tape the chocks together, base to base, with the rope holes at the same end. Cut about 18 inches of rope. If it’s plastic, use the soldering gun to melt through so that it won’t fray. If it’s a fiber rope, wrap it with electrical tape and the cut at the midpoint of the tape to stop the fraying. Insert the rope through the holes. If the rope is a snug fit through the hole, good. If not, tape the rope to the chocks. Don’t bother with a knot; the next step is the real anchor. Mix about one cup of epoxy and paint it liberally into all four ends of the chocks, soaking the rope ends as well. This wetting out ensures that the epoxyflox used in the next step will adhere to the chock and the rope. Add flox to the remaining epoxy to the consistency of cold peanut butter. You don’t want it to move when held on a stir stick. One of our guys left it rather thin and, though it made a nice, smooth end, it meant that he had to wait a day for it to set up before he could fill the other ends. Pack the mixture into the ends of the chocks so that it stands up about 1/8 inch above the edges. Don’t worry about get-

ting it smooth, just have it stick out. Place the chocks on a couple of stir sticks for 24 hours so that the flox will fully set. Forget the stir sticks, and you risk gluing the chocks to the workbench. “Nice set of work-stops you’ve got on the bench, Fred. But shouldn’t they be moveable?”

The rope on the right was cut with a knife; the one on the left was cut with a soldering gun and then massaged into shape with a wet towel. Don’t try it with a dry towel; you’ll pull plastic threads in all directions. And don’t do it barehanded; the neighbors will learn some new words when that hot plastic sticks to your fingers.

No knot needed. Just wet out the interior, mix up some stiff-as-cold-peanut-butter epoxy-micro, stuff it in both ends, let it set, and then sand it smooth.

Building Day 3 Untape the chocks and sand off all four ends to square and smooth. Here’s your excuse to go buy that inch-wide belt sander. Paint on or write your N number. If you don’t, these are guaranteed to grow legs and walk away. Be prepared to do it again for your buddy. See, I told you to make two sets. I built at Dave Saylor’s AirCrafters and then put it in the airplane to take it to my shop about an hour’s flight away. Mistake. The air within the foam expanded as I went to altitude with the result being rather large bulges where the wet glass was pushed away from the foam. Lesson: Don’t move the piece until it hardens. The saving grace is that holes, separations and bubbles are of no consequence here because these are nonstructural components. After all, I’m just going to throw them on the ground and park an airplane on them! I went on

to build a second set that came out perfect, though, so I’ll be able to make that Toastmasters talk after all. Even with bulges and voids, the upside is that you’ve learned a lot about how to work with composites. Aren’t you glad you did this before you started on that RV-10? The folks that came to the class were. After finishing, they all agreed that building this simple set of chocks left them rather proud of themselves and confident enough to tackle a more ambitious project…like glassing in an RV-10 windscreen. Next month we’ll take up vacuumbagging. No, that’s not a game played in Scotland by guys wearing skirts.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com. KITPLANES October 2007

43

Dave Saylor holds the finished window. Two pieces of carbon fiber were made using the bagging technique described, then bonded together in a bag. It’s super strong and light.

In Part 7, we tackle vacuum bagging.

V

acuum bagging…it sounds like the next stage of self-expression after you’ve gotten a couple of tattoos and been to the bodypiercing shop. But, no, it’s a process for creating very low weight, complex shapes using composite materials. In previous articles we looked at wet layups where one soaks the fabric in epoxy and then drapes it over the mold. You’ll recall that much effort was expended in getting air out from between the layers and reducing the percentage of epoxy in the product. You’ll also remember in our first article in this series that the strength of this technology comes primarily from the fibers; the epoxy’s job is to prevent shifting of the fibers relative to one another. It follows, then, that when it comes to the epoxy/fiber ratio, the adage, “If enough epoxy is OK, more is better, and 48

KITPLANES November 2007

BY BOB FRITZ

too much is just right” is wrong. Once the job of nailing the fibers together is done, more epoxy just adds weight. Is it easy to have too much epoxy? A beginner at wet layup might have an epoxy/fiber ratio of 90/10. Someone skilled in wet layup will reduce it to less than 70/30. In a sophisticated aerospace component this will drop to 40/60. However, achieving this requires excellent process control, without which you might find uneven distribution of the epoxy to the point of having dry spots. A reasonable target for us is in the neighborhood of 60/40, and vacuum bagging is a great way to get there. www.kitplanes.com

Jesus Gomez and Harold Bunyi demonstrate the value of teamwork in laying out the bottom side of the bag. Note that this is not exactly the same as the schematic/ cross-section drawing. Here, they’re going to squeeze two rigid parts together with epoxy in between rather than force a glass/epoxy matrix into conformance with a mold. The principles, though, are identical.

The vacuum tape consists of white paper separating a strip of putty-like adhesive.

The guys are creating the bag. You can see that Gomez is about to set the upper side of the bag into position over the tape that Bunyi has laid down.

But how important is it to keep the pounds off? If you’re building a Light Sport Aircraft you’re limited to 1320 pounds maximum gross weight. If you’re building an ultralight, well, the name says it all. Keep it under 254 pounds. Rule compliance aside, there is still the issue of performance. Kelly Johnson of the Lockheed Skunk Works quantified it best: “Each pound adding to the airplane’s overall weight cost us one foot Photos: Bob Fritz

of altitude, so in building the U-2 we were ruthless weight-watchers.” Even if you’re not building the U-3, if you’ve ever taken off with full tanks, all the seats full and the baggage area maxed out, you have a feel for the advantage of permanently removing 50 pounds from the airplane. (That’s why I’m on a diet.) Getting there consistently, though, requires a bit more than a pin to get the air out and a squeegee to remove the

excess resin. (That refers to the epoxy, not the diet.) That’s where vacuum bagging comes in. It uses air pressure to squeeze out the excess epoxy and trapped air. This improves the bonding between the layers, decreases the cure time, and will frequently get you down to that target of 60/40. You’re thinking, “Oh, sure, but I can’t do that in my little shop.” Ah, but you can, Grasshopper, and it’s neither difficult nor expensive. If you’ve already done a wet layup you’ll need just a few more items, the most expensive being a vacuum pump. However, even that can be had without too much pain. In fact, in a future article, we’ll show you how to build a nice vacuum pump that is powered by compressed air. You’ll notice that I did not say, “Use a vacuum pump to suck the air out of the bag.” This is so often stated that the engineer in me goes nearly apoplectic every time I hear it. Hold this thought: Air does not suck, it blows. A moment’s cogitation to consider the tensile strength of air should convince you that the vacuum cleaner you use on the carpet is really a pressure-differential cleaner. The “vacuum” in vacuum pump, as opposed to a compressor, is simply a sloppy way to describe whether KITPLANES November 2007

49

Composites, Part 7 continued the machine alters air pressure to above or below ambient. It’s important to understand this concept, because without it you’ll not understand the why if something goes wrong in this process.

The Setup The basic idea is simple: In doing a wet layup, we used peel ply and a brush to soak out the excess epoxy. This time, we’ll mechanically squeeze it out. You could try to sit on the bag, but as you do so you’ll remember the last time you tried to squeeze the air out of an air mattress. Tough to flatten out those random pockets of air wasn’t it? Sort of like playing Whack-a-Mole at the amusement park. Instead, let’s get Mother Nature to sit on the bag. All we have to do is push a bit of the air out of the bag with a vacuum pump, and the rest will take care of itself.

The two components received a generous layer of epoxy-flox and were taped together. They didn’t need the perforated plastic sheet, as there simply isn’t a lot of epoxy to squeeze out and it all comes to the edges. The bleeder cloth, though, is required to give the air a path for leaving the bag.

After placing plenty of bleeder cloth in positions that will channel all the air out of the bag, it’s time to close up. Again, a buddy to help in doing a part this big is essential to keeping the wrinkles out of the tape zone. A wrinkle there is a leak.

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KITPLANES November 2007

www.kitplanes.com

One more digression into science. Air pressure is simply the load exerted by the weight of the air above a specified area. Go up, and the air pressure decreases because you’re now above some of it. At sea level it’s 14.7 psi; at 18,000 feet above sea level it’s half that. If you don’t think air has a weight, put a flattened basketball on a gram scale, then pump it up and weigh it again. What that means is that unless you live in a very deep hole in the ground such that the air pressure is greater than at sea level, the absolute limit you’re going to place on the bag is 14.7 psi. Now, 14.7 psi is not a trivial load; on a part measuring 12 x 12 inches, you’d have to stack 42 50-pound bags of sand in perfect distribution to get the same effect. The reason you don’t feel that weight is because you’re 99% water, which simply transmits the load through you without crushing you. Taking a look at the diagram, we see that there are a lot of components to the bag. The vacuum bag, fabric, peel ply, wax and mold are all familiar by now, and a vacuum pump is obvious. But how do the sealant tape, perforated plastic and breather material fit in? Clearly, to transform the bag material into a bag you have to seal it. That’s the job of the sealant tape. It’s a putty-like material that adheres to the bag material. It comes as a roll and is roughly one-eighth by onehalf inch when laid out. You strip off the paper that protects its sticky surface and simply push the bag material into it. Wrinkles should be avoided as much as possible because they are leak paths. Vacuum tubing is vital. Obviously, Tygon tubing will simply collapse, so a more rigid material is required. Size is not critical; just keep it short, 4 to 5 feet maximum, and unkinked. Last is the pressure differential generator, aka vacuum pump. This can be anything from a vacuum generator (no moving parts) that hooks up to your compressor, to an aquarium air pump, to a big-bucks industrial vane or screw-type vacuum pump. The size of the pump is entirely dependent upon the size of the project and the integrity of the bag. You should be able to achieve a solid squeeze

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Composites, Part 7 continued in 5 to 8 minutes. Too small a pump and you won’t keep up with the inevitable leaks. To explain the function of the other parts, let’s go into the bag to see what happens when the pressure is on. First, the bag itself starts to collapse, pushing against the bleeder cloth. Remember that air mattress? Tough to get out those random pockets of air. That’s where the bleeder cloth comes in. It’s a thick, porous mat that gives the air a path by which to escape the outside push. The bleeder cloth has another use. The gases and excess epoxy pass into it with the liquids being trapped, and the gasses continue through the breather material and out through the vacuum pump. It’s somewhat akin to wearing socks on a wet carpet. The water gets squeezed out of the carpet and into your socks through a combination of capillary action and a pressure differential. Speaking of pressure, the bleeder cloth also serves to distribute it evenly. Without it, any wrinkles in the bag would define the surface contour of the finished part just like a wrinkle in your pillow leaves you looking like you’ve fought a saber dual. As the bleeder cloth collapses, it pushes against the perforated release film and the peel ply. Epoxy, being a liquid, is non-compressible, so it squeezes through the weave in the peel ply, through the small holes in the release film, and soaks into the bleeder cloth. The air bubbles in the epoxy also are forced through the release film and, as the layers descend, they push the fabric against the mold. The epoxy/air between the fabric and the mold gets pushed through the fabric, through the holes in the release film, and it also deposits into the bleeder cloth. Just like that, you’re at the 60/40 ratio. I should note here that some of my sources tell me that peel ply is optional, but that it’s good for fine-tuning the ratio of the epoxy to fiber. It’s also a good filter to keep strands of the breather material away from the epoxy-fabric, but it means the part will assume the texture of the peel ply.

The fabric has been soaked in resin and then assembled with all the rest of the components. All that remains is to turn on the pump, let the epoxy set, and then pull out the finished part.

They take the time to carefully push the upper bag into the tape.

The hose goes in with special attention to keeping the bag material away from the inlet. Gomez has even wrapped the end of it with bleeder cloth.

KITPLANES November 2007

53

Composites, Part 7 continued

Here’s what it looks like if the bleeder cloth is excluded. The inevitable wrinkles in the bag and release film define the surface finish.

Choices and Tradeoffs Is this the only way to vacuum bag a part? Of course not! That’s the fun of this process. It’s open to invention and variation once you understand the basics. For instance, to make a flat layup such as an extra thick spacer, you could use painter’s plastic for the bag, kitchen sponges to soak up the excess epoxy, Dacron material from the fabric store (that’s what peel ply is made from), duct tape to seal the bag, windshield washer hose for the vacuum line, an aquarium air pump for a vacuum source and skip the perforated release film if it’s a part smaller than about 10 square inches. I did it, and it worked great. Another variation was demonstrated by AirCrafters’ Guru of Glass, Harold Bunyi. He’s a bit more elegant in making up the windows for a Lancair when he bonds two existing shells into a single component. The bag goes completely around the assembly and, when the pump is turned on, it gets about 4000 pounds of perfectly distributed pressure while the adhesives do their thing. And that vacuum pump that uses compressed air? Here’s a hint: Bernoulli. We’ll show you how to build one next month. For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.  54

KITPLANES November 2007

Now they add wrinkles. The bag material will stretch just so much, and then it either displaces the parts or won’t fully collapse around the assembly. Placing wrinkles is a skill, though. You have to “see” where additional material will be needed.

It started out as a square bag and ended up looking like something from the Bat Cave, but these two parts are getting squeezed with about the same load as if a Hummer parked on them.

www.kitplanes.com

We can make a vacuum reservoir out of some large diameter ABS pipe and a few fittings. The hose on the right end goes into the bag.

This pump by Vaccon is about 5 inches long, uses about 60 psi air, costs about $80, and is more than adequate for use in a vacuum-bagging operation.

Part 8: Building a vacuum pump.

L

ast month we described vacuum bagging, a fairly simple process that uses atmospheric pressure to squeeze out excess epoxy and those nasty little (and sometimes large) air bubbles from your layup. But a vacuum pump is not something most folks have on the shelf, and though the one you bought for the instruments on your airplane will develop enough of a pressure differential to be effective, it’s way too expensive to be pumping epoxy vapors through. A quick survey on eBay revealed a dizzying array of pumps ranging in price from $20 to $2000 with some at the high end being inad-

BY BOB FRITZ

equate, and an occasional gem that fits what we want for as little as $75. But how do you know if that bargain is up to the job? I’m glad you asked! There are four ways to develop a vacuum. OK, there are several more, but they’re impractical for our application.

How to Get Vacuum The first way to get vacuum is with a rotary vane pump. That short beer-can size pump on the back of your aircraft

With all of the parts in place it looks like this. Air from the compressor enters at the fitting on the bottom left. It shoots through the blue pump, exits out the black noise muffler at the end, and creates a low-pressure area at midpoint. The air in the bag and vacuum reservoir flows toward that lowpressure area and past the gauge on top, which gives you a sense of progress. When the pressure gets low enough, the vacuum sensor opposite the gauge sends a signal to the valve, located between the air inlet and pump, and shuts off the flow of compressed air. It works well and can be built for about $100 if you scrounge a few parts.

Photos: Bob Fritz

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Composites, Part 8 continued engine is in this group. It has a set of vanes being spun inside a cylinder and can range from anemic to really powerful. It usually has the input power shaft mounted directly to the output power shaft of an electric motor and is, hence, cleverly described as a “direct drive rotary vane pump.” The second method is a piston pump and, as the name implies, it is simply a lawn mower engine with no spark plug, spun by an electric motor. The third method is a diaphragm/ flapper valve type pump. This uses a cam to push a diaphragm back and forth and expel the air through a simple rubber flapper that covers a hole. While seemingly fragile with all that material being flexed, you’ve had one for years and never noticed it because it was so reliable; it’s used most often in your refrigerator. If you’ve ever looked into the back of your appliance, you’ve seen a can that looked like a flattened soccer ball; inside that can, buried under a lot of soundproofing, is a pump about the size of your fist. Now, before you start to raid the junkyard for one, you should know that though they work great in a refrigerator, they won’t have the capacity to pump enough air to a hard enough vacuum if you’re making vacuum-bagged parts larger than a shoebox. All of these methods have one thing in common: They have moving parts. The fourth way of pumping out the air is to use compressed air. Right now you may be thinking, “Use high-pressure air to generate low air pressure? He’s gone weird on us.” Well, perhaps, but in this case I’m doing well, thanks for asking.

Do Try This at Home Go get two books of the same thickness, place them on the desk about 4 inches apart and lay a single sheet of paper on the gap. Now blow through the channel. Intuition would say that you’ve pumped air into the channel, so the paper should rise, but instead it drops. That phenomenon had puzzled folks as far back as the ancient Greeks, but it wasn’t until the mid-1700s that Daniel Bernoulli figured 84

KITPLANES December 2007

It’s only a simple solenoid-driven valve that can be had via a web search. You want to get one that is for vacuum, though, not pressure.

Showing signs of use, this hose end may be inelegant, but it works fine.

This vacuum sensor will send an electrical signal to the valve when the pressure rises. Again, an eBay search for one will likely be successful.

it out. What’s happening is that when a fluid accelerates, its pressure drops. The math is complex and it’s not necessary to understand the principle for our purposes, but we put it to use all the time. For instance, take a look at the throat of a carburetor. It starts large and then narrows. You’d think that forcing the air to flow through a small area would drive the pressure up, but the only way to get the same amount of air out as went in is to make it go faster. The pressure drops just as it does with our books and paper. It’s the phenomenon being used by that double funnel on the side of older airplanes. Why have a vacuum pump bolted to the rear of the engine if this is so effective? Well, first, it works when air is flowing at a fast rate; during takeoff you’re not going fast enough to get the vacuum-driven gyros up to speed. Second, it’s prone to icing, which is certainly not desirable in the chain of

machinery that’s most needed on a dark and stormy night. But if we can push air through it fast enough, and icing is not an issue, then it’s perfectly capable of developing the sort of pressure differential we need for vacuum bagging. Inasmuch as we’re unlikely to crash if it ices up, all we need to do is get the air flowing, and that’s where your shop compressor comes in. We’ll blow air through the right port of the pump and hook up a hose to the vacuum side.

Even Better We can improve on this, though. The goal is to keep a vacuum in a bag that probably has a few leaks. Instead of hooking the pump directly to the bag, let’s hook it up to a reservoir and then to the bag. This is somewhat like having the air tank on your compressor; without it you’d have to turn on the compressor every time you wanted to www.kitplanes.com

blow up a bicycle tire. Next, we need a hose from the reservoir to the bag. If you simply stuck an openended hose in the bag, it would tend to grab the plastic of the bag. Instead, we’ll plug the end of the hose and knock a bunch of holes in the sides of it. It’s also helpful to know just how effective your system is, so we’ll put a vacuum gauge on it between the pump and reservoir. You could check for leaks by watching the bag, but that’s hard to quantify: “Hey, John, does that bag look any bigger?” With a gauge you should watch for a rise of not more than about 5 psi in 5 minutes.

One More Thing That still leaves you either standing around watching the gauge and turning on the compressor, or letting the compressor cycle off and on for the 24 hours it can take for the epoxy to set. So let’s automate it by using the same setup as on the compressor and installing a pressure sensor and a solenoid. On your compressor, when the pressure rises to a set level, the sensor sends an electrical signal to a solenoid-controlled valve and shuts off the pump. We’re doing the same thing: When the pressure in the bag and reservoir rises due to leaks, the sensor sends a signal to the solenoid-controlled valve and opens up. That lets the compressed air go through the pump and restore the vacuum in the chamber and bag. When the pressure drops below the set level, the sensor shuts off the flow of air through the pump. Of course, if you don’t want to tie up your shop compressor, you could substitute the venturi vacuum pump by going on eBay and looking for a small pump built by Robinair. It is a low-cost brand, and you need a capacity of only 2 to 5 cubic feet per minute to keep up with the inevitable leaks. Although they sometimes show up for about $50, you’ll still have to add all the other bits to get the automatic off-on feature. Go have fun, and before long you’ll be whipping up some really neat parts, leaving all your buddies amazed and stopping by with a pizza in exchange for advice or help.  KITPLANES December 2007

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These pinholes show up only because they had a wipe of black glazing (above left). Small enough to ignore? No way! Chuy Gomez is pointing them out as unacceptable land mines that will explode through the paint. This one (left) is definitely too large to ignore.

Part 9: Dealing with the pinholes problem.

I

f you’re thinking of building a composite airplane, you should be aware that it’s built roughly in thirds. The first third of your time will be spent assembling the airframe. How long that takes is up to you and the designer of the kit. The second third is spent installing the systems, most notably the instrument panel. Again, that’s up to you. Are you a fast worker? Are you building it for day VFR around the patch or transcontinental competition with the airlines? The third third, if you will, has little to do with the first two: It’s the finishing. Of course you are proud of your accomplishment, but is the goal to build a sleek little gem that gathers trophies faster than bugs on a Kansas evening, or do you want air under you and runways disappearing behind? That old saw of being 90% done, 90% to go was never more apropos than in relation to the last third of the project. If it were an aluminum airplane, you’d just wash, dry, prime, paint and fly. (Sure, that’s a pretty broad generalization, but you get the idea.) You can do that with glass, too, if you’re not too particular about the finish. But if you want to be invited to park next to the runway at Oshkosh, you’d best Photos: Bob Fritz

BY BOB FRITZ

be prepared to spend some time coming into the house perfecting your JackNicholson psycho rendition of “They’re baaaack.” What’s back? Pinholes. I know, I know. At the outset of this series I said something along the lines of building a beautiful glass airplane can’t be that hard, after all there are so many of them. And I’ll stick by that. It doesn’t take a Ph.D. in polymer technology. It does take a little grunt work and a lot of working smarter by using the right technologies.

