Bud li n i g a metal kit airplane is, in many ways, simpler than building a tube and fabric airplane, a wood and fabric job, or, for that matter, a composite. This is because fewer and less demanding building skills are required of a metal kit builder. Here's the way I see it. How difficult a project is to build is really determined by the number of skills needed to construct the aircraft's basic structure. Of course, the complexity of the project is also a factor. Perhaps the most difficult aircraft to construct are the traditional steel tube, wood, and fabric designs. These projects require knowledge and skills (not easily acquired) in at least three basic fields of construction . . . welding, fabric work, and wood work. Even though only one broad skill area predominates in composite projects, the procedural and application methods are quite demanding and critical. And then, there is the ever present health hazard attendant with the use of epoxies. But, to me, the least appealing aspect of a composite project are the work intensive "fill and sand" chores that can tax even the most dedicated builder's patience and endurance. A metal aircraft kit project, on the other hand, requires the development of only one skill area. This basic metal working skill is easily acquired by most any builder because the work consists primarily of drilling holes and driving rivets. Incidentally, almost all projects, regardless of their type of construction, be it steel tube, fabric, wood or composite, require some metal working skill. The construction and installation of engine baffles is a good example. Fabrication of fittings, brackets and the like is another. So, why not go metal all the way? Besides, the cost of kits for high performance metal aircraft are considerably less expensive than kits for composite aircraft of similar performance. And I do, indeed, recommend that a first time builder build from a kit rather than from scratch (raw 66

materials). A lot of frustration will then be avoided. A kit, ordinarily, contains all the materials, many pre-formed parts, and most of the hardware you will need. More importantly, most of the metal skins, ribs and bulkheads will be preformed or at least cut to dimension. All this, naturally, represents a considerable reduction in the total building time that would otherwise be required - another bonus. A kit will do much to bolster confi-

dence and minimize construction confusion and difficulties. Furthermore, building from a proven kit generally results in a safer airplane. SO, LET'S BEGIN

When you receive your kit, inventory the contents of the plywood cartons within the time specified by the manufacturer. To get the packing crates out of the way, disassemble them care-

and steel banding, for safety's sake. Save the plywood and wood for future use in making temporary storage shelves, templates, jigs, stands, etc. Unwrap the contents carefully . . . you don't want to lose any of the small pieces and parts. I suggest you put all the wrapping material in one pile at some distance away from those nice new parts you intend to keep. Check off everything against the packing list or material list (usually enclosed). Take your time, inventorying the kit contents can be educational and useful because you will have to locate and identify each of the many pieces that will ultimately become your airplane. Each piece of metal, each part, is usually marked so you can identify it. If it isn't, search through the plans until you locate and identify it. Then, mark it yourself with a waterproof marking pen (Sharpie, or something similar). Undoubtedly, you are anxious to get on with the actual assembly work - but don't get into too big a hurry. In other words, don't rush. Proceed by taking care of the first things first. In the end this will save you a lot of time. Here, for example, listed in logical sequence, are a number of often slighted chores you should take care of before you start your construction and assembly work. f uly and remove all nails, staples, DEBURR ALL METAL EDGES

Many builders question the need to deburr or chamfer the sharp metal edges on all the sheared metal skins and formed parts before they are really needed. However, I find it to be a lot easier and quicker to perform most preparatory, and like operations, at one time. Deburring drilled holes and deburring cut or sheared metal edges are good examples. Expect your aluminum skins and other formed parts to have sharp and rough edges because most of the parts were stamped out, or sheared, during manufacture. Those razor sharp edges can draw blood. The kit manufacturer certainly doesn't feel obligated to smooth the holes and cut edges for you as he would have to raise the cost of his kits. Besides, according to the rules governing amateur built airf67

the burrs or chips removed, and you try to do it right the second time. You might as well take the time to deburr the edges of all of the metal parts that came with the kit. This would include all of the preformed wing ribs, bulkheads, tail ribs, and skins. Figure 2 shows several deburring tools you can use for deburring holes. All of them work quite well any place access is unrestricted. In other less accessible places, however, you may find that one particular tool works best for you. In time, you will develop other tool preferences and the technique for getting a particular drilling or deburring job done. Later, when you start drilling holes for the assembly of metal skins and other components, you will learn that it is necessary to take each assembly apart after the drilling is completed in order to properly deburr the holes ... a pain in the neck but, oh, so important. ABOUT THOSE STEEL COMPONENTS

craft, c < you are c 1 supposed 6 1 >to dot651 %h of the work. You should be very much aware that burrs and sharp edges are stress inducing areas that can develop cracks. These could, in time, progress to the point where complete failure of the part can occur. Figure 1 shows how you can deburr your metal edges. There is another good reason for smoothing and chamfering the edges of aluminum skins and formed parts. Burrs and sharp edges are risky to handle because they can inflict serious cuts on your hands and fingers. While working on a piece of metal or a part, it is only natural that you will slide your hands across its surfaces and otherwise handle the metal as you measure, bend, or drill it. Not only that, you will from time to time subconsciously brush off f68