Find the Pinhole Pinholes are the inevitable result of using a porous material. Some pinholes are voids in the final layer of surface, which might be primer or gel coat, but might also be nothing at all because KITPLANES January 2008

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Composites, Part 9 continued some fiberglass components’ outer surface is simply the last exterior layup done or the final layer of glass-and-glue next to the mold. So what does a pinhole look like? How big is big enough to ruin your paint? As it turns out, the answer is: It depends. It depends on your tolerance level of course, but it also depends on your patience to let the filler material cure before painting. It also depends on how hot the weather is. I’ll get into those factors a little later, but for now take a good look at the pictures to see what we’re talking about. I spent some time with the guys at AirCrafters in Watsonville, California, to get the point of view of those with years of experience and see what they liked. When I asked for the expert, they all pointed to Jesus Gomez, aka Chuy. He suggested several products, but his emphasis was on preparation and technique to first avoid creating more pinholes instead of trying to eliminate them later. Once that’s mastered, it makes filling the little monsters a bit easier.

Keep It Clean First, let’s look at the technique. The watchword is “clean.” Any oil or dirt will create a pinhole in the paint where there was none on the bare glass. It’s easy enough to remedy; just get it clean, really

Start with a pinhole, then cover it to cause a bump of compressed air. Now sand it smooth, and what do you get? A small opening to a cavern that is even harder to fill in.

clean. How clean? Start by sweeping and vacuuming the entire shop. Without this step you’ll only frustrate yourself by kicking dirt into the air as you walk around the part, going at it with acetone and new rag. Wipe the plane down at least twice, too. Then he likes to give the part a misting of black paint followed by a light sanding to remove the black on the surface. What’s left is in the pinhole. Another method is to squeegee a layer of red or black glazing compound over the surface. Assuming the surface is white, or

at least light colored, the pinholes will show up like acne the night before the high school prom.

Product Placement Gomez then likes to use a body filler called Rage. It’s a lightweight version of Bondo that’s usually employed to do fine fillets. In this case, he goes at with a squeegee to lay on a coating as thin as possible. “This is where most beginners goof,” he said. “They think they can just slap it on and sand it back down.” That’s a mistake, he says, because using this poor technique results in the Rage not being forced into the cavity; it’s just layering over it. He also advises that if you want to use it for filling the larger divots or creating a radius in a corner, it should not be applied more than one-eighth of an inch thick. “It’ll crack as it dries,” he says. I checked with another friend who’s flying his sixth composite airplane, Jim McCormick of Jabiru Pacific. He learned a lot in the course of all that

PolyFiber will even supply the roller for applying the product. They’ve also written a pretty good manual on the subject of finishing.

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work and was quick to admonish me when I suggested wet sanding. “No! No! No!” he said. “That creates a very good paste that fills in the pinholes just fine, but it doesn’t adhere to the base material. When it dries out, it falls out and your pinhole is back.” A search of web chat room discussions consistently confirmed the use of Rage and also uncovered the name PolyFiber. I called PolyFiber, explaining that I was researching the subject and would like to speak to someone with technical know-how. A conversation with Jon Goldenbaum, self described gofer, trash guy and janitor at Consolidated Aircraft Coatings, aka founder and CEO of the company that makes the PolyFiber line of products, was a wealth of information. “So, what’s the cause of pinholes?” I asked. “Three causes, Bob. First, there’s prepreg. Even though the fabric is completely soaked with resin, a lot of it is squeezed out, leaving a texture due to the weave, and you can feel the ridges of the yarn. Second, the epoxy sometimes outgases, leaving tiny bubbles at the surface. And third, a simple, unfilled void in the cloth where it didn’t get sufficient resin to wet out…a dry spot.”

The PolyFiber Solution The company has two products of interest: UV Super Fine and SuperFil. Goldenbaum explained that the most apparent difference between the two products is their viscosity: UV Super Fine is near water, and SuperFil is a bit thicker. He went on to explain that though SuperFil has been on the market longer and is, therefore, more popular, it could stand a couple of improvements. The result is that they concocted UV Super Fine not as a replacement, but as a dedicated filler of pinholes. The reason for this is that if the pinholes are large enough, most coatings—including paint, Bondo and even SuperFil— will simply follow the contour of the hole, resulting in a wing surface with the appearance of the tee-off point at a Smurf golf course: little divots everywhere. You could cover them, but com-

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Composites, Part 9 continued pressing that tiny amount of air will only cause a tiny bump that you’ll want to sand down. The result is a tiny pinhole with a cavern underneath. The solution is to fill the holes, not cover them, which is what Gomez said. The reverse of that problem was also explained. “Bondo-type products are too heavy in viscosity to get into those small cavities,” Goldenbaum said. “For SuperFil to do this job, you have to really squeegee it in. A lot of our customers didn’t understand this and were slathering it on like it was frosting on a cake. That’s fine if you want to fill seams and use it to create corner radii, but not for filling pinholes. So we came up with UV Super Fine.” The liberal use of wetting agents causes it to flow out, adhering to the surfaces and filling in the pinholes. This allows the user to apply it either by spray or to indulge in a heresy, applying it with a foam roller. Hey, it works, it’s cheap, it’s fast and clean-up is a snap. In fact, Goldenbaum says it works better

than spraying because it really squeezes the material into the holes. Just don’t let your flying buddies see you going at your Bugsmasher VI with a roller and then expect less than a razzing.

Don’t Race for the Cure There’s an adage that says, “If it sounds too good to be true, it probably is.” Here, too, that maxim applies. You have to wait for UV Super Fine to dry at least three weeks, longer in humid weather, and invest in a dehumidifier while you’re at it. “Our job is to watch paint dry,” Goldenbaum said. This is where the patience issue comes in. The UV Super Fine, along with many other products, has to be 100% cured before you apply the paint. Rush it and the filler will continue to shrink, pulling the paint down with it. Similarly, if it’s not fully cured and you leave the airplane out in the hot sun, the filler shrinks unduly and can force the pattern of the glass cloth up through the paint. But then maybe you like that effect…just add a Levi’s label and call

it a feature. The offset, though, is that you can watch the filler dry for three weeks, or you can fill-sand-fill-sand for three weeks or more if the paint gods are displeased. If you have the front-row-at-theairshow obsession, you’ll have to have a go with SuperFil to get the little radii that make the airframe appear to be blow-molded in one piece. After that, you will need to seal the surface with a two-part epoxy primer and then allow it to cure for another week. But that’s literally a discussion for next month, when we wrap up our Build Your Skills: Composites series by taking you from the done-with-pinholes stage right to the last steps in preparing the final skin of paint. Just as with pinholes, technique and patience play starring roles.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.

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Part 10: Repair of a twisted tail and more.

I

love working with aluminum. Having said that, fiberglass has its advantages, not the least of which is the invisibility of repairs. Sure, it’s sticky, slimy and can leave you scratching like a dog after an all-day possum hunt in the deep woods of southern Georgia, but for minimizing the aw-shucks factor, it’s hard to beat. How much so is illustrated by my current project— building a Jabiru J250 LSA. [Expect to see a full series on this project later this year.—Ed.] The kit maker says that the rear of the fuselage is molded around the horizontal stabilizer, so all I need to do is align it perpendicularly to the long axis of the fuselage, and then shift it laterally so that the same amount of stabilizer is on the left as is on the right. Seems simple enough…but whoa! Either this hangar is falling over or that horizontal stabilizer is, er, not quite horizontal. The first thing to do is make sure that the fuselage is level, and the best place to check it is right across the wing supports. The manual advised that the bottom of the doorsills is the point for checking for level. However, in that I would prefer to have flying surfaces parallel to flying surfaces, it seemed silly to select the doorsills. After all, I Photos: Bob Fritz

BY BOB FRITZ

It’s only 1.5 degrees low on the left, but that’s 2 inches lower than the right.

like to fly wings level, not doorsills level. As it turned out, though, my preferred reference agreed with theirs. A bit of scrounging and a long carpenter’s level allowed a confirmation of two things: 1. The fuselage is level. 2. The level is level. Digital levels are really fun, but they can be wrong. You must check them periodically for correct reading, and if they’re not telling true, recalibrate them. The KITPLANES February 2008

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Composites, Part 10 continued instructions for doing so are not exactly simple, but it’s absolutely vital to do this or you’ll be wrong to several decimal places.

How to Fix It With the wing attach points and door sills confirmed to be level—and therefore the horizontal stabilizer confirmed not to be—the procedure for correcting this had to be determined. The kit maker suggested that it simply be twisted to straight, temporarily locked down with sheetmetal screws and glassed in place. That brute-force approach didn’t appeal to me, nor did the potential of inducing residual loads. Thinking about it for a while, I suspected that when the system sat outside in the hot sun, the residual load would simply cause the tail to wind right back to out of alignment. But a second realization presented itself: I might take advantage of those loads and the susceptibility to heat by twisting it past level, and then applying a heat gun. If I could support it in just the right position while it cooled, I might get it straight and with no stresses built in. Carefully placed alignment marks were made, and then supports were positioned under the left end with weights on the right end. This induced a twist in the opposite direction, and, when it had

Step one: Is the fuselage level, and do all the tools agree? Steel machinists’ blocks on the wing-attach points make a platform for a long spirit level. Digital levels can be out of calibration, so a cross-check is in order.

gone past zero by 0.5°, clamps and then sheetmetal screws were added to guarantee I’d get back to this position. Bondo might cover a few screws, but nothing was going to hide the C clamps, so epoxy/flox would have to be added between the joining surfaces. This required that all of the hardware be pulled off followed by a wipe down with acetone. The stabilizer is removed by sliding it out sideways. Therefore, adding a coat of epoxy or flox would not be possible without smearing it down half the stabilizer. It was necessary to pry the gaps

open enough to paint some straight epoxy on the mating surfaces, add some epoxy/flox, and screw it together. Put it on the supports, add the weights and check that it’s back to 0.5°. Good. The criticality of this procedure had me frequently confirming that the fuselage was still level. With all of it looking good, and with a silent prayer to the gods of fiberglass, I started carefully painting around the interfacing surfaces with a heat gun. Now, this isn’t your mother’s hairdryer; this will take graffiti off a New York City subway station, so careful use is the order of the day. My technique was to heat the tail cone and its flanges while avoiding the stabilizer as much as possible. The last thing I needed was to turn the once-straight stabilizer into a Z shape. Doing it evenly, and not excessively, was controlled by placing my hand on the tail after applying a bit of heat. Bringing it up to a bit too hot to keep my hand on it seemed about right. Now came the hard part: leaving it alone. I worried that the torque I’d placed on it might be too much, and I’d

That little table on the left has a stack of wood under it, while that round, white container on the top right end is a 20pound stack of welding rod. The level on the right says that it’s low 0.5° on the right.

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come back the next morning to find it twisted the other way. It worked, though. It was at 0.4° the other way when I first checked it the next morning. Knocking out the blocks and taking the weight off let it snap back to 0.0°. The finishing touch was not anticipated; the gaps between the fuselage flange and the stabilizer disappeared under the pressure of the straps and the heat gun! Just add the required fiberglass strips, and it’s done.

Air in the Wing, Not Under It

With epoxy-flox in the joint and sheet-metal screws holding it down, the straps and clamps are there to get that last bit of shape conformance.

With all that torque, it’s time for the heat gun to let the entire tail cone twist the other way.

The next repair required the direct involvement of the manufacturer. The left wing showed what looked like a delamination in the upper surface. Unlike a tube-and-fabric aircraft, the wingskin carries a significant portion of the load, so an obvious defect like this is not something to ignore. Now, this is not a panic situation, but it’s not one to just paint over either. A careful examination top and bottom on both wings showed a similar blemish on the lower surface of the right wing. After a bit more data gathering, it appeared that what we had was a defect in the mold that caused a dry spot. We probably could have ignored it but preferred not to. It should be stressed at this point that this is not something to undertake without direct involvement from the manufacturer. It may belabor the obvious, but a misdiagnosis of the situation could lead to a solution that is not effective, so it is imperative that an expert evaluation be made to assure that this is the proper fi x for your flaw. But how to repair it? The concept was fairly simple: Put epoxy where it should be. Getting the epoxy below the surface without tearing up the whole wing was the problem. However, just as your doctor doesn’t have to saw off a piece of your arm to give you a medication, the fi x could be injected. And that’s what we decided to do. A visit It worked! There are no weights or supports other than the epoxy, and all of the surfaces are cool. Just glass over the interface and move on.

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Composites, Part 10 continued

To get epoxy into the area, we have to first get access to it by piercing the skin with a small punch.

to our local veterinarian supply store was necessary to obtain a syringe and a large-gauge needle. Fiberglass covered in a gelcoat, however, is a bit tougher than a horse’s hide, so another method was needed to get entry to the zone to be treated. A very small drill bit was considered, but in that we were aiming for a midlayer, sufficient depth-control made that a dicey proposition. But an awl, or in our case, a trammel-point, was just right for punching through the first layer while giving a feel for the cavity below. At first we tried a single injection point. That was not terribly successful, as it was not possible to determine if we were spreading the epoxy laterally. A lot of holes would let us keep trying until we found the void. The extra holes had the additional benefit that they not only allowed air to escape, they also bled epoxy, showing that the entire area was being filled. Frequent wiping to show the extent of coverage left us feeling pretty confident that we’d really solved the problem. All that was needed now was to finish the area as part of the pre-paint operations. Now before any of you aluminum builders get to smugly thinking that this sort of thing doesn’t happen to metal airplanes, I’d like to point out that when I started my RV-6, I was cautioned that first-time builders usually built between 1.2 and 1.5 airplanes. The stuff to the right of the decimal place was in a pile in the corner. I was right on target, too. But that’s another saga.  52

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Under thumb pressure, the area is distorted and wrinkled. This indicates a delamination, which, in a load-bearing area, is not desirable.

A large hypodermic needle and syringe were obtained from a veterinary supply house. The trick is to inject epoxy and see it come out the adjacent holes.

All that’s needed now is some finishing work with Bondo and some sanding.

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A plane flown as much as Stimple’s gets its dings. A proper paint job, though, makes repair a lot easier.

P 11 Part 11: Painting P i i the h bbeast. UUnderstand d d the h subtleties b l i off methods hd and materials before you try them on the real thing.

BY BOB FRITZ

“Paint your vehicle with thinned Rustoleum and a roller!” Well, that’s one bit of advice you’ll find on the web. At the other end of the spectrum is sending your several years’ worth of work to a distant shop, along with the deed to the house and your first-born child. How about a middle road? Say, do it yourself, but not with a roller? But not with a paint booth, either! You can do it. It’s not a slam dunk and will take a lot of work, but the satisfaction of getting a pretty fair job is enormous. Pride of parenthood covers a multitude of misgivings. As the conclusion of our “Build Your Skills: Composites” series, this installment will help you make the final preparations for painting and, should you so choose, give you a basic grounding in the painting process itself. A word (or more) of warning: Painting is not inherently difficult but can be unforgiving. Take your time, learn all you can, and practice on small components or test articles before pointing your gun at your airplane. Photos: Bob Fritz

Justification Should you go for it? Well, the price of having a $40,000 airplane painted is about the same as having it done to a $400,000 airplane—typically $5000 to $9000 depending on location, shop experience and quality, and the amount of prep work left to be done. Is it a good investment? For the $40,000 airplane you’ll spend 12% to 15% of the value of the airplane; for the $400,000 aircraft, less than 2%. The more expensive airplane will see the investment paid back when it comes time to sell. Regardless, many builders elect to paint on their own, for reasons of pride or economics. Before we go any further, you need to check your local environmental KITPLANES March 2008

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Composites, Part 11 continued regulations regarding spray-painting at home. If you live in a condo, be aware that, even if not harmful, some of these concoctions lack only Macbeth’s three witches for being an evil-smelling brew. Painting at home is often not an option.

Tapping the Source If you want an expert opinion, go see an expert, right? Joe Fields of Fields Aviation started by saying, “Preparation is everything. Shooting paint takes a bit of talent, but it’s not where the work is.” He pointed out an example of an aluminum aircraft with a beautiful paint job that, upon closer inspection, showed corrosion starting to lift the paint in spots. “Bad prep; you don’t have that problem with glass airplanes,” he said. “You have others.” What others? Why, pinholes of course. Jerry Stimple owned the premier paint supply shop in the San Francisco area for 30 years and finally retired. He is a guy of unbounded generosity when it comes to helping his friends paint their “precious.” He’s done at least four in just the past couple of years and is able to draw upon all of the big paint manufacturers for the latest products. He’s not wedded to any one process or product.

Preparation OK, back to the painting. In a previous article on composites I addressed pinholes, but in my research on this stage of the process, I kept being dragged back to the little monsters, so let’s have another go at them. Fields and Stimple both went to great

Here, a black guide coat has been applied so that when the next round of sanding begins all the high and low spots in the composite shell will be apparent.

Your Editor-in-Chief, younger and dumber. At the time, circa 1992, SuperFil was the new thing for filling pinholes in composite skins. He didn’t know to apply it thin, so an incredible amount of sanding was to follow.

lengths to give different approaches to the problem, so we’ll reiterate. Fields’ recipe is to sand down to smooth “…but not through the gel coat! It’ll only create more pinholes, and the fabric has gaps between the weave even if it’s resinsoaked, so the finished paint will show a pattern.” Stimple agreed, and joked that it would have more pinholes than the carpet in a tailor’s fitting room. Both had an answer. Fields likes to start with a light scuffing with 409 cleanser and Scotch-Brite, followed by rinsing with clear water…at least 10 times. He emphasized that you should use the original, purple-label 409, not the later versions with aroma additives. The rinse is needed to get the 409 off, so don’t make it harder by adding Divine Springtime to the mix. Simple Green–

Aircraft was also high on his list of acceptable cleaners. Stimple prefers a tiny squeeze of dishwashing soap in a gallon of water, followed by a rinse mixture of 10% rubbing alcohol/90% water. He emphasized that a tiny squeeze is all that’s needed for the same reasons Fields gave: You have to get it off. The alcohol/water helps, but don’t make it more difficult with too much soap. They both agreed that automotive wax and grease removers (not to be confused with engine degreaser) are great on aluminum, but fiberglass tends to soak up the stuff and leave a film; before going this route make sure that whatever you choose leaves absolutely no residue. If you’re curious, put a couple of drops on a piece of window glass to see if it leaves a film when it evaporates. If you want to be a bit more aggressive, “Abrade the surface with a dual-action sander pushing 150-grit paper,” Stimple added.

The Shop Air System Fields recommended finishing the cleaning process off with a high-pressure shot of air straight down on to the surface to blow off the dust. That brought up the subject of the air system, so let’s digress for a moment into preparing the shop. Even if you don’t plan on shooting the paint yourself, if you have the air system wrong, you’ll set the stage for disaster At some point, your project is going to look this bad. The motley appearance is from various coats of filler, guide coat and more filler having been sanded away to achieve a smooth finish.

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Composites, Part 11 continued when the color can is opened. (See the sidebar, “Compressor Come Clean,” Page 49.) With the hardware fi xed, let’s get back to the airplane.

Filling the Pinholes You’ve washed the surfaces, and now you have to dry them. Heat lamps are OK, but that big heat lamp in the sky is better, especially if you’re painting parts rather than an assembled airplane. However you do it, be sure to do it, or you’ll trap moisture under the next coatings. “Paint on a glass airplane sticks by mechanically bonding with the surface, so scuff it and make sure it’s clean and dry,” sayeth Fields. He also advises using the product type that was used to build the airframe. “If the airplane is epoxy, don’t use polyester-based products.” Stimple expanded on that idea and urged builders to, “Get to know your local paint supplier and rely on their expertise. Then buy the products of a single manufacturer; don’t mix them.” Among his product recommendations is Split-Second Rapid Sanding Glazing Putty from US Chemical. “It’s so lightweight you’ll think the can is empty.” In my previous article on fi lling pinholes, my sources were adamant that pinhole-filler should be applied as thinly as

possible, but Stimple said unequivocally that Split-Second could be slathered on up to 1/16 inch thick and then sanded to smooth out ridges as well as fill pinholes. The other sources didn’t mention this product, but both groups got fabulous results, so it must be the product. Once it sets, you can sand it down followed by a wipe with a clean rag lightly soaked with alcohol. The alcohol evaporates, hastening the drying time. Fields emphasized that whatever product you use, it should be both lightweight and flexible. “I’ve seen a lot of airplanes that looked great until after a few flights caused hairline cracks at the wingroot,” he said. “In fact, I prefer to leave a floating gap like a freeway expansion joint.” In the previous installment, we spoke of a guide-coat, and both Stimple and Fields agreed that it was a good idea, so I’ll reprise it here. When you think you have a smooth surface, clean it and spray it with a high-contrast paint. A rattle-can is fine. Then, sand that off using a long board to expose low spots. Don’t use a short block or, worse still, your hand to back up the paper. Either of those will just follow the curves. You may have to do this a couple of times to get that jaw-dropping, mirror-flat finish. Skip this step, and you’ll have an adequate, but not spectacular, result.

The left side of this Pulsar’s fuselage has had a thick primer/surfacer rolled on; the right side shows the first attempts to sand and smooth the filler.

Lighting Here’s an area where most of us go wrong: We add a bunch of shop lights up high over the project and consider it adequate. Not according to Fields. He strongly recommends that you set a 4-foot fluorescent lamp on a tripod and position it at the height of the surface you’re working on. At the other end you should have a dark background. The combination, he says, makes discontinuities show up faster than the end of a three-day vacation.

Sandpaper All sandpaper is not alike. The cheap stuff is likely to have varying sizes of par-

A proper regulator-particulate filter-dryer is not cheap, but a cheap one is more expensive. Don’t forget to use the drain every week or so.

ticles, and you’ll only create scratches while trying to eliminate them. Buy it from your local automotive paint store, not the hardware store; hardware customers are sanding on wood and laying down heavy coats with a brush.

The Spray Gun If you’re like me, you’re hoping that a

Here’s a cheater valve. It’s useful for knowing the air pressure at the spray gun, but don’t use it to adjust line pressure; set that at the regulator.

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This gun from Binks is the gold standard in HVLP. That black cylinder at the base is a line filter/insurance.

better gun will offset the need to sand, sand, sand. It won’t. But with that desire in mind, I’ve saved the spray gun for last. You’ve cleaned up the air delivery system, so let’s talk about the business end. Now hear this! A compressor, 50 feet of ¼ inch line and a $25 spray gun is not adequate. If you can’t or won’t build a good air system, stop right now and send your project out to be painted. Let’s take a look at spray guns and work back toward the compressor. There are lots of ways of spraying paint onto your airplane, but that airless sprayer you used on the house is not one of them. The three most common ways of spraying paint are high-pressure guns, turbine systems and HVLP guns. High-pressure guns. These are the cheapies that operate at about 45 to 60 psi and 8 cfm. Paint has become expensive, and the EPA put a halt to some of the really nasty chemistries—especially to the quantity of solvents that were being dispersed—so these guns are relegated to water-based paints. That led to turbine guns. These use a small turbine blowing warm air through a large diameter hose to generate 3 to 8 psi and 100 cfm. The first versions of these were built using vacuum cleaner components, and they worked well. The low pressure means that they gently lay the paint down instead of blasting it all over the shop, so, with practice, you can get as much as 85% transfer efficiency. With paint costing $200 to $400 per gallon, that’s an important number. To learn more about these, visit www.fujispray.com/what_is_hvlp.html or www.

turbinaire.com. Those vacuum cleaner parts led the high-pressure gun guys to figure out how to do it without the turbine. Although a turbine and an HVLP system work to produce a gentle spray, they have different approaches and advantages. The turbine doesn’t overly compress and then expand the air, hence there’s no entrained condensation, and what little heating there is tends to aid the atomization of the paint. Forget about entrained oil. Point scored for turbines. Getting set up with HVLP is cheaper, though, assuming you already have an adequate compressor with dryers and traps for your other air tools, all you

Compressor Come Clean Do not spray your work with anything other than clean, dry air. What’s clean? The compressor uses oil, and some of it will become entrained in the compressed air. If you use air tools, you might also have an in-line oiler. That means you’re spraying vaporized oil all over the surface you just washed. Not good. Even if you don’t have an in-line oiler, the pistons within the compressor may be lubricated with oil from the sump, and some will get into the lines. The solution is to get the oil out of the compressed air by, at the minimum, putting a tap upstream of the oiler and installing a filter on the new branch, then using hoses that are dedicated to spraying. The inside of those other hoses is coated with oil, so don’t use them for anything other than air tools that require oil. Now add in the fact that air gets hot when it’s compressed and, when it cools, the water vapor in it condenses into liquid. Don’t believe it? Go open the drain at the bottom of the tank, but put a pan under it first. You can get the water out by running a hose up the

need is 3/8 inch ID or larger air hose and a gun. A turbine system goes for about $800, while a top quality HVLP gun is less than half that. Point for HVLP. Those $50 HVLP guns? Stimple won’t touch them. Visit www.binks.com to see the gold standard in this area. By the way, you should have those dryers and traps even if you don’t have HVLP. How do the experts vote? Fields likes his turbine, and Stimple likes his HVLP. Stimple then tackled the setup of the HVLP and cleared up some misconceptions. HVLP does not mean low-pressure air is supplied to the gun. “The gun itself reduces the pressure,” he said. “You need about 45 psi at the gun, and the line

wall in a zig-zag pattern that has every zig and zag going downhill, back to the tank. The air will cool and, when it’s not flowing, the condensation will drain back to the tank where you can dump it.That’s just a start, though. That filter we put in the branch line will also remove water as well as oil and particulate. It’s not rocket science, but if you want to do it right, go to your local auto-paint supply store. You can frequently get a diagram of a good system along with the parts to build it. I’ll slip my two cents in here, and, although I’ll get a lot of “I did it and it works fine” letters, I’d advise against using PVC for the distribution lines. My shop was equipped with 2-inch PVC all around the ceiling with eight drops to plug-in fittings. It had more leaks than a Congressional committee in an election year, so the compressor kept coming on at night. One of those leaky joints finally failed with a thunderous explosion that sent large shards of sharp PVC all over the shop. I shudder to think of the consequences had I been nearby instead of in the hangar. Now you have clean, dry air ready for the final hose to the sprayer. Quarterinch inner diameter hose is cheap , but it has too much of a pressure drop; use 3/ inch or larger. —B.F. 8

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Composites, Part 11 continued loss is about 1 psi per 10 feet of 3/8 inch hose. So, for a 50-foot hose, crank up the pressure to 50 psi and let the gun do the conversion from pressure to volume.” He also described a “cheater,” a small gauge with a valve that can be attached to the base of the gun. “Don’t bother with them,” he said. “They reduce the pressure but don’t regulate it, so as soon as you let off the trigger the line pressure goes back up to tank pressure. Just set it at the tank where you’ve got a true regulator.” Of course, a portable compressor used to drive a nail gun won’t keep up with an HVLP gun, so when you’re shopping for a gun, be sure to check that requirement. As for gun configuration, there are some with one-quart cups on top, with a cup on bottom, and others with a siphon tube leading to a separate paint container. Stimple says that a top cup has the advantage of gravity feed and may, therefore, require less pressure and use 100% of the paint. If you have a gravity feed cup, shooting upward onto the bottom of the wing is nigh-on impossible. 3M came up with a dandy accessory, though. It’s a bag that, once filled with paint, fits in the bucket. You invert the gun so that all the air in the bag is blown out; then it matters not what angle you’re shooting. The turbine gun is a bit different. Read the manual, plug it into 110 VAC, and go spray.

Protective Clothing The EPA may have gotten rid of the worst of the concoctions, but you should still not go cheap on a respirator. Here’s one good reason to get to know your paint supplier. If he/she advises a full suit and external air, either buy it or change paints. You don’t want the paint job to last longer than you do.

Let’s Start Painting: The Primer Two-part primers were recommended by both of our experts. Better adhesion with a smoother surface was the advantage. But check with the manufacturer of the final paint system to ensure compatibility. Sand the surface lightly with 180 grit, blow it off and then shoot it again. Then give it a light sanding with 400 grit and water, and let it dry. Take your time here. Be sure there are no visible flaws after the final surface sanding or they’ll show through the final coat of gloss paint. When in doubt, sand smooth and re-prime.

Finish Coat Paint schemes and colors are totally subjective, except that most composite-kit manufacturers specify white (or at least light colored) hues to reduce temperature buildup in direct sunlight. Moreover, our experts said to avoid metal-flake paints; many of them contain copious quantities of aluminum that will interfere with internal antennas. Fields advises that, if possible, paint the wings separately from the fuselage. Two good reasons include the obvious fact that doing it that way is much easier than shooting straight up to the bottom of a wing; it’s also easier to control overspray. But if you can’t, cover the wings with bed sheets and spray the fuselage from nose to tail. Before you do that, look for obvious start/stop points such as seams, and use a fine-line tape to tape off what you’re not painting. When that area has set up in 24 hours or so, you can switch over to painting what’s still bare. “Don’t spray over the overspray!” Fields warns. “Pass your clean hand over a sur-

Stimple’s Lancair is no hangar queen. It’s a working plane, flown on a 2-hour daily commute, and with four transcontinental trips. The paint has to hold up.

face and if it makes a rough noise, use a fine Scotch-Brite pad to smooth it.” Layout of a multi-color system has its own perils. Stimple advises that if you can get your hands on one of those old overhead projectors, put a half-black sheet on it and you’ll get a perfectly straight line on the side of the fuselage. The paint job will also require a clearcoat to blend the ridges at the color interfaces. Therefore, each color should be thin, and that requires a very good base or any less-than-perfect sanding will show through. Now, here’s the crazy part: These twopart paints are made to be buffed to a glass-like smooth surface, but to keep the weight down we have to keep the thickness down. When we put our final clear coat on and buff it down to level, the scratches in the color show through. That sounds like a fool’s errand, but Stimple’s Lancair is a four-color job with a clear coat that is unbelievably flat and glossy. “It’s all in the sanding” he says. “If that’s smooth, the colors will be smooth.” He also unequivocally recommends the 3M product line. “You can’t get in trouble,” he says. (I’m going to test that bet and will report the result.) Fields’ favorite supplier is Poly-Fiber. The color coat, if it’s the final one, and the clear coat require buffing. But don’t use anything larger than a 7-inch wool or foam pad at 1200 rpm. “Do not use a 90° grinder with a pad; you’ll burn the paint!” Stimple says. One last tip: You could wet the floor down to suppress dust, but a better idea is to use SherwinWilliams Dust Free #55. Spray the walls and floor, and it will keep down the dust by adhesion and electrostatic attraction.

Joe Fields stands by, both figuratively and literally, his turbine gun—no water vapor, no oil, just clean, warm air and even a fresh-air source if you use a fully enclosed paint suit.

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Painting Outside Stimple has done several aircraft right out between the hangars and within 100 yards of a freeway with astonishing results. “First, understand that dirt is unavoidable and that these new paints are, as I’ve said, designed to be buffed,” he explained. “Still, you should pick a calm-air day, early in the morning, and with a heavy dew fall to suppress the dust. 70° F is ideal, but if it’s cold or windy, forget it.” Incredulity alternates with skepticism until one sees the result. Then astonishment kicks in.

Vinyl Striping If you’re going for a bit less complex process, Stimple highly endorses the use of vinyl stripes and overlays. These allow you to paint one or two colors that cover the entire aircraft, and then add the accent color where you want it. They are usually computer-cut, so almost any design is doable. Those jazzy wavelike patterns on a Cessna 172 are a good example. “Just make sure the surface is smooth, because the tape will magnify every bump,” he says. (Applying these is a topic in itself that I’ll cover when I get to that stage with my own project.) Is that all you need to know? Although there are books on the subject, both large and small, some near to “Dummies Guide” level and many that are cures for insomnia, there’s no substitute for practice and patience in doing the prep work. As a Lancair owner said, “Everyone has a fi xed quantity of sanding strokes built into them. When you reach your quantity, you’re done.” So procrastinate later. Get a big box of sandpaper, a good spray gun, and remember the words of Dizzy Dean: “It ain’t bragging if you done it.” 

Clear leading-edge tape is applied as protection against bugs, rocks and other sources of dings. It’s replaceable, too. KITPLANES March 2008

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Aircrafters’ Dave Saylor (left), with author Bob Fritz, discuss the differences in basic composite materials.

In Part 1 of our newest hands-on series, we’ll open your eyes to the wonders of modern composite materials. BY BOB FRITZ

W

hat do Burt Rutan, the police and a trickedout Toyota have in common? An off-axis question no doubt, and the answer is not the one Benjamin Braddock received in The Graduate, (“I want to say one word to you. Just one word. Plastics.”). No, the linkage here is composite materials, which are everywhere you look. Burt makes magic airplanes with them—has done so since the 1960s and continues to even today—the police wear Kevlar vests, and the guy that customized his Toyota loves the look of a carbon-fiber panel or two on his car. Sure, you’re familiar with composite’s use in skateboards, bicycles and bulletproof vests, and you’ve seen it in the body of Corvettes since 1953. But did you know that it was in use 5000 years ago? 36

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Peel-ply is used to help absorb excess resin in fiberglass or carbon-fiber cloth and helps create a clean (but comparatively rough) surface for secondary bonding.

Bricks and Mortar OK, I’ll back up a bit. What’s a composite? Answer: It is two or more materials that are bonded into a single structure where the originating materials retain their identity and contribute to the creation of new material. Simply put? Combine the materials such that neither dissolves and the sum is greater than the parts. By that definition bellybutton lint mixed into Elmer’s Glue qualifies, though I doubt it enjoys wide usage. More realistically, composites didn’t www.kitplanes.com

Let’s start with the materials: What are they, why use them, and how do you select which to use? Actually, this decision is made for you by the kit manufacturer, but let’s take a look at some of the elements that went into their thinking. For aircraft use, we start with what is popularly known as fiberglass. There are two basic types: E-glass and S-glass. The most common is E-glass. It’s strong, water resistant, easily formed and easy to repair. It’s cheap to manufacture and was initially optimized for its electrical properties, hence the E. It’s now found its way into surfboards and boats. The materials science guys wanted to keep their jobs, so they tweaked the

Fiberglass cloth comes in a variety of weights and weaves. The variations accommodate not only differing strength considerations but the ways in which the finished composite structure will be strong—it’s possible to fine-tune composites to an amazing degree.

change much from the mud and straw used to make bricks 5000 years ago in Sumeria until the Romans came up with concrete (cement and gravel). The next leap, although one that has yet to be adopted, was Pykrete. That was a dandy material composed of water and sawdust frozen into blocks. It came within a year or two of being the material from which torpedo-proof ships would be built for WW-II in the North Atlantic. You’ve probably come across terms such as E-glass, S-glass, Kevlar, carbon fiber, ceramic and a host of options that boggle the beginner and make the entrepreneur salivate. These materials have come to common use in homebuilt aircraft because they give the designer significant freedom in choosing shapes—both for aesthetics and aerodynamics—but also because they are comparatively easier to use than traditional materials. We say comparatively because while composite construction is most often used in large chunks—reducing the total number of pieces in any given airframe design—the medium requires its own set of skills and specialized understanding. Photos: Marc Cook and Bob Fritz

Looking Ahead In this series we’ll look at those options as they relate to amateur-built aircraft and take some of the mystery out of them. We’ll define the terms, explain the options, and offer some hints, help and advice. Moreover, for this first installment, we have worked through a small composites project to give you a general lay of the land in photographs. As we move forward in this series, we’ll connect the dots a bit more thoroughly, and offer you the benefit of experience and craftsmanship from several sources, including Dave Saylor, who owns Aircrafters in Watsonville, California, a thriving builder-assist shop that also does inspections and repairs on Experimentals.

Carbon fiber, mysteriously black, is dramatically stronger than fiberglass but considerably more expensive.

recipe a bit. What they got was S-glass. It’s a tiny bit lighter, but has 20% more tensile strength, hence the S for strength. Great stuff! However, as with all things scientific, there’s always a better version just around the corner. In this case, the wonder material was carbon fiber. The fibers themselves are stiffer than steel and are about 25% the weight of the grey stuff. But the matrix material in which the fibers are embedded lowers the modulus (engineer speak for stiffness) to being equal to or lower than that of steel while not losing the KITPLANES May 2007

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BYS Composites, Part 1 continued weight advantage. The result is a finished part that weighs a quarter as much as steel while being in the same strength range as steel. Compare CF to aluminum, and it still has the advantage. That CF part is almost twice as stiff as aluminum at about half the weight. This stuff has “airplane” written all over it…until you see the cost. The next ratchet on that wheel is Kevlar. Manufactured by DuPont, Kevlar has five times the strength of steel on a per-weight basis, 60% of the weight of fiberglass, is fire resistant, electrically insulating, sound damping and, like Superman, can even stop a speeding bullet. But, alas, each of these products is more expensive than the previous, with Kevlar being realistic only if you want a bulletproof airplane to carry you to your Swiss bank.

Saylor begins our sample project at a table dedicated to resin/epoxy preparation.

Underpinning It All: Technique There are trade-offs to these wonder materials. All of them have to be carefully handled in the construction of the finished product to make sure that air is not trapped between layers, and none of them is especially abrasion resistant. They will all cause your cutting tools to dull quickly, carbon fiber is not as good in impact resistance as the others, none is nearly as heat-resistant as metal, and even the wonder material Kevlar—also called Aramid—falls behind the others when in compression. A quick explanation is in order here: Tension means pulling on the individual fiber; compression is as if we had one end of a fiber being pushed directly towards the opposite end; shear is what one does to a fiber with scissors. Keep in mind that a cloth made of fibers has to be seen as individual fibers, each loaded individually in tension, compression or shear. That said, the strength-to-weight ratio, ease of fabrication and the ability to mold these materials into compound curves make them the first choice for the airframes of the latest go-fast aircraft. So, what goes into these wonder materials? In modern terms, the two compo38

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Using a specialized pump, Saylor is able to dispense the precise proportions of resin and hardener for the job.

nents are the binder, also known as the matrix, and the reinforcement. It’s the reinforcement (the fibers) that gives the strength, while the binder holds the fibers in position and transmits the load among them. Let’s simplify that a bit. As you can see from the photos, the reinforcement isn’t very impressive; wet noodle comes to mind. But tensile strength is where composites shine. Generally speaking, resistance to breaking when

Mixing takes place in a wax-free cup using wood tongue depressors to emulsify the resin and hardener. The purpose is to have both components mixed thoroughly in a way that introduces as few air bubbles as possible.

being pulled is known as tensile strength and it is key to these materials. “But we’re not pulling on them in an airplane, we’re bending them,” you say? Ah, but we are pulling. Try this: Take four similar boards, each a halfinch thick, stack them loosely on a pair of sawhorses and then stand on them. www.kitplanes.com

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They bend quite a bit. Now drive a few nails through them and stand on them. They’re a lot stiffer, right? That’s because they cannot slide against one another anymore; the bottom two boards are being stretched (tension loading) and the top two boards are being squeezed (compression loaded). The nails combine the strength of the boards just as using a solid, 2-inch-thick board is stronger than a group of four half-inch boards. Now substitute a stack of fibers and epoxy for the boards and nails and you see the mechanism of tension.

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Aligned with the Load To take advantage of the tensile strength of the material we have to align the fibers with the load. An airplane sees stresses in many directions, so we have to lay the fibers in many directions, too. There are a couple of ways of doing that. You can build the fuselage like they do at the super-high-tech shops with the entire fuselage on a rotisserie and a machine that lays down a tape that winds around the circumference on its way to the tail, and then continues the same filament back to the nose. It’s done that way on cylindrical shapes because it KITPLANES May 2007

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BYS Composites, Part 1 continued gives a fantastic strength-to-weight ratio on pressurized containers. It’s a couple of million bucks for the equipment, so if your airplane fuselage isn’t a cylinder or pressurized, it’s not an option for the do-it-yourselfer—or even an extremely well-funded kit manufacturer, were there such a thing. The practical way for homebuilders is hand layup. It can, if done well, be just as strong as a filament wound structure. It takes a bit longer, but it requires only a few basic tools and the right format for the fibers, and that format is usually a cloth. Before we get started on this discussion, let’s define some terms. • Weight: That’s in ounces per square yard of dry cloth. • Thickness: This is primarily a function of weight and fiber type, but also depends on the weave. • Filament: A single fiber sometimes known as a monofilament. • Strand or End: A group of monofilaments. • Roving or Tow: A group of monofil-

Our practice project is a hardpoint—in this case a #10 screw embedded in a foam sandwich—but the techniques will carry through many types of composite construction. Here, the foam is being wetted with plain epoxy.

The next step is to slather on a thin layer of microballoon mixture.

The two pieces of foam can be placed together. The critical concern here is to watch for sufficient “push out” so you know that the surfaces have sufficient micro/epoxy for a good bond. 40

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Now the screw stud is placed in the hole and surrounded with a thick flox/epoxy mixture.

The flox can be smoothed with a brush dipped in the plain epoxy.

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Next we create a micro/epoxy fillet where the two foam pieces join. Fiberglass cloth doesn’t like to go around sharp corners, so this transition is important.

After smoothing the fillet with a brush dipped in epoxy, it’s time to lay down the first layer of cloth.

aments all parallel to one another. These are usually designated by the number of filaments, e.g. 3K or 12K. • Yarn: A group of monofilaments twisted to hold the fibers together. Twisting lowers the strength of the fibers a bit. • Denier: A weight in grams of 9000 meters of yarn. The lower the number, the finer the yarn. • Warp or Ends: These are the yarns that run in the direction of the roll of material. • Fill or Picks: These yarns are shorter because they go across the roll. • Count: This refers to the number of warp yarns (ends) and fill yarns (picks) per inch. For example, a 24 x 22 fabric has 24 ends in every inch of fill direction and 22 picks in every inch of warp direction. • Drape-ability: This is a bit subjective, but it describes how easily the cloth conforms to a non-flat shape. Aluminum foil, for instance, has poor drapeability in that it has to be folded to wrap around a sphere. • Weave: This describes how the warp and fill yarns are interlaced to make what we see as cloth. It determines the drapeability and the strongest direction of the cloth if the cloth is woven with a bias to the warp or fill direction. To complicate the issue further, there are three classifications of reinforcement: particulates, discontinuous fiber and continuous fiber. An example of a particulate composite would be concrete, with the gravel being the particulate. If the key to the strength of composites is the tensile strength of the fibers, then longer strands are better because there are more places for the epoxy to grab on, loading the individual fiber to its maximum. That’s why they put long rather than short steel bars in concrete. Discontinuous-type construction uses short fibers and is, therefore, stron-

After the cloth is positioned, it can be wetted out with a brush. Work from the center out, and try to avoid using too much epoxy. Give the cloth time to wet out.