> e chips with e yourr hands and, l OUCH! . . . you guessed it, a painful cut from a metal burr that wouldn't let go. How smooth do the edges of a metal sheet need to be? Well, if after passing your fingers along the edges you draw blood, you didn't quite finish the job. Almost every hole drilled in metal will have small metal chips or burrs around its edges on both the top and bottom sides. Drilling a hole with a sharp drill bit produces a minimum of burrs but you can still expect to find some. All burrs must be removed prior to riveting or the burrs will keep mating parts from fitting flush. When this happens, a rivet installed in such a hole will not seat properly and will tend to swell up between the sheets when it is being set. Should that happen, the improp-

Steel parts such as the landing gear, engine mount, control stick assembly, rudder pedal assemblies and the like, will begin to rust almost as soon as they arrive. It is the wise builder who will immediately prime these assemblies to protect them from rusting. If you delay doing so until you need to install a particular component, you may be faced with the additional chore of laboriously removing a lot of accumulated rust y seated must and b e drilled out, before you rivets can prime paint the stuff. You might even have to have all of the components sandblasted . . . another time consuming expense. Bare steel parts must be treated with a good metal treatment system. How you do this will depend on the paint brand you prefer to use. Whichever it is, follow the manufacturer's instructions to the letter. For example, if you plan to use DuPont products, you can clean the steel parts with their enamel Reducer Cleaner 3812S or 3832S and spray on their VARIPRIME SelfEtching Primer. This should preserve the parts until you are ready to paint them. Of course, this is only one of the several systems DuPont has for treating various kinds of metal. Other companies have similar products.

EASILY, SO ...

Handle those large floppy skins carefully, they will pick up scratches in spite of all your efforts. Here's how it happens and what you can do about it: 1. Cutting an aluminum sheet on the bandsaw invariably results in some light scratches on the under surface as you slide the part through the saw. Be sure the saw table is free of chips and dirt before you start cutting. Putting masking tape strips across the bottom of the sheet along the cut line helps. 2. Sliding aluminum parts around on the workbench also can result in scratches because of the fallen chips and burrs produced by various trimming or drilling operations. A large piece of cardboard (mattress carton, etc.) laid across the workbench surface helps reduce scuffing. However, you must con- them out. stanty brush its surface clean of However, if these rings are not chips or your precautionary efforts cut out, riveting the wing skins will be wasted. could become an impossible task Some builders have had consid- because you wouldn't be able to erable success in reducing reach inside the wing with a buckaluminum skin damage by laying a ing bar to do the skin riveting. rubberized mesh across the workTo cut out the holes, use a heavy bench surface. This material is duty adjustable circle cutter or "flysimilar to the rubber mesh used cutter" (Circle Cutters .. . Sears has under throw rugs to keep them them). Clamp the rib securely to your from sliding. The stuff looks like hardware cloth but is made of rub- drill press and use a slow speed for ber or plastic. It will, I am told, cutting. Those fly cutters are danprotect an aluminum sheet from gerous to use so be careful! scratches because it will be eleI don't believe an adjustable fly vated above the stuff that cutter should be used in a hand accumulates on the bench. Don't drill for cutting out large holes as it write, I know of no source ... try is hard to keep the circle cutter Rubbermaid™ or your local carpet from jamming in the aluminum durdealer. ALUMINUM < c c1 SKINS 6 1> SCRATCH T 6h a ing > the t i scutting. good e enough areason t oc c u t 3. If you really intend to have a After the centers are all cut out, shiny polished aluminum airplane free of scratches, you might be motivated enough to keep brown wrapping paper taped under all your aluminum skins as you work on them. Otherwise, exercise reasonable care and don't worry about minor scuff marks. They will not show after the structure has been properly prepared, primed and painted . . .

smoothed. A good finishing tool for this is a small Scotch Brite wheel chucked in a drill press, preferably. STRAIGHTENING RIBS/BULKHEADS

The wing ribs as furnished in an all-metal kit, most likely, will have been stamped out of 2024-0 alclad aluminum in a hydraulic press. After forming, the ribs are placed in an oven and heat treated to a T-4 condition. This makes them stronger but also harder and more brittle. That being the case, it is very important that all edges be smoothly

chamfered and free of saw or file

marks lestshould they create a starting h hole b e deburred a n d

place for cracks.

so, don't panic.