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BYS Composites, Part 1 continued ger than particulate. But because the fibers are short, their strength is not fully employed. Instead, the load goes the length of a fiber and is then transmitted through the epoxy to the next fiber. However, it can be sprayed to make the layup, as is done in boat hulls. Spraying also means the fibers are randomly oriented; therefore, the strength is not directionally dependent. The last group, continuous fiber, is what we want for maximum strength. It’s our very long tug-of-war rope where enough guys can get their hands on it to really use its strength.

A clipped brush is effective for stippling out unwanted air bubbles.

Fabric, Meet Your Binder That analogy of guys grabbing a rope can be extended to the other half of this material, the binder. There’s no point in having a strong rope held by weak people, and the more hands on the rope the better. If we compare two teams, one called Vinylester Resin and the other Epoxy Resin, and look at how they grab the rope, it becomes obvious that the ER team is stronger. The VR molecules are long fellows that wrap around the fiber, while the ER molecules connect not only to the fiber in many places, but also to each other. The advantage is that the ER molecules put more “hands” on the fiber. It’s as if one team had extra players pulling on the belts of the guys holding the rope. The ER hands are a lot stronger as well. In fact, they’re about four times as strong as the VR guys. Not only that, ER will bond dissimilar or already cured materials where VR sometimes has difficulty. The real icing on the cake is that ER also has better resistance to microcracking. What’s the downside to ER? It’s a bit tougher to work with in that to achieve as many 3D bonds as possible, you have to mix the constituent components very accurately. If not, the excess material is just mixed in without finding a partner and weakens the finished product. VR, on the other hand, is a long chain that can be made longer, so mixing is a bit more forgiving. Did I mention that the 42

KITPLANES May 2007

Peel-ply can be used to absorb excess resin. Apply it like another layer of fiberglass.

One trick to doing good fiberglass work (true with carbon and Kevlar, too) is to know when to stop fussing. This project’s peel-ply is just about perfectly wetted out. Stop now, have a cup of coffee and let it cure.

cost is about half that of ER? Consequently, the kit makers use ER where it’s needed for strength. VR is the binder of choice for Glasair, Glastar and the Van’s series, which uses it wrapped around a metal frame. Lancair, with no metal, opted for ER. That’s what makes so many great airplanes possible: options. The bottom line is that if we lay down really long fibers of glass, and bind them with a resin, we get an affordable airplane with phenomenal strength, light

weight and, because the cloth is, well, cloth, it can be easily formed into compound curves that make love to the air rather than just shoving it out of the way. Right now you’re probably feeling like someone just dumped a 1000-piece jigsaw puzzle on the table. Not to worry. In future installments we’ll show you the box and point out all the edge pieces. You’ll find that handling the materials is actually much easier than you think. In fact, this will be fun.  www.kitplanes.com

The humble but noble tongue depressor. Shape it into a custom tool and let the epoxy coat it for extreme use.

In Part 2, we’ll discuss what you’ll need in the way of tools, supplies and a workbench.

D

BY BOB FRITZ

o you find operating a ballpoint pen a difficult task? Is a layer cake with canned frosting a tough assembly? No? Then working with composites is within your grasp. Last month we reviewed the basic materials and found that one end of the fiber spectrum is made up of boat hulls built of chopped E-glass held together with polyester resin; the other is multi-million-dollar aircraft made of single-strand Kevlar and carbon fiber laid down by multi-million-dollar machines. We homebuilders are in the middle of those extremes, doing layups of cloth made of S-glass or E-glass, and using either epoxy resin (ER) when needed for strength and vinylester resin (VR) the rest of the time—or when the kit manufacturers choose to use each of those materials. Typically, the choice is dictated easily by simply choosing the lowest cost material that meets the mission profile. An excellent example is Van’s RV aircraft, which use vinylester resin on the wingtips and epoxy resin on the cowling. You can spot the difference easily enough; the VR parts are white, and the ER parts are a transparent greenish color. But with that choice comes a new set of decisions: What tools are needed? Ever get really ticked off when you see your screwdrivers being used to dig weeds? “^@#$%, use the right tool for that!” Uh, huh. Use the right tool. So it is with composites, and though a lot of these items look like refugees from a Chinese kitchen, 38

KITPLANES June 2007

Not likely to be mistaken for paint rollers, these are used in much the same way to smooth out the fiberglass.

No, it’s not a pizza cutter. It’s a rotary cutter (aka circular razor blade).

The scissors are pricey, so labeling them will make them less likely to walk away.

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The table and cloth rack are essential. The hat is optional.

should cover it with a smooth, plastic sheet. Don’t use wood; the splinters will embed into the cloth. And forget metal; it will dull the cutters. Although Melamine looks nice, it too will cut and release contaminants. Plastic…that’s the ticket. The trick to cutting the fabric is to understand that it has a coating that encourages the movement of the fibers over themselves; the stuff is slippery. It can be a curse when cutting, but it’s what makes it conform to complex shapes.

The Blade they are no more so than a screwdriver is a weed digger. It works, but the right tool works better. So what’s different about working with fiber? Well, you have two basic materials to work with: cloth and resin. Unlike working with aluminum, you also have a timing issue. You can’t stop in mid operation and take it up tomorrow without expecting some back up and restart. There are also health issues that are unknown in metal work. And while the rivet bangers like a big workbench as much as anyone, for the glass guys it’s not an option.

assists the epoxy in wetting the fiber; water removes that coating. You’ve also noticed the black diagonal lines on the table. You might have heard your mother refer to a dress as having been cut on the bias. The term simply means cutting at an angle instead of straight across the material. It’s a way of keeping the cloth from de-weaving. The marks on the table are at 45° to the cloth and serve as a guide. The second reason for a large, flat workspace is that cut material tends to de-weave itself, so the less handling the better. More on this later. As for the surface of the table, you

Now that you have a table surface, you’ll want a cutting wheel. No, this is not a refugee from a pizza joint; it’s a circular razor blade. These come in several sizes, but the 45mm diameter is the best allaround tool for cutting long, straight lines and curved corners. Keep it on the worktable and don’t use it for any material that’s been soaked with resin. The solvents are hard on the handle, and though there are aluminum-handle versions available, any contaminants remaining on the blade will ruin it for further use. There are all-aluminum models that can be stored in a can of solvent, but that requires keeping a liquid in the cutting area or doing a careful job of clean-

Getting Started Let’s begin with how to handle the materials. The first thing you’ll need is a large, flat, smooth surface devoted to cutting cloth. If you try working this stuff on your regular workbench you’re asking for trouble for a couple of reasons: One, the material needs to be kept clean. That means no oils, water or general dirt should come in contact with the glass. Oil and dirt seem obvious as contaminants, but water? Yes, water. There’s a coating on the fibers that

Once you’re done cutting, you have to be careful moving the strips so that they don’t de-weave.

Photos: Bob Fritz and Courtesy the Manufacturers

KITPLANES June 2007

39

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Composites, Part 2 continued ing and drying prior to cutting on the table. Better to simply keep your cutting wheel clean and dry, advises Dave Saylor of Aircrafters. Now that you see the tool, let’s go back to the table and explain why we want this tool and that table. Once you’ve marked your cut with a Sharpie, you need to cut it out. But because the glass cloth is a loose weave, lifting it off the table to get at it with scissors distorts the fabric. Remember that term “drapeability” from the first installment of this series? It’s helpful when you want the fabric to conform to a compound curve, but in this operation it’s a hindrance. The solution is to cut the fabric without lifting it off the table. Using a cutting wheel against a flat table does just that, and having a backing of firm plastic ensures that the wheel cuts through the glass without dulling the blade. Transporting the pieces to the worksite is not a grab-n-go operation; you’ll get to your destination with loose threads all over the shop as the fabric deweaves itself. Instead, have a large board on which you can place the cut pieces and carry them to the worksite. A large sheet of paper will do if you have a helper to grab the corners. This is especially critical when doing a multi-piece layup such as the one around the leading edge of the windshield on the RV-10. (Hint:

We’re demonstrating that operation in the next issue.) Scissors seem like an obvious requirement. But don’t plan on appropriating them from the sewing kit; there’s a difference in the hardness and the edge profile on glass scissors that makes them work better. Good ones will cost about $30, so that should tell you something. You’ll find these are useful to cut the wetted glass that overlaps into areas it shouldn’t. If you’re going to add a bit of Kevlar to the project, then plan on buying special scissors for that as well. They look just like all the other scissors, so painting the handgrips a nice bright color would be a good way of differentiating your $60 purchase. The blades have tiny diamonds adhered to the edges. One use of these scissors on some material that’s been wetted with resin and the neighbors will learn a whole new set of words not to be repeated in polite company when you discover that resin has coated the sharp corners of the diamonds and now they won’t cut hard butter.

Dabbling in Fluids Now that you have the material to the worksite, you’re ready to start playing with the liquids. That means latex gloves and lots of them. Ultra-cheap ones tear easily, and you’ll find that they are more expensive because of it. But you don’t need surgery-qualified ones either. Dave

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Using the rotary cutter is surprisingly easy. Just don’t trim your fingernails with it.

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You want air over the wing, not in it.

Saylor recommends powdered gloves. “They’re easier to put on, and the powder is on the inside so it’s not a problem for the material,” he says. Rollers are used to squeeze out the bubbles and force the epoxy into the yarns on large, relatively flat surfaces. At first glance it would seem tempting to think these are great for use in curved areas, but in working on the RV-10 windshield I found it easy to push too hard and displace the layer of cloth. The flexibility of the paint brush reduced that tendency. However, you will need several sizes for large and small areas, and you don’t get these puppies at the local hardware store; they are not overpriced paint rollers. For instance, there are various lengths and diameters, but also curved ones, finned ones, and some that look like a pizza wheel gone insane. We’ll describe the uses of each as we

progress with this series. At first glance, peel ply may seem to be a material more than a tool. However, its function is the same as the paint brush or roller. It’s a 2- to 3-inch-wide strip of Dacron that laminates onto the layup as the last layer and is used to squeeze out excess resin. It cures along with the rest of the material, but it doesn’t adhere to the resin. Saylor was adamant about its use: “More epoxy does not make a stronger component,” he said. “Using peel ply correctly will let you squeeze out the excess resin and give you a lighter, smoother part.” An added benefit is that when you peel it off you have a surface ready for further work without sanding. Handy stuff. Among the essential tools is the humble 1-inch-wide paint brush. Buy the cheapest ones you can, and buy them by the box because you’ll throw them away after one use. The obvious uses are slathering on the epoxy and pushing the cloth into position; the not so obvious use is that when you cut them to about 3/8-inch, they also are perfect for smoothing out the bubbles under the peel ply. That white-ish dot (see photo) by the handle of the brush is an air bub-

ble. Resin has a hard time sticking to air, so white is not right. Some of my favorite tools are not only the least costly, but they also get better with use. They’re ordinary tongue depressors filed and shaped to conform to most any job. They get coated with the epoxy and when it dries, they’re harder and smoother than the original. Create a few of these in your shop and keep some in your shop coat. But wait until the epoxy has hardened or you’re in for a surprise when you grab one. Here’s a disaster scenario: The epoxy is setting, and you see the need for a bit more cloth. There’s only one problem. You suddenly find that the tool you want grew legs and walked off. In 30 minutes that epoxy will be jelling, and you are about to panic.

Your Workshop The answer to this problem is not two or three of everything; that’s an unnecessary cost and means you have to keep track of more tools. You wouldn’t make dinner on your workbench; you’d go to the workshop specifically suited to the task, the kitchen. Therefore, your composites workshop needs one thing: a workbench sized so that the parts and all the right tools you need are close at hand. There’s the key: close at hand. This will solve a lot of the problems. Build yourself a mobile work station devoted to composite work. It only needs to be about 2x3 feet, on wheels, and with enough room to carry all the chemicals, tools, materials, gloves, measuring and clean-up supplies in one place. Next month we’ll start to discuss the specifics of how to use all of this in the fabrication of an actual aircraft. We’ll be following various stages of construction of a Lancair IV-P owned by Bill George (that’s him behind the cutting table), and whatever Dave Saylor of Aircrafters in Watsonville, California, happens to be working on. To learn more about Aircrafters, I’d recommend a visit to his web site: www.aircraftersllc.com.  A short brush is useful for getting the air bubbles out of the laminate.

KITPLANES June 2007

41

Dave Saylor and his crew at work at Aircrafters.

In Part 3, we look at how to go about basic preparations.

Y

BY BOB FRITZ

ou walk past an airplane that has wings so smooth and free of ripples that it’s hard to see where the air ends and the paint surface begins. It seems to say, “Go ahead, punk, make my day” to every bug and bird in the vicinity, just daring them to try something that might stick to it. And you wonder how many gazillion hours were spent sanding that puppy. Well, there’s good news and there’s bad news. The good news is that it’s all in the preparation. The bad news is that it’s all in the preparation. But unlike painting aluminum where no amount of paint will make a bad surface look good, composite construction allows you to sneak up on perfection. Now that I’ve got you discouraged, go back out to the tiedowns and look at the percentage of really nice composite 38

KITPLANES July 2007

airplanes compared to the bad looking composite airplanes. Don’t see any real dogs, do you? Hmmm…must not be that hard.

Preparation Begins OK, let’s get to it. What’s it take to get off to a good start in this venture? Well, start with the right tools—as we have been saying all along—a dedicated cutting table, a portable composites bench, a lot of rubber gloves and the patience of a turtle with an arthritic hip…at least at the beginning. If you want to ensure success, start small with a guaranteed win. What’s the worst that can happen? Somebody might challenge you to something more difficult. With that in mind, let’s follow Dave Saylor of Aircrafters in the glass layup around the front of an RV-10’s windscreen. It’s a task almost everyone has to do, be it on an all (well, mostly) aluminum RV or www.kitplanes.com

Here are the three pimary fillers, left to right: millfiber, microballons and flox.

A fit like this makes for a winner.

mega-plastic Lancair IV. The first thing to do is get out the blue painter’s tape and tape off the bottom 3 inches or so of the inside of the windscreen. We’re going to be applying glass to the outside of this area, so painting the inside flat black means you don’t look at the inside of the layup from the pilot’s seat when we’re done. You’ll also find that, unlike the tan tape, the bright blue color makes finding torn bits of it really easy. Next, we need to fill in the gaps and irregularities on the area to which we

microscope you’ll see tiny glass spheres, hence the “balloons.” If you’re familiar with Bondo as used on cars, you’ve already used micro; it consists of micro mixed into polyester resin. We want something a bit stronger, so we’re using micro mixed with either vinylester resin or epoxy resin. Millfiber: That’s short for milled fiber. It’s just fiberglass that’s been chopped into tiny pieces. Flox (are you ready for this?) is really short-length strands of cotton. Each of these has its own unique use based primarily on strength, with millfiber the strongest and micro the weakest. Therefore, we would use micro for non-load-bearing filler around a windshield. To mix the micro into the resin, we have to first mix the resin. It’s critical that you follow the directions of your

intend to apply glass. Don’t get too wrapped up in achieving super smoothness here; there will be multiple layers of glass over this zone, so the small discontinuities that result can be addressed later.

Filler Material Here Over the course of this series we’re going to be working with three similar materials: millfiber, microballoons and flox. Micro, or Q-cell, as it’s sometimes called, looks like powdered sugar and flows almost like water. Under a

Even this tab can be buried and lost under properly applied glass.

Micro mixed to the consistency of creamy peanut butter is great for filling sharp corners.

Photos: Bob Fritz

kit manufacturer for this. As was discussed in a previous issue, kit aircraft use either vinylester resin or epoxy resin. VR is a three-part mix consisting of the base resin, a promoter and a catalyst. Buying promoted VR makes the mixing KITPLANES July 2007

39

Composites, Part 3 continued

A couple hundred grand for an airplane, and it all comes down to a little wood block to ensure that you get the right amount of epoxy each time.

The by-volume mixing pump is an indispensable dispenser. So to speak...

easier, but it does have a shorter shelf life of three to six months. Un-promoted VR, however, has an almost indefinite shelf life. Care must be taken when mixing un-promoted VR, as combining promoter and catalyst results in an exothermic reaction—that’s a six-bit word meaning it gets hot, dangerously hot. The procedure is to mix the promoter into the base resin, then add the catalyst. It’s not a difficult job, just another step that must be done correctly. Epoxy resin is a two-part system and, while this is a bit simpler, it does tend to cause skin sensitization so gloves are necessary. And once sensitized, you could be stuck with an airplane to which you are allergic! Gloves should be worn with both types of resin simply to keep the sticky mess off your skin. Now that the resin is mixed, we can add the micro to different consistencies based on their use. Here, we’ll mix about five parts micro to one part resin by volume. Micro is of such a low density that mixing it thoroughly can be difficult, but you have about an hour before it sets up, so keep at it until it’s a uniform color. The resulting mix can then be easily 40

KITPLANES July 2007

applied with a wood tongue depressor. Two safety notes here: First, dry micro is extremely lightweight; a sneeze can blow it all over the shop and you

don’t want to inhale it. Second, never use it between layers of fiberglass; it has poor structural strength and you’ll be building in a weak point. Yes, here it’s the base layer, but this application is not load bearing, that is, it’s not holding anything in place other than itself. On the subject of mixing, there are two schools of thought: Measure the materials by weighing them on a scale, or measure them out with a pump. Both methods are perfectly valid, and your kit manufacturer will most likely recommend one or the other. However, be

Work the first layer gently, and don’t worry about loose threads. They can be fixed later.

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aware that both have advantages and disadvantages: A pump is fast because you simply adjust it to the ratio required, and then lower and lift the handle to get the perfect amount of two components. The downside is that a pump can clog, so you’ll have to periodically check that it’s clear. Scales are clog-proof, but are slower and require active attention to the numbers (Let’s see, was it 3:2 or 2:3?). Here’s a neat tip: Once the ratios are adjusted, you can set the quantity dispensed by placing a wood block under the pump handle. It will limit the down-

in interleaved layers like a deck of cards that have been shuffled but not aligned. Shorter pieces are easier to position. Each layer went on dry and was gently slathered with resin. Gently is the operative word here; you can and will undoubtedly pull some threads out of the weave. Not to worry, though. Gentle use of the brush in a poking motion will smooth out the wrinkles, and you’ll be able to sand the loose threads smooth

when everything is nice and hard. Take a close look at the photo, though, and notice that in addition to loose threads and wrinkles, there are whiteish spots. That’s air. You want air under the wings, not under the glass. Get the air out by continuing to gently smooth the layer. This is a step-and-repeat operation until you get all the layers down. It’s also an opportunity to have a friend

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stroke so that when you release the lever, the amount you get in that non-waxed cup will always be the same. Label the sides of the block with the quantities you use most often and then simply flip the block to the required quantity.

Laying It On As was described last month, cut your glass strips to length and transport them carefully to the work site. Using the first of a boxful of brushes, Saylor applied a liberal coating of the resin directly to the masked area. I was then surprised to see him lay the strips onto the resin base without first soaking them in resin. His reasoning was that it is easier to position dry strips on a non-horizontal surface. (Hmmm, seems to work well.) Here’s where the big advantage of composites shows up. Those nasty looking little aluminum tabs that position the windshield simply become lost under the layers of cloth. Van’s specifies seven layers in this area and, while you could do it with seven strips, Saylor used 14 strips laid down KITPLANES July 2007

41

KB

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A classic book Kent Paser. If you're willing to spend the time, you can significantly increase the speed of most any aircraft. This book takes you through every aspect of drag reduction using a Mustang-II as an example. Flat plate and other types of aerodynamic drag are discussed in depth as is cooling drag, exhaust systems, and over 100 other hidden contributors of slow flight. Available as a standard book or instantly as a fully searchable e-Book download.

Composites, Part 3 continued help. Promise them some flight time when you’re done, promise them dinner, promise to help on their airplane, but do this with a buddy. It’s much easier and more than twice as fast. We’ve been fairly liberal in slapping on the resin, and now we’re going to get rid of some it by adding a layer of peel ply. This is a 3- to 4-inch-wide strip of Dacron cloth that we simply apply over the entire layup. Previously we noted that the strength of the assembly comes primarily from the fiber not the resin. In fact, more resin won’t make the assembly stronger, it’ll just be heavier. Peel ply won’t stick to the resin, so we can use a stubby brush to squeeze out the excess resin. Again, this is a good time to go after the bubbles. Don’t go overboard, though; you have to leave some resin in. Just smooth it out and give a rough shape to the entire layup. An option with large, flat surfaces is to put a layer of thin plastic over the peel ply and drag a squeegee across the surface to squeeze out the excess. Leave the plastic, if you use it, and the peel ply in place for 24 hours and go clean up the work area. I hope you put a

liner in the trash can. A friend of mine, who shall remain nameless, did not, and at the end of the week had to throw away the can. There was a solid mass of brushes and remnants stuck to the sides, much to the amusement of observers.

Ready, Set, Peel Now we get to why it’s called peel ply; you peel it off when the resin is fully set. Don’t forget to do this! I’ve heard of one poor soul who did not realize that it has only slightly better adhesion than air, and wondered why parts kept falling off every time he washed his plane. One last tip: If you want to get a perfect fit between two existing parts, such as the plastic that wraps around the aluminum surfaces of the tail, put some release tape (super-thin packaging tape works well) on the aluminum and a bit of resin on the mating part. Slip the two parts together and hold them in place until the resin has set. When you separate the parts you can remove the release tape and install the part, and you’ll find that the gap is gone. That’s all there is to it. Go give it try on something simple, and next time we’ll take up a larger project. 

A layer of filler was used between the fairing and the wing leading edge. After completion, this wedge will be filled on the outside, never to be seen.

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If the mold you’re working on is small, it can move all over the table if not secured. A bit of thin plywood makes a great base.

“I

Ordinary packing tape is great for holding the mold to its base. Just remember to stay outside the boundary of the finished part.

In Part 4, we make parts from molds.

t ain’t so much the things we don’t know that get us into trouble, it’s the things we know that just ain’t so.”—Artemus Ward With that admonition in mind, and with an understanding that there’s more than one way to do most anything in composites, let’s dive back in. Last time we laid down gently curving fiberglass strips on the leading edge of an RV-10’s windshield. This month we’re going to get a bit more advanced. We’ll make some parts from curved molds and then go to the opposite extreme, showing you a technique for making flat-side parts. As you’ll see, the previous advice about workspace preparation and proper tooling really comes into play. There’s that word: play. With the right tools, it will be just that. First, let’s do some mold preparation. We’re going to make some bits for a Falco. It’s called an all-wood, plansbuilt aircraft, but let’s face it, nothing is all-anything, so unless you’re an over-the-top expert in wood, you’re still going to have to do some work in composites. In this case, owner Mark Wainwright, a first-time builder, went to AirCrafters in Watsonville, California, for some assistance, and he was kind enough to allow us to document the process.

BY BOB FRITZ

Those two shiny spots are valleys on each side of a ridge. Fill them with micro so that the shape doesn’t transfer to this side of the finished part.

Note that this is not auto polish; it’s specifically for mold release.

Where to Begin Let’s start with some fairings for the control rods on the ailerons. This is a small part, so it needs to be held in position, and a nice way to do that is to use packaging tape to hold it onto a thin piece of plywood. It’s obvious that the mold in the photo isn’t the same as the 38

KITPLANES August 2007

one we just taped down, but it’s a good example of the next step: keeping discontinuities off of the visible surface of the finished part. Look closely and you’ll notice two shiny areas near the www.kitplanes.com

Micro and epoxy. You won’t need much, and it should be pretty stiff.

Here you can see the mold peeking out at the 6 o’clock position. Note that the threads of the fabric are also along that line, which is incorrect.

Smooth the micro on to the discontinuity and then don’t worry about it. The epoxy will fill in the small holes.

As you can see, the valleys were not wide, but they would have been seen in the finished part.

apex. Those are valleys with a large ridge in between. The first step toward fi xing this is to apply three coats of moldrelease wax, letting each dry for at least 15 minutes before polishing it off. Now whip up some micro with a bit of epoxy to about the consistency of cold peanut butter, and smear it onto the offending area, smoothing it with a wood depressor. There’s no need to get fancy. Let’s step off to the side for a moment and think this through. This is a male mold, so when you see the mold, you’re seeing the part. In this case, when the glass pops off the mold, the surface that does not Photos: Bob Fritz

touch the mold is the important one. But if I leave the discontinuity on the mold, the glass will follow the curve, generating an area to be fi xed later. So I’ll fill the valley with something that will stick to the glass but not to the mold. When the part comes off the mold, so does the micro. The result is that the discontinuity is still on your finished part, but only on the side not seen. At this point, the astute reader will be thoughtfully musing: “Meathead! Why don’t you just fi x the mold?” The question was put to Dave Saylor, owner of AirCrafters. “The surface of the mold is finished as necessary,” he said. “It can be nice and smooth or not. Smooth surfaces wax and release easier. High quantity production molds are usually smooth for ease of finishing and releasing. When the mold is hard to release, it gets dinged and scratched every time it’s used, and has to be repaired in order to maintain the surface finish. It’s like a woodsman sharpening his ax—the more time he spends sharpening, the less time he spends swinging.” Now, if this were a female mold, that is, a mold with a pocket to be filled, and had a similar discontinuity, we could do the same thing as long as the surface to be seen is on the side not touching the mold. Just keep in mind that the micro is going to stick to the glass, release from

Now we’ve rotated the fabric 45° so that the thread lines don’t cross the mold at right angles.

the mold and have the profile of the mold’s discontinuity. We also asked Saylor how to make a female mold. “A male mold is not the same as a plug,” he explained. “A plug is used to make a female mold. For example, to build a cowl, first you might build a plug that simulates the cowl, then ‘splash’ a female mold over the plug. The plug will not usually look exactly like the cowl, but rather it will look sort of blank or plain. Details are added later.”

Enter the Fabric Having waxed the mold and filled the discontinuities, let’s take a look at the fabric. The threads are at 90° to one another (warp and weave, remember?). To get the strength to be balanced we want to place the fabric onto the mold with as many threads as possible KITPLANES August 2007

39

KB

27 Years of the RVator 27 years of technical articles from Van’s aircraft accurately reproduced and organized in sequence as you build, test, fly, & maintain your RV.

Composites, Part 4 continued crossing the long axis of the mold. As much as you might like to use that rotary knife (assuming your teenager hasn’t snagged it for cutting pizza), be aware that it works only on very smooth surfaces. If there are any grooves or bumps on the table surface, you won’t get a 100% cut and the piece of material will simply unravel. Scissors are the ticket, but make sure to use the ones you bought for cutting glass, and if you get any wet epoxy on them, clean them immediately. In fact, go clean them anyway; they work best when you use the metal rather than an epoxy coating as the cutting edge. Push and gently pull the fabric until it wraps down and stretches over the mold. This is an example of what we spoke of in the first article: drapeability. The process is much like last month in that you want to wet the fabric without displacing or over-stretching it. It’s important that you see the fabric change from shiny, silver-white, to dark, wet and translucent. If it’s not, add some epoxy.

These molds are about 10 inches long, 2.5 inches high and only 3/8 inch thick. It’s amazing how nicely the flat fabric will conform to that extreme shape.

Pushing the material to conform to the surface of the mold is easier than it seems. But you can take it only so far without it springing back. At some point it’s best to continue the forming with a very wet brush.

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KITPLANES August 2007

Here comes our peel-ply, whose purpose is to chase out the excess epoxy and leave a surface finish with what a painter would call “tooth,” or roughness that allows the next coating to adhere. Just keep in mind that peel-ply is not a structural component; you’re going to remove it (we hope), so the fact that it won’t conform to the shape is immaterial. A patchwork of separate pieces is fine. Just make sure to cover the entire surface. On this mold you might try draping long, narrow strips first down the longest edges, followed by going diagonally over the glass. They stay in place better than 1-inch squares. Then just stipple it down with the brush, forcing the excess epoxy out through the peel-ply. Don’t go nuts trying to remove it from the peel-ply; you can let it dry there. Before we move on to the flat-side mold, let’s take a time-out to look at an interesting comparison. These two parts are identical except that one weighs

Like we showed last month, wet the fabric with the epoxy, being careful not to displace the fabric.

Don’t worry that the peel-ply is too stiff to go on in one piece; a patchwork works just fine.

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Here you see the start of the peel-ply and also the orientation of the fabric.

Harold Bunyi’s six years at the Lancair factory shows when he emphasizes scraping the table to get the old epoxy off. He uses an ordinary wood chisel for the job.

Peel-ply is all finished. Because they’re on a board, we can move the parts to a warm spot in the shop and have our bench clear for another project.

almost twice as much as the other. “Why is that?” you ask. The thick one weighs 49.5 grams, and the thin one weighs exactly 25 grams. The only difference is that the lightweight one had peel-ply treatment, and the heavy one did not. Now we all know someone who was

completely fanatical about the weight of his project during the building process and, after a couple of flights, started carrying an extra toolbox and a couple of quarts of oil in the plane. Or there’s the guy who spends 300 hours cutting out lightening holes and weighing the

Other than the glossy sheen, these pieces don’t look hugely different, but they are. The one on the left weighs double what the right one weighs.

Here you can see why there is such a weight differential even though they came off of the same mold.

KITPLANES August 2007

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Composites, Part 4 continued scrap to make sure he has a really trim airplane. Only thing is, he had a couple of six-packs while doing it! The point is that if you squeeze out the excess epoxy, you’ll have room for the oil or the sixpack. But there is also the option of addressing the system weight. Both you and the airplane will fly better.

Working with Flat-Side Molds Anyway, speaking of making things flatter, let’s look at a different technique that works for flat-side molds. The operative word here is flat, so the first thing to do is to get the workbench clear of hardened epoxy drips and other blips. How flat? Follow through on this and it’ll become apparent. In this technique, we’re going to eliminate the excess epoxy even before we get to the mold. Start by measuring the flat area of the mold. Well, OK, it’s not flat. It curves, but not simultaneously in multiple planes, so simply measure one side, over the top and down the other side. Add an inch, and that’s the length of the fabric. For the width, measure the length of one side, plus one end, and then add an inch. This happens to be a component that’s going to take a lot of load, so we’ll need to cut two pieces of fabric, plus two sheets of plastic wrap 1 inch larger than the fabric. When cutting the fabric, remember to cut on a diagonal relative to the threads. If you can find a diagonal on the plastic wrap, let me know. Next we’ll lay down one sheet of the plastic wrap on our smooth workbench, and center a single sheet of fabric on it. Although it’s tempting to pour the epoxy, you should brush it on, smoothing and spreading it over the entire surface. Now look for the thin threads in the fabric and orient the second cloth with the thin thread at 90° to those of the first cloth, then center the second cloth over the first. If the cloth isn’t square, you’ll go nuts trying to do this by rotating the second cloth—ain’t gonna happen, Bunky. Simply flip the second cloth over, and there you go. You’re probably getting fuzzy-headed about now, 42

KITPLANES August 2007

Here’s how the sandwich begins. Not the eating kind, though you’ll use plastic wrap and a pizza wheel.

The sides are flat, and, ignoring the saddle at which Mark Wainwright is pointing, the curve over the top means a rectangular shape of material will cover it all.

but hang in there. This takes longer to explain than to do, and it’s obvious once you go through it the first time. As before, spread a bit of epoxy onto the second cloth; it won’t take much because the cloth below is already wet. Lay the second plastic wrap in place and, using a roller but not pressing excessively hard, squeeze the epoxy into the fabric and out beyond the plastic wrap. You could also put a thin layer of epoxy on top of this sandwich and then, using a squeegee, clean that epoxy off. That last coat of epoxy is a lubricant to keep from pulling the wrap.

Neat and Trim Next, you finally get to use the circular razor. Trim all around, cutting through the bottom layer of plastic wrap. Right now this sandwich looks like something you might put on your lower back for sore muscles. Word of advice, though— don’t. Instead, carry it over to the mold and set it aside while you give the mold

The sandwich is opened. It’s two layers of cloth placed 90° to one another between two sheets of plastic wrap.

Look closely and you’ll see that the fabric has thick threads and, at 90°, thin ones.

a thin coat of epoxy. Now peel off one layer of the plastic wrap, and drape the exposed cloth over the mold. You can center it, and work it around a bit, but eventually you’ll want to peel off that second plastic wrap. From here on you only need to fold the ends around and then, using your brush, push the whole thing into shape over the mold. Add some peel-ply, work out the excess epoxy (there won’t be much because of the sandwich method), and let the whole thing sit for a couple of days in a warm area. How long to cure depends partly on how you mixed your epoxy, but more so on the temperature. If your shop is around 50° F, as it was when we did this, you’ll need to let it sit for 24 hours before taking a small part from a simple mold, and three days for a full cure. As I said, it’s easier than you first thought and takes longer to describe than to do.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com. www.kitplanes.com

To create space for the epoxy-flox Harold Bunyi of AirCrafters created a tool to compress the paper honeycomb.

Part 5: working with carbon fiber.

L

ast month, we learned how to construct flat and curved parts from molds. This time, let’s see what it takes to build a gas tank out of carbon fiber. In this case, it’s a 325-gallon tank. Six pounds per gallon (OK, 5.85) multiplied by 325 gallons…that’s heavier than the 1320 pounds allowed for an entire Light Sport Aircraft! While this seems like a real “out there” project, consider that the construction techniques are the same as for a 32-gallon tank. Once you know the basics, it’s frequently just a matter of scaling up or down the projects that look intimidating. First off, the materials here were about $3500 for several sheets of two-ply/side carbon fiber with a 3/8-inch honeycomb core—sometimes called a two-core-two, meaning two plies of material then the core and then two more plies. Now aren’t you glad I said it could be scaled down? It sort of reminds me of the guy who said, “I’ve an unlimited budget and I’ve already exceeded it.” The advantage to us is that it’s a lot easier to see what’s going on with a project of this size. The first step is, of course, to have a set of drawings that 36

KITPLANES September 2007

BY BOB FRITZ

Take a good look at the green flange. It’s the secret to putting a glass fillet on all interior corners of a closed box.

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can be transferred onto the CF using a compass, straight edge, ordinary pencils and a lot of blue painter’s tape. Don’t use a grease pencil; it’ll contaminate the surface for later gluing. If it’s a large (spelled “expensive”) project, a mock-up in cheaper, easier-to-manage materials is a good idea.

The Value of Carbon Fiber You’ll remember in an earlier issue that we spoke of taking a stack of four 1-inch-thick boards and standing on them. They bent fairly easily until we drove a couple of nails through them, and then they didn’t sag so much. Those nails forced the bottom board to take a much larger radius turn and, hence, placed almost the entire tension load on that board. Inversely, the top board was being compressed. Thinking through this stack, you’ll realize that the second board was getting compressed less than the top board, and the third board was getting less tension than the bottom board. Now imagine pulling those two middle boards out of the stack while keeping the gap between the top and bottom boards. You’d still have the top board being compressed and the bottom board taking the entire tension load. This is exactly what CF honeycomb is doing. The paper in between keeps the top and bottom separated so that the bottom takes the entire tension load. This is completely reversible, of course. Top and bottom are just identifiers of

Jesus “Chuy” Gomez is using two sticks to aid in packing the epoxy-flox completely into the gap. You can see that he has periodically scraped the excess off the outer surfaces. It’s OK to pack those scrapings back into the gap.

Photos: Bob Fritz

Newtonian gravitational enhancement modules (bricks) are used to hold the top down.

the layers; flex the CF in the other direction and it works the same way. From just this example you can see why all modern composite structures are built like this, using some form of a core to separate the load-bearing fiberglass or carbon-fiber layers. Not only is it a strong way to build, it can be made surprisingly light.

Making the Cut We now have the pattern laid out, but how do you cut this stuff? The core is

just paper, so anything that cuts the CF is going to slice through the core like it was, well...paper. A toothed blade will do it, but a grit-edge blade does it better, especially if the part is too big to go on the bandsaw. Sanding the edges is also a bit different. Orbital or reciprocating sanders tend to lift the CF away from the paper and fuzz the edges. A hand-size belt sander is the ticket, but position it so that it doesn’t lift the CF. When it comes to needing a tight fit,

Basketball on a ping-pong table? No, laying out the pieces with painter’s tape. KITPLANES September 2007

37

Composites, Part 5 continued consider a plastic bag; it can be hard to tear until you create a tiny notch. That notch is known as a stress riser and serves to direct all the force into a small area, making the tearing pretty easy. That’s why your teeth, though they don’t cut the material, are more effective than your soft, round fingers at tearing open a bag of potato chips. It seems pretty obvious that to eliminate stress risers we have to spread out the load. In this case, that means getting the adhesive epoxy-flox to both sides of as many surfaces as possible. To do this, simply crush the paper core down about 3/ inch, exposing the interior sides of 8 the two CF sheets. This operation is surprising the first time you see it simply because going at the CF with a hammer seems anti-tech. As I said, don’t be intimidated by the technology; part of it may be rocket science, but not the part we’re dealing with. Once the interior sides of the CF are exposed, we have to add the epoxy-flox, but to ensure adhesion it’s best to paint the surfaces with straight epoxy. The procedure is referred to as wetting out, and is a simple job of slopping the epoxy into the groove. Again, this is to prevent stress risers in the form of gaps between the CF and the epoxy-flox. As a quick review, recall that flox is short cotton fiber and is quite strong when mixed with epoxy. Micro, on the other hand, is microscopic glass balloons that are not nearly as strong. They’re great when a lightweight material is needed as a filler in a corner or gap.

The cut is ready for sanding.

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KITPLANES September 2007

Get in the Groove Once the material is wetted out, we can pack the groove with epoxy-flox. It should be a thick consistency, as we’ll be installing the part on-edge and don’t want the epoxy-flox to drip and run. It’s important to fill the groove completely, creating a mound that protrudes above the edge of the CF about half the distance between the CF layers. That is, if the gap between the layers of CF is 3/8 inch, the epoxy-flox should be about 3/16 inch above the edge. This general rule will tell you how much epoxy-flox to mix. The excess will squeeze out and be removed, so don’t go overboard. You’ll notice that glass, not CF, is used to radius-out the joints between the flat surfaces. This is because the advantage

Miss your line? Not to worry, just re-cut it. If the variation is less than about double the width of the blade you’ll find it difficult to keep the blade cutting on the line. Go for a belt sander instead.

Dual interactive self-referential layout modules with eco-friendly digital baseline (two guys with a stick). Lesson: Don’t be intimidated by techno-speak.

When the carbon fiber costs $3500, a mockup of the fuel tank is a wise precaution.

Doing the layout with an ordinary pencil works fine, but the carbon in the pencil lead all but disappears on the carbon fiber material. Follow up with blue painter’s tape, and then mark the tape to indicate the cutting edge.

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Note the orientation of the 1-inch-wide belt-sander Gomez is using. This is to avoid creating fuzz at the edge.

Cutting a straight line is aided by having a guide fence. Just remember to cut to the correct side of the tape

The 2x2-inch boards over the top are to hold the walls in place while the epoxy dries. It’s important that the surfaces are clean when placing the walls.

This vertical wall (the one with the hole) is an internal reinforcement as well as an anti-slosh barrier. Note how well the contour fits.

Isn’t that a clean, tidy junction? Hard to believe it’s so strong.

of CF is negated by not being able to vacuum-bag the junction. Once the wall is in place and the epoxy-flox has had time to harden, we only have to add a tape of fiberglass to the juncture. We covered that operation in a previous issue, and the procedure is the same here: Radius the corner with epoxy-micro, wet the surface, lay out the tape, smooth out any bubbles, and let it dry. This is getting easy, isn’t it? The top is the tricky part. Once you’ve laid out the pattern of the walls onto the bottom side of the top and placed a few triangles, you can paint on some epoxy and hope all the parts drop into place.

The Inside Story “So how could the killer have done it?” asked Dr. Watson. “All the doors and windows are locked from the inside.”

A bandsaw will cut CF honeycomb, but it also tends to tear the fibers, creating a fuzzy edge that has to be sanded. KITPLANES September 2007

39

Composites, Part 5 continued

Use of the tool is simple: Hit it with a hammer. The paper honeycomb is simply compressed out of the way.

The construction of the tool is equally simple: a piece of aluminum of a thickness to match the spacing of the CF and a pair of plates riveted to it to limit the depth to a distance equal to the distance between the layers of the CF.

It’s messy, but completely filling the groove is essential. Wetting out the groove ensures that the epoxy-flox comes in contact with all surfaces.

Just as was done prior to packing the groove full of epoxy-flox, it’s necessary to wet out the surface. That small, lightcolored triangle is held down with hot glue and is for positioning the bottom edge of the wall.

Scrape off the excess as best as you can.

Adding a layer of glass to radius-out the corner makes it even stronger.

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KITPLANES September 2007

The wall (left) is in place and checked with roofing squares. When the epoxyflox has hardened, the triangles can be scraped off.

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#1 In Customer Service: We’re done with the interior walls. Putting it all together takes some planning but not a Ph.D.

The same incredulity applies here: How do you reinforce all of the interior junctions without being inside a closed box? You start by putting release tape on the last surface to be installed. Then build the walls into it, glassing all of the junctions. Then remove that taped last surface, and you find you’ve created glass flanges. Put the opposite wall in place, glass the corners, and let it harden. Finally, apply a coat of epoxy to the flange and that so-called last surface, and put it together. Elementary, my dear Watson. By now you’re wondering what a 325-gallon tank fits into. Well, here’s a non-clue: It’s one of four, and they’re all different shapes. But it’s a secret project. We’ll tell you all about it later when the owner gives the go-ahead. Next month we’ll tackle a smaller project, a set of fiberglass wheel chocks.

If you want to build with me I’ll be using the materials available in the Composite Materials Practice Kit from Aircraft Spruce, part number 01-15000. (There is also a p/n 01-15100 that’s the same kit without the book. I recommend you get the book because your education should never stop and, besides, it’s worth the extra money just to see photos of a very much younger Burt Rutan.) For this project, you’ll also need a circular razor, a couple of feet of rope, and a plastic or disposable surface about 24x36 inches. The first set I built got appropriated by my wife for her Cessna, so I’m making another. Come by and watch them come together.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.

Whatever you’re looking for, we have the qualified and experienced stafff to make your visit the most enjoyable ever.

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41

Stripping off the peel-ply. Don’t expect a flash of light, though.

You’ve rounded the corners to about a quarter-inch radius, now vacuum off the dust. Brushing it off just doesn’t remove it all, and it’ll mix into the epoxy.

Cutting the foam is easy even with a dull blade.

In Part 6, enough theory! Let’s build some wheel chocks.

I

recently sat in on Dave Saylor’s Fiberglass for RV10 Builders class (see the sidebar, Page 40) in which he had everyone build a set of wheel chocks. It was such a good experience I thought I would shamelessly pirate it for this article. I was also fascinated to see the different interpretations of seemingly straightforward instructions. More on that later. What we’ll do here is a dandy little application of what’s come before in this series. The nice part is that the result is not only useful, but you’ll be the envy of your neighbors and invited to give talks at the local Toastmasters. OK, that’s a bit over the top, but you will have a useful set of wheel chocks that weigh about a tenth of what those old wood wedges you’re lugging around weigh. I’d suggest you build at least two sets because your buddy will want one. 38

KITPLANES October 2007

BY BOB FRITZ

Materials List Glass cloth: 24 x 7 inches. Don’t forget to cut this on the bias. Flox: 1 cup. Microballoons: 1 cup. Epoxy: 2 cups. Mix per the manufacturer’s instructions…but not all at once! Stir sticks: a dozen or so. Rubber gloves: at least six pairs. Cups: Paper, not plastic, and definitely not wax coated. A dozen or so will do. Foam for mold: 2-inch square by 24 inches long. It should be small cell; the type that flower arrangers like to use is perfect. We’re going to split it into two www.kitplanes.com

Warped, isn’t it? That’s the bad half of the good/bad characteristic of drapeability.

Cut on the bias with the circular cutter, but be careful. It also helps to have a friend or a couple of clamps holding the straightedge, as the cloth is slippery and the straightedge moves.

A bit of gentle pulling, though, will move it right back into a nice rectangle.

Paint on a layer of epoxy-micro to seal the pores of the foam. One glove? The camera is on a timer, and I don’t want to risk epoxying the shutter button. A bare hand will know when it’s contaminated.

triangles so if you can buy a triangular piece, all the better. Rope: 18 inches long and any diameter between 1/8 and 3/8 inch is fine. Any material is OK, but plastic is best. Clear plastic-wrap: One 24-inchwide roll will do. Heavy gauge is best. Kitchen wrap clings to itself too easily and tears apart instead of coming off the hardened glass. Roller: Should be 3 to 4 inches wide, any diameter. Even a piece of broom handle will work, but you like tools so here’s an excuse. Circular razor, aka pizza cutter. Photos: Bob Fritz

1-inch brushes: a dozen of the cheap disposable kind. 60-weight sandpaper: One sheet is enough, really rough grit. One old, large, flat-blade screwdriver or chisel for digging out foam. Peel-ply: One piece about 3 inches wide by 24 inches long. Needle or pushpin for piercing bubbles. Bandsaw or cut-off wheel to cut the hard fiberglass. Step drill for use on hard fiberglass. Drill motor for that step drill. Safety glasses.

36-inch straightedge.

Other Requirements This project will take a couple of hours on day one, an hour on day two, 30 minutes on day three. You’ll also need warm weather or a warm workshop. Below 65° F, and the epoxy won’t harden. Use a small workbench with a plastic, or at least disposable, top. The epoxy will get stuck to it, ruining a wood surface.

Building Day 1 Split the foam square into two triangles about 24 inches long. Sand the three KITPLANES October 2007

39

Composites, Part 6 continued edges to about a 1/4 -inch radius and vacuum off the dust. You are wearing the safety glasses, aren’t you? Mix epoxy and add micro until it has the consistency of mayonnaise; the brand is not critical. Paint this mixture onto foam and set foam aside. This fills the pores of the foam. Lay out 30 inches of clear wrap. Lay out fiberglass cloth onto wrap. Work out the distortions so that the piece is square and straight. Lay second fiberglass on first and

straighten it. Mix about one cup of epoxy. Pour over cloth. Spread with brush or squeegee until completely wet. The cloth will be bright until soaked with epoxy. Lay the second sheet of plastic wrap over the wetted cloth. Use the roller to work out bubbles and excess epoxy. You need only a light pressure; don’t distort the glass. Use the straightedge and the circular razor to cut through the plastic and glass. You want to have a sandwich that will wrap around the foam, overlapping

RV-10 Builders Workshop

Aluminum seems “safe” for most of us compared to composite construction. After all, if metal shop is taught in high school (at least it was when I was there) it can’t be too tough, but composites? The rub of modern kit aircraft is that even the most ardent aluminum lover is going to have to pull on the rubber gloves and grab a stir stick, and it’s more than a little intimidating when the part to be worked with is so big that four people are needed to fit it to the fuselage. So it is with the roof/door/window section of an RV-10, one of the most popular projects on the market. Yet, after spending two full days at the RV-10 composites class at AirCrafters in Watsonville, California, the task of taking a saw to the single most expensive component in the kit was not so intimidating. At lunch several of the 10 participants, none of whom had prior experience in kit aircraft construction, commented that the class had really been worthwhile. “I’d have spent two weeks staring at it saying, ‘There’s no way I’m going to cut that much off,’” explained one builder. After the cabin, the item of most interest was the alignment of the wheel fairings. The cowling and doors were discussed, and a simple project of making a set of glass wheel chocks gave everyone some experience in using the materials. —B.F.

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KITPLANES October 2007

Epoxy-flox is poured onto the cloth. Some advocate brushing only, but this seems to work just fine.

at least one face, and be about 1/2 inch longer than the foam. Use the handle of the brush to push/ lift the corner of the upper plastic. Gently pull away the upper plastic and dispose of it. Place foam stick on the cloth 21/2 inches from one edge of the cloth. (Read the next step to understand this positioning.) Lift plastic so that it wraps the cloth over one side of the foam mold and halfway down the second side. Brush the surface of the plastic at 45° to smooth out air bubbles. This angle minimizes distortions in the cloth. Use the pin/needle to pierce any bubbles and continue smoothing out the air. Use the handle of the brush to push and lift the corner of the plastic. Lay plastic back down to original position leaving the glass cloth in place. Lift the other plastic/glass over and around the foam. Roll the foam/epoxy over if needed to get all the cloth in contact with foam/ epoxy. Be careful to not roll the plastic wrap in between the layers of glass. Brush the surface of the plastic at 45° to smooth out air bubbles. Don’t forget to use the pin if needed. Unwrap the plastic back down to original position, leaving glass cloth exposed on two sides of the chock. Lay the strip of peel-ply onto one side of the foam mold. Use a brush to smooth out the peel-ply. www.kitplanes.com

Not much point to using a rubber squeegee. It would become rigid when the epoxy dries and couldn’t be cleaned as easily as a steel blade. Also examine the bottom-right corner: You’ll see that there are two layers of glass. It’s referred to as two-bid for two layers of bi-directional cloth. Bi-directional means that the cloth has approximately the same number of threads in both the X and Y directions.

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Use the needle to pierce any bubbles and then continue smoothing. Rotate the part so that the peel-ply is down; remove the remaining plastic wrap and dispose of it. Place it on a flat sheet of new plastic wrap and let it dry 24 hours.

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Building Day 2 Pull off peel-ply. It will be a firm pull. Note that this side is now the “down” side. Use the saw to cut off both ends of the stick so that they’re square. Draw a cutline at the midpoint of the length of the stick. Place a mark on each side of the cutline about 1/2 inch away from the cutline. This is where you’ll drill holes for the rope. Cut the stick in half. No, not lengthwise! Use the screwdriver to dig out 1 inch of the foam from the end of both chocks where you marked the rope hole. That’s why I said it should be an old screwdriver. The foam is soft stuff, and we’ll fill this in with epoxy later. Dig out 1/4 inch of the foam from the other end of both chocks. Drill one hole, barely large enough for the rope, where you made the mark on each chock.

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Composites, Part 6 continued

The end of the brush works well for lifting the corner of the top layer of plastic sheet.

Don’t forget to peel back the clear plastic before wrapping the glass over itself. You have to have a friend help lay down the top layer of plastic. If you think kitchen wrap is frustrating, try this alone!

Bubbles, always bubbles. It’s surprising, though, how effective a simple pin is in getting rid of them.

Roll out the bubbles. It’ll take about 10 minutes of gentle work the first time you try this.

One sheet of peel-ply on the side where the glass wraps over itself, and we’re almost done with this phase.

A bit of brushing helps the peel-ply soak up the excess epoxy. Going all the way around the chock with peel-ply is possible, but it would leave the top two sides rough, and we’re looking for some aesthetics here.

Back to the circular razor to cut the sandwich to rectangular pattern just a bit longer than the length and about 3.5 times the “circumference” of the foam.

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KITPLANES October 2007

If you have to transport the stick, wrap it in heavyweight plastic. Lightweight plastic will stick to the epoxy, and there go the aesthetics. Guess how I found that out.

www.kitplanes.com

I had to transport the stick so the ends were distorted, and that required cutting off a bit more than planned. No big deal. Then find the middle and cut the stick in half.

These Unibit drills are the hot ticket for drilling through just about any thin material. You want a hole just a bit larger than the rope. It should be in the two ends from which you dug out that inch of foam.

Dig about half an inch of the foam out of one end of each chock and an inch out of the other two ends.

Tape the chocks together, base to base, with the rope holes at the same end. Cut about 18 inches of rope. If it’s plastic, use the soldering gun to melt through so that it won’t fray. If it’s a fiber rope, wrap it with electrical tape and the cut at the midpoint of the tape to stop the fraying. Insert the rope through the holes. If the rope is a snug fit through the hole, good. If not, tape the rope to the chocks. Don’t bother with a knot; the next step is the real anchor. Mix about one cup of epoxy and paint it liberally into all four ends of the chocks, soaking the rope ends as well. This wetting out ensures that the epoxyflox used in the next step will adhere to the chock and the rope. Add flox to the remaining epoxy to the consistency of cold peanut butter. You don’t want it to move when held on a stir stick. One of our guys left it rather thin and, though it made a nice, smooth end, it meant that he had to wait a day for it to set up before he could fill the other ends. Pack the mixture into the ends of the chocks so that it stands up about 1/8 inch above the edges. Don’t worry about get-

ting it smooth, just have it stick out. Place the chocks on a couple of stir sticks for 24 hours so that the flox will fully set. Forget the stir sticks, and you risk gluing the chocks to the workbench. “Nice set of work-stops you’ve got on the bench, Fred. But shouldn’t they be moveable?”

The rope on the right was cut with a knife; the one on the left was cut with a soldering gun and then massaged into shape with a wet towel. Don’t try it with a dry towel; you’ll pull plastic threads in all directions. And don’t do it barehanded; the neighbors will learn some new words when that hot plastic sticks to your fingers.

No knot needed. Just wet out the interior, mix up some stiff-as-cold-peanut-butter epoxy-micro, stuff it in both ends, let it set, and then sand it smooth.

Building Day 3 Untape the chocks and sand off all four ends to square and smooth. Here’s your excuse to go buy that inch-wide belt sander. Paint on or write your N number. If you don’t, these are guaranteed to grow legs and walk away. Be prepared to do it again for your buddy. See, I told you to make two sets. I built at Dave Saylor’s AirCrafters and then put it in the airplane to take it to my shop about an hour’s flight away. Mistake. The air within the foam expanded as I went to altitude with the result being rather large bulges where the wet glass was pushed away from the foam. Lesson: Don’t move the piece until it hardens. The saving grace is that holes, separations and bubbles are of no consequence here because these are nonstructural components. After all, I’m just going to throw them on the ground and park an airplane on them! I went on

to build a second set that came out perfect, though, so I’ll be able to make that Toastmasters talk after all. Even with bulges and voids, the upside is that you’ve learned a lot about how to work with composites. Aren’t you glad you did this before you started on that RV-10? The folks that came to the class were. After finishing, they all agreed that building this simple set of chocks left them rather proud of themselves and confident enough to tackle a more ambitious project…like glassing in an RV-10 windscreen. Next month we’ll take up vacuumbagging. No, that’s not a game played in Scotland by guys wearing skirts.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com. KITPLANES October 2007

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Dave Saylor holds the finished window. Two pieces of carbon fiber were made using the bagging technique described, then bonded together in a bag. It’s super strong and light.

In Part 7, we tackle vacuum bagging.

V

acuum bagging…it sounds like the next stage of self-expression after you’ve gotten a couple of tattoos and been to the bodypiercing shop. But, no, it’s a process for creating very low weight, complex shapes using composite materials. In previous articles we looked at wet layups where one soaks the fabric in epoxy and then drapes it over the mold. You’ll recall that much effort was expended in getting air out from between the layers and reducing the percentage of epoxy in the product. You’ll also remember in our first article in this series that the strength of this technology comes primarily from the fibers; the epoxy’s job is to prevent shifting of the fibers relative to one another. It follows, then, that when it comes to the epoxy/fiber ratio, the adage, “If enough epoxy is OK, more is better, and 48

KITPLANES November 2007

BY BOB FRITZ

too much is just right” is wrong. Once the job of nailing the fibers together is done, more epoxy just adds weight. Is it easy to have too much epoxy? A beginner at wet layup might have an epoxy/fiber ratio of 90/10. Someone skilled in wet layup will reduce it to less than 70/30. In a sophisticated aerospace component this will drop to 40/60. However, achieving this requires excellent process control, without which you might find uneven distribution of the epoxy to the point of having dry spots. A reasonable target for us is in the neighborhood of 60/40, and vacuum bagging is a great way to get there. www.kitplanes.com

Jesus Gomez and Harold Bunyi demonstrate the value of teamwork in laying out the bottom side of the bag. Note that this is not exactly the same as the schematic/ cross-section drawing. Here, they’re going to squeeze two rigid parts together with epoxy in between rather than force a glass/epoxy matrix into conformance with a mold. The principles, though, are identical.

The vacuum tape consists of white paper separating a strip of putty-like adhesive.

The guys are creating the bag. You can see that Gomez is about to set the upper side of the bag into position over the tape that Bunyi has laid down.

But how important is it to keep the pounds off? If you’re building a Light Sport Aircraft you’re limited to 1320 pounds maximum gross weight. If you’re building an ultralight, well, the name says it all. Keep it under 254 pounds. Rule compliance aside, there is still the issue of performance. Kelly Johnson of the Lockheed Skunk Works quantified it best: “Each pound adding to the airplane’s overall weight cost us one foot Photos: Bob Fritz

of altitude, so in building the U-2 we were ruthless weight-watchers.” Even if you’re not building the U-3, if you’ve ever taken off with full tanks, all the seats full and the baggage area maxed out, you have a feel for the advantage of permanently removing 50 pounds from the airplane. (That’s why I’m on a diet.) Getting there consistently, though, requires a bit more than a pin to get the air out and a squeegee to remove the

excess resin. (That refers to the epoxy, not the diet.) That’s where vacuum bagging comes in. It uses air pressure to squeeze out the excess epoxy and trapped air. This improves the bonding between the layers, decreases the cure time, and will frequently get you down to that target of 60/40. You’re thinking, “Oh, sure, but I can’t do that in my little shop.” Ah, but you can, Grasshopper, and it’s neither difficult nor expensive. If you’ve already done a wet layup you’ll need just a few more items, the most expensive being a vacuum pump. However, even that can be had without too much pain. In fact, in a future article, we’ll show you how to build a nice vacuum pump that is powered by compressed air. You’ll notice that I did not say, “Use a vacuum pump to suck the air out of the bag.” This is so often stated that the engineer in me goes nearly apoplectic every time I hear it. Hold this thought: Air does not suck, it blows. A moment’s cogitation to consider the tensile strength of air should convince you that the vacuum cleaner you use on the carpet is really a pressure-differential cleaner. The “vacuum” in vacuum pump, as opposed to a compressor, is simply a sloppy way to describe whether KITPLANES November 2007

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Composites, Part 7 continued the machine alters air pressure to above or below ambient. It’s important to understand this concept, because without it you’ll not understand the why if something goes wrong in this process.

The Setup The basic idea is simple: In doing a wet layup, we used peel ply and a brush to soak out the excess epoxy. This time, we’ll mechanically squeeze it out. You could try to sit on the bag, but as you do so you’ll remember the last time you tried to squeeze the air out of an air mattress. Tough to flatten out those random pockets of air wasn’t it? Sort of like playing Whack-a-Mole at the amusement park. Instead, let’s get Mother Nature to sit on the bag. All we have to do is push a bit of the air out of the bag with a vacuum pump, and the rest will take care of itself.

The two components received a generous layer of epoxy-flox and were taped together. They didn’t need the perforated plastic sheet, as there simply isn’t a lot of epoxy to squeeze out and it all comes to the edges. The bleeder cloth, though, is required to give the air a path for leaving the bag.

After placing plenty of bleeder cloth in positions that will channel all the air out of the bag, it’s time to close up. Again, a buddy to help in doing a part this big is essential to keeping the wrinkles out of the tape zone. A wrinkle there is a leak.

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One more digression into science. Air pressure is simply the load exerted by the weight of the air above a specified area. Go up, and the air pressure decreases because you’re now above some of it. At sea level it’s 14.7 psi; at 18,000 feet above sea level it’s half that. If you don’t think air has a weight, put a flattened basketball on a gram scale, then pump it up and weigh it again. What that means is that unless you live in a very deep hole in the ground such that the air pressure is greater than at sea level, the absolute limit you’re going to place on the bag is 14.7 psi. Now, 14.7 psi is not a trivial load; on a part measuring 12 x 12 inches, you’d have to stack 42 50-pound bags of sand in perfect distribution to get the same effect. The reason you don’t feel that weight is because you’re 99% water, which simply transmits the load through you without crushing you. Taking a look at the diagram, we see that there are a lot of components to the bag. The vacuum bag, fabric, peel ply, wax and mold are all familiar by now, and a vacuum pump is obvious. But how do the sealant tape, perforated plastic and breather material fit in? Clearly, to transform the bag material into a bag you have to seal it. That’s the job of the sealant tape. It’s a putty-like material that adheres to the bag material. It comes as a roll and is roughly one-eighth by onehalf inch when laid out. You strip off the paper that protects its sticky surface and simply push the bag material into it. Wrinkles should be avoided as much as possible because they are leak paths. Vacuum tubing is vital. Obviously, Tygon tubing will simply collapse, so a more rigid material is required. Size is not critical; just keep it short, 4 to 5 feet maximum, and unkinked. Last is the pressure differential generator, aka vacuum pump. This can be anything from a vacuum generator (no moving parts) that hooks up to your compressor, to an aquarium air pump, to a big-bucks industrial vane or screw-type vacuum pump. The size of the pump is entirely dependent upon the size of the project and the integrity of the bag. You should be able to achieve a solid squeeze

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Composites, Part 7 continued in 5 to 8 minutes. Too small a pump and you won’t keep up with the inevitable leaks. To explain the function of the other parts, let’s go into the bag to see what happens when the pressure is on. First, the bag itself starts to collapse, pushing against the bleeder cloth. Remember that air mattress? Tough to get out those random pockets of air. That’s where the bleeder cloth comes in. It’s a thick, porous mat that gives the air a path by which to escape the outside push. The bleeder cloth has another use. The gases and excess epoxy pass into it with the liquids being trapped, and the gasses continue through the breather material and out through the vacuum pump. It’s somewhat akin to wearing socks on a wet carpet. The water gets squeezed out of the carpet and into your socks through a combination of capillary action and a pressure differential. Speaking of pressure, the bleeder cloth also serves to distribute it evenly. Without it, any wrinkles in the bag would define the surface contour of the finished part just like a wrinkle in your pillow leaves you looking like you’ve fought a saber dual. As the bleeder cloth collapses, it pushes against the perforated release film and the peel ply. Epoxy, being a liquid, is non-compressible, so it squeezes through the weave in the peel ply, through the small holes in the release film, and soaks into the bleeder cloth. The air bubbles in the epoxy also are forced through the release film and, as the layers descend, they push the fabric against the mold. The epoxy/air between the fabric and the mold gets pushed through the fabric, through the holes in the release film, and it also deposits into the bleeder cloth. Just like that, you’re at the 60/40 ratio. I should note here that some of my sources tell me that peel ply is optional, but that it’s good for fine-tuning the ratio of the epoxy to fiber. It’s also a good filter to keep strands of the breather material away from the epoxy-fabric, but it means the part will assume the texture of the peel ply.

The fabric has been soaked in resin and then assembled with all the rest of the components. All that remains is to turn on the pump, let the epoxy set, and then pull out the finished part.

They take the time to carefully push the upper bag into the tape.

The hose goes in with special attention to keeping the bag material away from the inlet. Gomez has even wrapped the end of it with bleeder cloth.

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Composites, Part 7 continued

Here’s what it looks like if the bleeder cloth is excluded. The inevitable wrinkles in the bag and release film define the surface finish.

Choices and Tradeoffs Is this the only way to vacuum bag a part? Of course not! That’s the fun of this process. It’s open to invention and variation once you understand the basics. For instance, to make a flat layup such as an extra thick spacer, you could use painter’s plastic for the bag, kitchen sponges to soak up the excess epoxy, Dacron material from the fabric store (that’s what peel ply is made from), duct tape to seal the bag, windshield washer hose for the vacuum line, an aquarium air pump for a vacuum source and skip the perforated release film if it’s a part smaller than about 10 square inches. I did it, and it worked great. Another variation was demonstrated by AirCrafters’ Guru of Glass, Harold Bunyi. He’s a bit more elegant in making up the windows for a Lancair when he bonds two existing shells into a single component. The bag goes completely around the assembly and, when the pump is turned on, it gets about 4000 pounds of perfectly distributed pressure while the adhesives do their thing. And that vacuum pump that uses compressed air? Here’s a hint: Bernoulli. We’ll show you how to build one next month. For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.  54

KITPLANES November 2007

Now they add wrinkles. The bag material will stretch just so much, and then it either displaces the parts or won’t fully collapse around the assembly. Placing wrinkles is a skill, though. You have to “see” where additional material will be needed.

It started out as a square bag and ended up looking like something from the Bat Cave, but these two parts are getting squeezed with about the same load as if a Hummer parked on them.

www.kitplanes.com

We can make a vacuum reservoir out of some large diameter ABS pipe and a few fittings. The hose on the right end goes into the bag.

This pump by Vaccon is about 5 inches long, uses about 60 psi air, costs about $80, and is more than adequate for use in a vacuum-bagging operation.

Part 8: Building a vacuum pump.

L

ast month we described vacuum bagging, a fairly simple process that uses atmospheric pressure to squeeze out excess epoxy and those nasty little (and sometimes large) air bubbles from your layup. But a vacuum pump is not something most folks have on the shelf, and though the one you bought for the instruments on your airplane will develop enough of a pressure differential to be effective, it’s way too expensive to be pumping epoxy vapors through. A quick survey on eBay revealed a dizzying array of pumps ranging in price from $20 to $2000 with some at the high end being inad-

BY BOB FRITZ

equate, and an occasional gem that fits what we want for as little as $75. But how do you know if that bargain is up to the job? I’m glad you asked! There are four ways to develop a vacuum. OK, there are several more, but they’re impractical for our application.

How to Get Vacuum The first way to get vacuum is with a rotary vane pump. That short beer-can size pump on the back of your aircraft

With all of the parts in place it looks like this. Air from the compressor enters at the fitting on the bottom left. It shoots through the blue pump, exits out the black noise muffler at the end, and creates a low-pressure area at midpoint. The air in the bag and vacuum reservoir flows toward that lowpressure area and past the gauge on top, which gives you a sense of progress. When the pressure gets low enough, the vacuum sensor opposite the gauge sends a signal to the valve, located between the air inlet and pump, and shuts off the flow of compressed air. It works well and can be built for about $100 if you scrounge a few parts.

Photos: Bob Fritz

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Composites, Part 8 continued engine is in this group. It has a set of vanes being spun inside a cylinder and can range from anemic to really powerful. It usually has the input power shaft mounted directly to the output power shaft of an electric motor and is, hence, cleverly described as a “direct drive rotary vane pump.” The second method is a piston pump and, as the name implies, it is simply a lawn mower engine with no spark plug, spun by an electric motor. The third method is a diaphragm/ flapper valve type pump. This uses a cam to push a diaphragm back and forth and expel the air through a simple rubber flapper that covers a hole. While seemingly fragile with all that material being flexed, you’ve had one for years and never noticed it because it was so reliable; it’s used most often in your refrigerator. If you’ve ever looked into the back of your appliance, you’ve seen a can that looked like a flattened soccer ball; inside that can, buried under a lot of soundproofing, is a pump about the size of your fist. Now, before you start to raid the junkyard for one, you should know that though they work great in a refrigerator, they won’t have the capacity to pump enough air to a hard enough vacuum if you’re making vacuum-bagged parts larger than a shoebox. All of these methods have one thing in common: They have moving parts. The fourth way of pumping out the air is to use compressed air. Right now you may be thinking, “Use high-pressure air to generate low air pressure? He’s gone weird on us.” Well, perhaps, but in this case I’m doing well, thanks for asking.

Do Try This at Home Go get two books of the same thickness, place them on the desk about 4 inches apart and lay a single sheet of paper on the gap. Now blow through the channel. Intuition would say that you’ve pumped air into the channel, so the paper should rise, but instead it drops. That phenomenon had puzzled folks as far back as the ancient Greeks, but it wasn’t until the mid-1700s that Daniel Bernoulli figured 84

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It’s only a simple solenoid-driven valve that can be had via a web search. You want to get one that is for vacuum, though, not pressure.

Showing signs of use, this hose end may be inelegant, but it works fine.

This vacuum sensor will send an electrical signal to the valve when the pressure rises. Again, an eBay search for one will likely be successful.

it out. What’s happening is that when a fluid accelerates, its pressure drops. The math is complex and it’s not necessary to understand the principle for our purposes, but we put it to use all the time. For instance, take a look at the throat of a carburetor. It starts large and then narrows. You’d think that forcing the air to flow through a small area would drive the pressure up, but the only way to get the same amount of air out as went in is to make it go faster. The pressure drops just as it does with our books and paper. It’s the phenomenon being used by that double funnel on the side of older airplanes. Why have a vacuum pump bolted to the rear of the engine if this is so effective? Well, first, it works when air is flowing at a fast rate; during takeoff you’re not going fast enough to get the vacuum-driven gyros up to speed. Second, it’s prone to icing, which is certainly not desirable in the chain of

machinery that’s most needed on a dark and stormy night. But if we can push air through it fast enough, and icing is not an issue, then it’s perfectly capable of developing the sort of pressure differential we need for vacuum bagging. Inasmuch as we’re unlikely to crash if it ices up, all we need to do is get the air flowing, and that’s where your shop compressor comes in. We’ll blow air through the right port of the pump and hook up a hose to the vacuum side.

Even Better We can improve on this, though. The goal is to keep a vacuum in a bag that probably has a few leaks. Instead of hooking the pump directly to the bag, let’s hook it up to a reservoir and then to the bag. This is somewhat like having the air tank on your compressor; without it you’d have to turn on the compressor every time you wanted to www.kitplanes.com

blow up a bicycle tire. Next, we need a hose from the reservoir to the bag. If you simply stuck an openended hose in the bag, it would tend to grab the plastic of the bag. Instead, we’ll plug the end of the hose and knock a bunch of holes in the sides of it. It’s also helpful to know just how effective your system is, so we’ll put a vacuum gauge on it between the pump and reservoir. You could check for leaks by watching the bag, but that’s hard to quantify: “Hey, John, does that bag look any bigger?” With a gauge you should watch for a rise of not more than about 5 psi in 5 minutes.

One More Thing That still leaves you either standing around watching the gauge and turning on the compressor, or letting the compressor cycle off and on for the 24 hours it can take for the epoxy to set. So let’s automate it by using the same setup as on the compressor and installing a pressure sensor and a solenoid. On your compressor, when the pressure rises to a set level, the sensor sends an electrical signal to a solenoid-controlled valve and shuts off the pump. We’re doing the same thing: When the pressure in the bag and reservoir rises due to leaks, the sensor sends a signal to the solenoid-controlled valve and opens up. That lets the compressed air go through the pump and restore the vacuum in the chamber and bag. When the pressure drops below the set level, the sensor shuts off the flow of air through the pump. Of course, if you don’t want to tie up your shop compressor, you could substitute the venturi vacuum pump by going on eBay and looking for a small pump built by Robinair. It is a low-cost brand, and you need a capacity of only 2 to 5 cubic feet per minute to keep up with the inevitable leaks. Although they sometimes show up for about $50, you’ll still have to add all the other bits to get the automatic off-on feature. Go have fun, and before long you’ll be whipping up some really neat parts, leaving all your buddies amazed and stopping by with a pizza in exchange for advice or help.  KITPLANES December 2007

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These pinholes show up only because they had a wipe of black glazing (above left). Small enough to ignore? No way! Chuy Gomez is pointing them out as unacceptable land mines that will explode through the paint. This one (left) is definitely too large to ignore.

Part 9: Dealing with the pinholes problem.

I

f you’re thinking of building a composite airplane, you should be aware that it’s built roughly in thirds. The first third of your time will be spent assembling the airframe. How long that takes is up to you and the designer of the kit. The second third is spent installing the systems, most notably the instrument panel. Again, that’s up to you. Are you a fast worker? Are you building it for day VFR around the patch or transcontinental competition with the airlines? The third third, if you will, has little to do with the first two: It’s the finishing. Of course you are proud of your accomplishment, but is the goal to build a sleek little gem that gathers trophies faster than bugs on a Kansas evening, or do you want air under you and runways disappearing behind? That old saw of being 90% done, 90% to go was never more apropos than in relation to the last third of the project. If it were an aluminum airplane, you’d just wash, dry, prime, paint and fly. (Sure, that’s a pretty broad generalization, but you get the idea.) You can do that with glass, too, if you’re not too particular about the finish. But if you want to be invited to park next to the runway at Oshkosh, you’d best Photos: Bob Fritz

BY BOB FRITZ

be prepared to spend some time coming into the house perfecting your JackNicholson psycho rendition of “They’re baaaack.” What’s back? Pinholes. I know, I know. At the outset of this series I said something along the lines of building a beautiful glass airplane can’t be that hard, after all there are so many of them. And I’ll stick by that. It doesn’t take a Ph.D. in polymer technology. It does take a little grunt work and a lot of working smarter by using the right technologies.

Find the Pinhole Pinholes are the inevitable result of using a porous material. Some pinholes are voids in the final layer of surface, which might be primer or gel coat, but might also be nothing at all because KITPLANES January 2008

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Composites, Part 9 continued some fiberglass components’ outer surface is simply the last exterior layup done or the final layer of glass-and-glue next to the mold. So what does a pinhole look like? How big is big enough to ruin your paint? As it turns out, the answer is: It depends. It depends on your tolerance level of course, but it also depends on your patience to let the filler material cure before painting. It also depends on how hot the weather is. I’ll get into those factors a little later, but for now take a good look at the pictures to see what we’re talking about. I spent some time with the guys at AirCrafters in Watsonville, California, to get the point of view of those with years of experience and see what they liked. When I asked for the expert, they all pointed to Jesus Gomez, aka Chuy. He suggested several products, but his emphasis was on preparation and technique to first avoid creating more pinholes instead of trying to eliminate them later. Once that’s mastered, it makes filling the little monsters a bit easier.

Keep It Clean First, let’s look at the technique. The watchword is “clean.” Any oil or dirt will create a pinhole in the paint where there was none on the bare glass. It’s easy enough to remedy; just get it clean, really

Start with a pinhole, then cover it to cause a bump of compressed air. Now sand it smooth, and what do you get? A small opening to a cavern that is even harder to fill in.

clean. How clean? Start by sweeping and vacuuming the entire shop. Without this step you’ll only frustrate yourself by kicking dirt into the air as you walk around the part, going at it with acetone and new rag. Wipe the plane down at least twice, too. Then he likes to give the part a misting of black paint followed by a light sanding to remove the black on the surface. What’s left is in the pinhole. Another method is to squeegee a layer of red or black glazing compound over the surface. Assuming the surface is white, or

at least light colored, the pinholes will show up like acne the night before the high school prom.

Product Placement Gomez then likes to use a body filler called Rage. It’s a lightweight version of Bondo that’s usually employed to do fine fillets. In this case, he goes at with a squeegee to lay on a coating as thin as possible. “This is where most beginners goof,” he said. “They think they can just slap it on and sand it back down.” That’s a mistake, he says, because using this poor technique results in the Rage not being forced into the cavity; it’s just layering over it. He also advises that if you want to use it for filling the larger divots or creating a radius in a corner, it should not be applied more than one-eighth of an inch thick. “It’ll crack as it dries,” he says. I checked with another friend who’s flying his sixth composite airplane, Jim McCormick of Jabiru Pacific. He learned a lot in the course of all that

PolyFiber will even supply the roller for applying the product. They’ve also written a pretty good manual on the subject of finishing.

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work and was quick to admonish me when I suggested wet sanding. “No! No! No!” he said. “That creates a very good paste that fills in the pinholes just fine, but it doesn’t adhere to the base material. When it dries out, it falls out and your pinhole is back.” A search of web chat room discussions consistently confirmed the use of Rage and also uncovered the name PolyFiber. I called PolyFiber, explaining that I was researching the subject and would like to speak to someone with technical know-how. A conversation with Jon Goldenbaum, self described gofer, trash guy and janitor at Consolidated Aircraft Coatings, aka founder and CEO of the company that makes the PolyFiber line of products, was a wealth of information. “So, what’s the cause of pinholes?” I asked. “Three causes, Bob. First, there’s prepreg. Even though the fabric is completely soaked with resin, a lot of it is squeezed out, leaving a texture due to the weave, and you can feel the ridges of the yarn. Second, the epoxy sometimes outgases, leaving tiny bubbles at the surface. And third, a simple, unfilled void in the cloth where it didn’t get sufficient resin to wet out…a dry spot.”

The PolyFiber Solution The company has two products of interest: UV Super Fine and SuperFil. Goldenbaum explained that the most apparent difference between the two products is their viscosity: UV Super Fine is near water, and SuperFil is a bit thicker. He went on to explain that though SuperFil has been on the market longer and is, therefore, more popular, it could stand a couple of improvements. The result is that they concocted UV Super Fine not as a replacement, but as a dedicated filler of pinholes. The reason for this is that if the pinholes are large enough, most coatings—including paint, Bondo and even SuperFil— will simply follow the contour of the hole, resulting in a wing surface with the appearance of the tee-off point at a Smurf golf course: little divots everywhere. You could cover them, but com-

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Composites, Part 9 continued pressing that tiny amount of air will only cause a tiny bump that you’ll want to sand down. The result is a tiny pinhole with a cavern underneath. The solution is to fill the holes, not cover them, which is what Gomez said. The reverse of that problem was also explained. “Bondo-type products are too heavy in viscosity to get into those small cavities,” Goldenbaum said. “For SuperFil to do this job, you have to really squeegee it in. A lot of our customers didn’t understand this and were slathering it on like it was frosting on a cake. That’s fine if you want to fill seams and use it to create corner radii, but not for filling pinholes. So we came up with UV Super Fine.” The liberal use of wetting agents causes it to flow out, adhering to the surfaces and filling in the pinholes. This allows the user to apply it either by spray or to indulge in a heresy, applying it with a foam roller. Hey, it works, it’s cheap, it’s fast and clean-up is a snap. In fact, Goldenbaum says it works better

than spraying because it really squeezes the material into the holes. Just don’t let your flying buddies see you going at your Bugsmasher VI with a roller and then expect less than a razzing.

Don’t Race for the Cure There’s an adage that says, “If it sounds too good to be true, it probably is.” Here, too, that maxim applies. You have to wait for UV Super Fine to dry at least three weeks, longer in humid weather, and invest in a dehumidifier while you’re at it. “Our job is to watch paint dry,” Goldenbaum said. This is where the patience issue comes in. The UV Super Fine, along with many other products, has to be 100% cured before you apply the paint. Rush it and the filler will continue to shrink, pulling the paint down with it. Similarly, if it’s not fully cured and you leave the airplane out in the hot sun, the filler shrinks unduly and can force the pattern of the glass cloth up through the paint. But then maybe you like that effect…just add a Levi’s label and call

it a feature. The offset, though, is that you can watch the filler dry for three weeks, or you can fill-sand-fill-sand for three weeks or more if the paint gods are displeased. If you have the front-row-at-theairshow obsession, you’ll have to have a go with SuperFil to get the little radii that make the airframe appear to be blow-molded in one piece. After that, you will need to seal the surface with a two-part epoxy primer and then allow it to cure for another week. But that’s literally a discussion for next month, when we wrap up our Build Your Skills: Composites series by taking you from the done-with-pinholes stage right to the last steps in preparing the final skin of paint. Just as with pinholes, technique and patience play starring roles.  For more information about AirCrafters builder assistance, call 831/722-9141, or visit www.aircraftersllc.com. A direct link can be found at www.kitplanes.com.

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Part 10: Repair of a twisted tail and more.

I

love working with aluminum. Having said that, fiberglass has its advantages, not the least of which is the invisibility of repairs. Sure, it’s sticky, slimy and can leave you scratching like a dog after an all-day possum hunt in the deep woods of southern Georgia, but for minimizing the aw-shucks factor, it’s hard to beat. How much so is illustrated by my current project— building a Jabiru J250 LSA. [Expect to see a full series on this project later this year.—Ed.] The kit maker says that the rear of the fuselage is molded around the horizontal stabilizer, so all I need to do is align it perpendicularly to the long axis of the fuselage, and then shift it laterally so that the same amount of stabilizer is on the left as is on the right. Seems simple enough…but whoa! Either this hangar is falling over or that horizontal stabilizer is, er, not quite horizontal. The first thing to do is make sure that the fuselage is level, and the best place to check it is right across the wing supports. The manual advised that the bottom of the doorsills is the point for checking for level. However, in that I would prefer to have flying surfaces parallel to flying surfaces, it seemed silly to select the doorsills. After all, I Photos: Bob Fritz

BY BOB FRITZ

It’s only 1.5 degrees low on the left, but that’s 2 inches lower than the right.

like to fly wings level, not doorsills level. As it turned out, though, my preferred reference agreed with theirs. A bit of scrounging and a long carpenter’s level allowed a confirmation of two things: 1. The fuselage is level. 2. The level is level. Digital levels are really fun, but they can be wrong. You must check them periodically for correct reading, and if they’re not telling true, recalibrate them. The KITPLANES February 2008

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Composites, Part 10 continued instructions for doing so are not exactly simple, but it’s absolutely vital to do this or you’ll be wrong to several decimal places.

How to Fix It With the wing attach points and door sills confirmed to be level—and therefore the horizontal stabilizer confirmed not to be—the procedure for correcting this had to be determined. The kit maker suggested that it simply be twisted to straight, temporarily locked down with sheetmetal screws and glassed in place. That brute-force approach didn’t appeal to me, nor did the potential of inducing residual loads. Thinking about it for a while, I suspected that when the system sat outside in the hot sun, the residual load would simply cause the tail to wind right back to out of alignment. But a second realization presented itself: I might take advantage of those loads and the susceptibility to heat by twisting it past level, and then applying a heat gun. If I could support it in just the right position while it cooled, I might get it straight and with no stresses built in. Carefully placed alignment marks were made, and then supports were positioned under the left end with weights on the right end. This induced a twist in the opposite direction, and, when it had

Step one: Is the fuselage level, and do all the tools agree? Steel machinists’ blocks on the wing-attach points make a platform for a long spirit level. Digital levels can be out of calibration, so a cross-check is in order.

gone past zero by 0.5°, clamps and then sheetmetal screws were added to guarantee I’d get back to this position. Bondo might cover a few screws, but nothing was going to hide the C clamps, so epoxy/flox would have to be added between the joining surfaces. This required that all of the hardware be pulled off followed by a wipe down with acetone. The stabilizer is removed by sliding it out sideways. Therefore, adding a coat of epoxy or flox would not be possible without smearing it down half the stabilizer. It was necessary to pry the gaps

open enough to paint some straight epoxy on the mating surfaces, add some epoxy/flox, and screw it together. Put it on the supports, add the weights and check that it’s back to 0.5°. Good. The criticality of this procedure had me frequently confirming that the fuselage was still level. With all of it looking good, and with a silent prayer to the gods of fiberglass, I started carefully painting around the interfacing surfaces with a heat gun. Now, this isn’t your mother’s hairdryer; this will take graffiti off a New York City subway station, so careful use is the order of the day. My technique was to heat the tail cone and its flanges while avoiding the stabilizer as much as possible. The last thing I needed was to turn the once-straight stabilizer into a Z shape. Doing it evenly, and not excessively, was controlled by placing my hand on the tail after applying a bit of heat. Bringing it up to a bit too hot to keep my hand on it seemed about right. Now came the hard part: leaving it alone. I worried that the torque I’d placed on it might be too much, and I’d

That little table on the left has a stack of wood under it, while that round, white container on the top right end is a 20pound stack of welding rod. The level on the right says that it’s low 0.5° on the right.

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come back the next morning to find it twisted the other way. It worked, though. It was at 0.4° the other way when I first checked it the next morning. Knocking out the blocks and taking the weight off let it snap back to 0.0°. The finishing touch was not anticipated; the gaps between the fuselage flange and the stabilizer disappeared under the pressure of the straps and the heat gun! Just add the required fiberglass strips, and it’s done.

Air in the Wing, Not Under It

With epoxy-flox in the joint and sheet-metal screws holding it down, the straps and clamps are there to get that last bit of shape conformance.

With all that torque, it’s time for the heat gun to let the entire tail cone twist the other way.

The next repair required the direct involvement of the manufacturer. The left wing showed what looked like a delamination in the upper surface. Unlike a tube-and-fabric aircraft, the wingskin carries a significant portion of the load, so an obvious defect like this is not something to ignore. Now, this is not a panic situation, but it’s not one to just paint over either. A careful examination top and bottom on both wings showed a similar blemish on the lower surface of the right wing. After a bit more data gathering, it appeared that what we had was a defect in the mold that caused a dry spot. We probably could have ignored it but preferred not to. It should be stressed at this point that this is not something to undertake without direct involvement from the manufacturer. It may belabor the obvious, but a misdiagnosis of the situation could lead to a solution that is not effective, so it is imperative that an expert evaluation be made to assure that this is the proper fi x for your flaw. But how to repair it? The concept was fairly simple: Put epoxy where it should be. Getting the epoxy below the surface without tearing up the whole wing was the problem. However, just as your doctor doesn’t have to saw off a piece of your arm to give you a medication, the fi x could be injected. And that’s what we decided to do. A visit It worked! There are no weights or supports other than the epoxy, and all of the surfaces are cool. Just glass over the interface and move on.

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Composites, Part 10 continued

To get epoxy into the area, we have to first get access to it by piercing the skin with a small punch.

to our local veterinarian supply store was necessary to obtain a syringe and a large-gauge needle. Fiberglass covered in a gelcoat, however, is a bit tougher than a horse’s hide, so another method was needed to get entry to the zone to be treated. A very small drill bit was considered, but in that we were aiming for a midlayer, sufficient depth-control made that a dicey proposition. But an awl, or in our case, a trammel-point, was just right for punching through the first layer while giving a feel for the cavity below. At first we tried a single injection point. That was not terribly successful, as it was not possible to determine if we were spreading the epoxy laterally. A lot of holes would let us keep trying until we found the void. The extra holes had the additional benefit that they not only allowed air to escape, they also bled epoxy, showing that the entire area was being filled. Frequent wiping to show the extent of coverage left us feeling pretty confident that we’d really solved the problem. All that was needed now was to finish the area as part of the pre-paint operations. Now before any of you aluminum builders get to smugly thinking that this sort of thing doesn’t happen to metal airplanes, I’d like to point out that when I started my RV-6, I was cautioned that first-time builders usually built between 1.2 and 1.5 airplanes. The stuff to the right of the decimal place was in a pile in the corner. I was right on target, too. But that’s another saga.  52

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Under thumb pressure, the area is distorted and wrinkled. This indicates a delamination, which, in a load-bearing area, is not desirable.

A large hypodermic needle and syringe were obtained from a veterinary supply house. The trick is to inject epoxy and see it come out the adjacent holes.

All that’s needed now is some finishing work with Bondo and some sanding.

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A plane flown as much as Stimple’s gets its dings. A proper paint job, though, makes repair a lot easier.

P 11 Part 11: Painting P i i the h bbeast. UUnderstand d d the h subtleties b l i off methods hd and materials before you try them on the real thing.

BY BOB FRITZ

“Paint your vehicle with thinned Rustoleum and a roller!” Well, that’s one bit of advice you’ll find on the web. At the other end of the spectrum is sending your several years’ worth of work to a distant shop, along with the deed to the house and your first-born child. How about a middle road? Say, do it yourself, but not with a roller? But not with a paint booth, either! You can do it. It’s not a slam dunk and will take a lot of work, but the satisfaction of getting a pretty fair job is enormous. Pride of parenthood covers a multitude of misgivings. As the conclusion of our “Build Your Skills: Composites” series, this installment will help you make the final preparations for painting and, should you so choose, give you a basic grounding in the painting process itself. A word (or more) of warning: Painting is not inherently difficult but can be unforgiving. Take your time, learn all you can, and practice on small components or test articles before pointing your gun at your airplane. Photos: Bob Fritz

Justification Should you go for it? Well, the price of having a $40,000 airplane painted is about the same as having it done to a $400,000 airplane—typically $5000 to $9000 depending on location, shop experience and quality, and the amount of prep work left to be done. Is it a good investment? For the $40,000 airplane you’ll spend 12% to 15% of the value of the airplane; for the $400,000 aircraft, less than 2%. The more expensive airplane will see the investment paid back when it comes time to sell. Regardless, many builders elect to paint on their own, for reasons of pride or economics. Before we go any further, you need to check your local environmental KITPLANES March 2008

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Composites, Part 11 continued regulations regarding spray-painting at home. If you live in a condo, be aware that, even if not harmful, some of these concoctions lack only Macbeth’s three witches for being an evil-smelling brew. Painting at home is often not an option.

Tapping the Source If you want an expert opinion, go see an expert, right? Joe Fields of Fields Aviation started by saying, “Preparation is everything. Shooting paint takes a bit of talent, but it’s not where the work is.” He pointed out an example of an aluminum aircraft with a beautiful paint job that, upon closer inspection, showed corrosion starting to lift the paint in spots. “Bad prep; you don’t have that problem with glass airplanes,” he said. “You have others.” What others? Why, pinholes of course. Jerry Stimple owned the premier paint supply shop in the San Francisco area for 30 years and finally retired. He is a guy of unbounded generosity when it comes to helping his friends paint their “precious.” He’s done at least four in just the past couple of years and is able to draw upon all of the big paint manufacturers for the latest products. He’s not wedded to any one process or product.

Preparation OK, back to the painting. In a previous article on composites I addressed pinholes, but in my research on this stage of the process, I kept being dragged back to the little monsters, so let’s have another go at them. Fields and Stimple both went to great

Here, a black guide coat has been applied so that when the next round of sanding begins all the high and low spots in the composite shell will be apparent.

Your Editor-in-Chief, younger and dumber. At the time, circa 1992, SuperFil was the new thing for filling pinholes in composite skins. He didn’t know to apply it thin, so an incredible amount of sanding was to follow.

lengths to give different approaches to the problem, so we’ll reiterate. Fields’ recipe is to sand down to smooth “…but not through the gel coat! It’ll only create more pinholes, and the fabric has gaps between the weave even if it’s resinsoaked, so the finished paint will show a pattern.” Stimple agreed, and joked that it would have more pinholes than the carpet in a tailor’s fitting room. Both had an answer. Fields likes to start with a light scuffing with 409 cleanser and Scotch-Brite, followed by rinsing with clear water…at least 10 times. He emphasized that you should use the original, purple-label 409, not the later versions with aroma additives. The rinse is needed to get the 409 off, so don’t make it harder by adding Divine Springtime to the mix. Simple Green–

Aircraft was also high on his list of acceptable cleaners. Stimple prefers a tiny squeeze of dishwashing soap in a gallon of water, followed by a rinse mixture of 10% rubbing alcohol/90% water. He emphasized that a tiny squeeze is all that’s needed for the same reasons Fields gave: You have to get it off. The alcohol/water helps, but don’t make it more difficult with too much soap. They both agreed that automotive wax and grease removers (not to be confused with engine degreaser) are great on aluminum, but fiberglass tends to soak up the stuff and leave a film; before going this route make sure that whatever you choose leaves absolutely no residue. If you’re curious, put a couple of drops on a piece of window glass to see if it leaves a film when it evaporates. If you want to be a bit more aggressive, “Abrade the surface with a dual-action sander pushing 150-grit paper,” Stimple added.

The Shop Air System Fields recommended finishing the cleaning process off with a high-pressure shot of air straight down on to the surface to blow off the dust. That brought up the subject of the air system, so let’s digress for a moment into preparing the shop. Even if you don’t plan on shooting the paint yourself, if you have the air system wrong, you’ll set the stage for disaster At some point, your project is going to look this bad. The motley appearance is from various coats of filler, guide coat and more filler having been sanded away to achieve a smooth finish.

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Composites, Part 11 continued when the color can is opened. (See the sidebar, “Compressor Come Clean,” Page 49.) With the hardware fi xed, let’s get back to the airplane.

Filling the Pinholes You’ve washed the surfaces, and now you have to dry them. Heat lamps are OK, but that big heat lamp in the sky is better, especially if you’re painting parts rather than an assembled airplane. However you do it, be sure to do it, or you’ll trap moisture under the next coatings. “Paint on a glass airplane sticks by mechanically bonding with the surface, so scuff it and make sure it’s clean and dry,” sayeth Fields. He also advises using the product type that was used to build the airframe. “If the airplane is epoxy, don’t use polyester-based products.” Stimple expanded on that idea and urged builders to, “Get to know your local paint supplier and rely on their expertise. Then buy the products of a single manufacturer; don’t mix them.” Among his product recommendations is Split-Second Rapid Sanding Glazing Putty from US Chemical. “It’s so lightweight you’ll think the can is empty.” In my previous article on fi lling pinholes, my sources were adamant that pinhole-filler should be applied as thinly as

possible, but Stimple said unequivocally that Split-Second could be slathered on up to 1/16 inch thick and then sanded to smooth out ridges as well as fill pinholes. The other sources didn’t mention this product, but both groups got fabulous results, so it must be the product. Once it sets, you can sand it down followed by a wipe with a clean rag lightly soaked with alcohol. The alcohol evaporates, hastening the drying time. Fields emphasized that whatever product you use, it should be both lightweight and flexible. “I’ve seen a lot of airplanes that looked great until after a few flights caused hairline cracks at the wingroot,” he said. “In fact, I prefer to leave a floating gap like a freeway expansion joint.” In the previous installment, we spoke of a guide-coat, and both Stimple and Fields agreed that it was a good idea, so I’ll reprise it here. When you think you have a smooth surface, clean it and spray it with a high-contrast paint. A rattle-can is fine. Then, sand that off using a long board to expose low spots. Don’t use a short block or, worse still, your hand to back up the paper. Either of those will just follow the curves. You may have to do this a couple of times to get that jaw-dropping, mirror-flat finish. Skip this step, and you’ll have an adequate, but not spectacular, result.

The left side of this Pulsar’s fuselage has had a thick primer/surfacer rolled on; the right side shows the first attempts to sand and smooth the filler.

Lighting Here’s an area where most of us go wrong: We add a bunch of shop lights up high over the project and consider it adequate. Not according to Fields. He strongly recommends that you set a 4-foot fluorescent lamp on a tripod and position it at the height of the surface you’re working on. At the other end you should have a dark background. The combination, he says, makes discontinuities show up faster than the end of a three-day vacation.

Sandpaper All sandpaper is not alike. The cheap stuff is likely to have varying sizes of par-

A proper regulator-particulate filter-dryer is not cheap, but a cheap one is more expensive. Don’t forget to use the drain every week or so.

ticles, and you’ll only create scratches while trying to eliminate them. Buy it from your local automotive paint store, not the hardware store; hardware customers are sanding on wood and laying down heavy coats with a brush.

The Spray Gun If you’re like me, you’re hoping that a

Here’s a cheater valve. It’s useful for knowing the air pressure at the spray gun, but don’t use it to adjust line pressure; set that at the regulator.

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KITPLANES March 2008

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This gun from Binks is the gold standard in HVLP. That black cylinder at the base is a line filter/insurance.

better gun will offset the need to sand, sand, sand. It won’t. But with that desire in mind, I’ve saved the spray gun for last. You’ve cleaned up the air delivery system, so let’s talk about the business end. Now hear this! A compressor, 50 feet of ¼ inch line and a $25 spray gun is not adequate. If you can’t or won’t build a good air system, stop right now and send your project out to be painted. Let’s take a look at spray guns and work back toward the compressor. There are lots of ways of spraying paint onto your airplane, but that airless sprayer you used on the house is not one of them. The three most common ways of spraying paint are high-pressure guns, turbine systems and HVLP guns. High-pressure guns. These are the cheapies that operate at about 45 to 60 psi and 8 cfm. Paint has become expensive, and the EPA put a halt to some of the really nasty chemistries—especially to the quantity of solvents that were being dispersed—so these guns are relegated to water-based paints. That led to turbine guns. These use a small turbine blowing warm air through a large diameter hose to generate 3 to 8 psi and 100 cfm. The first versions of these were built using vacuum cleaner components, and they worked well. The low pressure means that they gently lay the paint down instead of blasting it all over the shop, so, with practice, you can get as much as 85% transfer efficiency. With paint costing $200 to $400 per gallon, that’s an important number. To learn more about these, visit www.fujispray.com/what_is_hvlp.html or www.

turbinaire.com. Those vacuum cleaner parts led the high-pressure gun guys to figure out how to do it without the turbine. Although a turbine and an HVLP system work to produce a gentle spray, they have different approaches and advantages. The turbine doesn’t overly compress and then expand the air, hence there’s no entrained condensation, and what little heating there is tends to aid the atomization of the paint. Forget about entrained oil. Point scored for turbines. Getting set up with HVLP is cheaper, though, assuming you already have an adequate compressor with dryers and traps for your other air tools, all you

Compressor Come Clean Do not spray your work with anything other than clean, dry air. What’s clean? The compressor uses oil, and some of it will become entrained in the compressed air. If you use air tools, you might also have an in-line oiler. That means you’re spraying vaporized oil all over the surface you just washed. Not good. Even if you don’t have an in-line oiler, the pistons within the compressor may be lubricated with oil from the sump, and some will get into the lines. The solution is to get the oil out of the compressed air by, at the minimum, putting a tap upstream of the oiler and installing a filter on the new branch, then using hoses that are dedicated to spraying. The inside of those other hoses is coated with oil, so don’t use them for anything other than air tools that require oil. Now add in the fact that air gets hot when it’s compressed and, when it cools, the water vapor in it condenses into liquid. Don’t believe it? Go open the drain at the bottom of the tank, but put a pan under it first. You can get the water out by running a hose up the

need is 3/8 inch ID or larger air hose and a gun. A turbine system goes for about $800, while a top quality HVLP gun is less than half that. Point for HVLP. Those $50 HVLP guns? Stimple won’t touch them. Visit www.binks.com to see the gold standard in this area. By the way, you should have those dryers and traps even if you don’t have HVLP. How do the experts vote? Fields likes his turbine, and Stimple likes his HVLP. Stimple then tackled the setup of the HVLP and cleared up some misconceptions. HVLP does not mean low-pressure air is supplied to the gun. “The gun itself reduces the pressure,” he said. “You need about 45 psi at the gun, and the line

wall in a zig-zag pattern that has every zig and zag going downhill, back to the tank. The air will cool and, when it’s not flowing, the condensation will drain back to the tank where you can dump it.That’s just a start, though. That filter we put in the branch line will also remove water as well as oil and particulate. It’s not rocket science, but if you want to do it right, go to your local auto-paint supply store. You can frequently get a diagram of a good system along with the parts to build it. I’ll slip my two cents in here, and, although I’ll get a lot of “I did it and it works fine” letters, I’d advise against using PVC for the distribution lines. My shop was equipped with 2-inch PVC all around the ceiling with eight drops to plug-in fittings. It had more leaks than a Congressional committee in an election year, so the compressor kept coming on at night. One of those leaky joints finally failed with a thunderous explosion that sent large shards of sharp PVC all over the shop. I shudder to think of the consequences had I been nearby instead of in the hangar. Now you have clean, dry air ready for the final hose to the sprayer. Quarterinch inner diameter hose is cheap , but it has too much of a pressure drop; use 3/ inch or larger. —B.F. 8

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Composites, Part 11 continued loss is about 1 psi per 10 feet of 3/8 inch hose. So, for a 50-foot hose, crank up the pressure to 50 psi and let the gun do the conversion from pressure to volume.” He also described a “cheater,” a small gauge with a valve that can be attached to the base of the gun. “Don’t bother with them,” he said. “They reduce the pressure but don’t regulate it, so as soon as you let off the trigger the line pressure goes back up to tank pressure. Just set it at the tank where you’ve got a true regulator.” Of course, a portable compressor used to drive a nail gun won’t keep up with an HVLP gun, so when you’re shopping for a gun, be sure to check that requirement. As for gun configuration, there are some with one-quart cups on top, with a cup on bottom, and others with a siphon tube leading to a separate paint container. Stimple says that a top cup has the advantage of gravity feed and may, therefore, require less pressure and use 100% of the paint. If you have a gravity feed cup, shooting upward onto the bottom of the wing is nigh-on impossible. 3M came up with a dandy accessory, though. It’s a bag that, once filled with paint, fits in the bucket. You invert the gun so that all the air in the bag is blown out; then it matters not what angle you’re shooting. The turbine gun is a bit different. Read the manual, plug it into 110 VAC, and go spray.

Protective Clothing The EPA may have gotten rid of the worst of the concoctions, but you should still not go cheap on a respirator. Here’s one good reason to get to know your paint supplier. If he/she advises a full suit and external air, either buy it or change paints. You don’t want the paint job to last longer than you do.

Let’s Start Painting: The Primer Two-part primers were recommended by both of our experts. Better adhesion with a smoother surface was the advantage. But check with the manufacturer of the final paint system to ensure compatibility. Sand the surface lightly with 180 grit, blow it off and then shoot it again. Then give it a light sanding with 400 grit and water, and let it dry. Take your time here. Be sure there are no visible flaws after the final surface sanding or they’ll show through the final coat of gloss paint. When in doubt, sand smooth and re-prime.

Finish Coat Paint schemes and colors are totally subjective, except that most composite-kit manufacturers specify white (or at least light colored) hues to reduce temperature buildup in direct sunlight. Moreover, our experts said to avoid metal-flake paints; many of them contain copious quantities of aluminum that will interfere with internal antennas. Fields advises that, if possible, paint the wings separately from the fuselage. Two good reasons include the obvious fact that doing it that way is much easier than shooting straight up to the bottom of a wing; it’s also easier to control overspray. But if you can’t, cover the wings with bed sheets and spray the fuselage from nose to tail. Before you do that, look for obvious start/stop points such as seams, and use a fine-line tape to tape off what you’re not painting. When that area has set up in 24 hours or so, you can switch over to painting what’s still bare. “Don’t spray over the overspray!” Fields warns. “Pass your clean hand over a sur-

Stimple’s Lancair is no hangar queen. It’s a working plane, flown on a 2-hour daily commute, and with four transcontinental trips. The paint has to hold up.

face and if it makes a rough noise, use a fine Scotch-Brite pad to smooth it.” Layout of a multi-color system has its own perils. Stimple advises that if you can get your hands on one of those old overhead projectors, put a half-black sheet on it and you’ll get a perfectly straight line on the side of the fuselage. The paint job will also require a clearcoat to blend the ridges at the color interfaces. Therefore, each color should be thin, and that requires a very good base or any less-than-perfect sanding will show through. Now, here’s the crazy part: These twopart paints are made to be buffed to a glass-like smooth surface, but to keep the weight down we have to keep the thickness down. When we put our final clear coat on and buff it down to level, the scratches in the color show through. That sounds like a fool’s errand, but Stimple’s Lancair is a four-color job with a clear coat that is unbelievably flat and glossy. “It’s all in the sanding” he says. “If that’s smooth, the colors will be smooth.” He also unequivocally recommends the 3M product line. “You can’t get in trouble,” he says. (I’m going to test that bet and will report the result.) Fields’ favorite supplier is Poly-Fiber. The color coat, if it’s the final one, and the clear coat require buffing. But don’t use anything larger than a 7-inch wool or foam pad at 1200 rpm. “Do not use a 90° grinder with a pad; you’ll burn the paint!” Stimple says. One last tip: You could wet the floor down to suppress dust, but a better idea is to use SherwinWilliams Dust Free #55. Spray the walls and floor, and it will keep down the dust by adhesion and electrostatic attraction.

Joe Fields stands by, both figuratively and literally, his turbine gun—no water vapor, no oil, just clean, warm air and even a fresh-air source if you use a fully enclosed paint suit.

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Painting Outside Stimple has done several aircraft right out between the hangars and within 100 yards of a freeway with astonishing results. “First, understand that dirt is unavoidable and that these new paints are, as I’ve said, designed to be buffed,” he explained. “Still, you should pick a calm-air day, early in the morning, and with a heavy dew fall to suppress the dust. 70° F is ideal, but if it’s cold or windy, forget it.” Incredulity alternates with skepticism until one sees the result. Then astonishment kicks in.

Vinyl Striping If you’re going for a bit less complex process, Stimple highly endorses the use of vinyl stripes and overlays. These allow you to paint one or two colors that cover the entire aircraft, and then add the accent color where you want it. They are usually computer-cut, so almost any design is doable. Those jazzy wavelike patterns on a Cessna 172 are a good example. “Just make sure the surface is smooth, because the tape will magnify every bump,” he says. (Applying these is a topic in itself that I’ll cover when I get to that stage with my own project.) Is that all you need to know? Although there are books on the subject, both large and small, some near to “Dummies Guide” level and many that are cures for insomnia, there’s no substitute for practice and patience in doing the prep work. As a Lancair owner said, “Everyone has a fi xed quantity of sanding strokes built into them. When you reach your quantity, you’re done.” So procrastinate later. Get a big box of sandpaper, a good spray gun, and remember the words of Dizzy Dean: “It ain’t bragging if you done it.” 

Clear leading-edge tape is applied as protection against bugs, rocks and other sources of dings. It’s replaceable, too. KITPLANES March 2008

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