CUTTING OUT LIGHTENING HOLES

Aluminum wing ribs furnished with a kit usually have several large reinforcing rings embossed in the rib metal. The centers of these embossed rings (lightening holes) may be cut out to save several pounds. f69

ment process the ribs often come out of flute crimps to a minimum neceswarped and bowed . . . especially in sary to straighten the rib. the flanged leading edge areas. You can lay a flexible straightThe most important rib prepara- edge on the drawn reference line to tion you can make is to remove as check its Straightness as you flute much of the distortion and bowing your way toward the nose of the rib. Unfortunately, the extreme foras you can. The bow or curvature in the wing ward portion of the rib may defy ribs can be completely straightened your straightening efforts so don't except, perhaps, near the leading get unduly upset when you learn that for yourself. Do the best you edges. Straight each rib with the help of can. Later, the rivet holes can be drilled offset a bit to avoid drilling fluting pliers. The commercial fluting pliers them too close to the edge of the form flutes that are quite wide. This rib flange. may cause a problem when close rivet spacing is called for. Many builders, therefore make their own fluting pliers by modifying a cheap pair of pliers. They simply Unfortunately, < c or 1 weld c 6 > sin pieces the 1t rheat aof treati6 g h t > e n t h eMrib. Keep o t h e number r braze short 3/16" rod to the jaws of the pliers. Short pieces cut from an AN3 bolt work quite well. It is essential that the rivet spacing be precisely established before crimping in the rib straightening flutes. In order to provide a good alignment reference for straightening the rib flanges, you should first draw a centerline along the flanges using a Sharpie pen or any suitable permanent marker. Drawing the reference line along the flange may be done free hand using your fingers as a spacing guide. Naturally the reference line will be curved because it follows the curvature of the rib. To straighten the rib, start the fluting at the rear end and work forward. Don't make the initial flute crimps too deep . . . only as necessary to f70

time, why not reread your manual? A Few Tool Sources Avery Enterprises, P.O. Box 387, Bedford, TX 76095, 817/267-9407.

Aircraft Spruce & Specialty, Box 424,

Fullerton, CA 92632, 1-

800/824-1930 (Order Hotline). Enco Mfg. Co., 5000 W. Bloomingdale, Chicago, IL 60639, 1 -800/860-3400.

St.,

Wicks Aircraft Supply, 410 Pine Highland, IL 62249, 1-800/221-

9425.

e next month. I n t h e mean-

Tony Bingelis

SPORTPLANE BUILDER

Part 2 Working With Rivet Holes It's true, anyone can drill a hole. However, not everyone can drill a nice round hole - in thin metal where it needs to be - and do it right every time. After all, with thousands of holes to be drilled in a typical all-metal aircraft project, the "opportunity" exists for some of those holes to be: • Drilled with the wrong size drill bit.

Your control (either air driven or electric) will very definitely have access limitations. For example, you will find that you

cannot drill closer than, maybe, 3/4" to a corner, or a perpendicular rib surface or bulkhead. At best, an ordinary electric drill will allow you to

get within 1" of a corner but certainly no more. In addition, access to the area where the hole is to be drilled must

be accessible enough to allow you

to maneuver and position the drill with the bit installed. Even with a short bit installed, this means you will need approximately 10" to 12". It is in a situation like this that a right angle drill becomes necessary for making those almost inaccessible areas accessible. You can count on i t . . . a desperate need for some sort of an angle drill will arise from time to time.

• Drilled off your center mark. • Drilled too close to the edge. • Drilled sloppily and elongated.

• Drilled in the wrong place. • Drilled O.K., but only after scratching the skin.

However, as if all that isn't bad enough - after the hole is drilled you can still create other problems by: • Forgetting to deburr the holes in both skins. • Forgetting to dimple the top

and/or bottom skin for flush riveting. • Making machine countersunk holes for flush rivets (where needed) too shallow or too deep. Read on to see how you can avoid

or at least limit your encounters with these, all too common, hole preparation problems.

UNIBITs STEP DRILL SEVERAL SIZES - HOLE DIAMETERS TO 3/4" (IN 1/16" INCREMENTS)

HIGH SPEED METAL

COUNTERSINK TOOL DIAMETERS 1/2" AND 3/4"

A Bit About Drills and Bits The type of drill bit you will proba-

bly use the most is the High Speed Drill (jobbers length) because of its

COUNTERSINK CUTTERS (DIAMETERS 3/8", 7/16", 1/2",3/4")

ready availability and low initial cost.

However, the tougher, more expensive, Cobalt bits should be used to drill hardened steel parts like landing gear legs and engine mounts. In addition, you should acquire a couple of 12" long (#30 and #40) drill bits for drilling in hard to reach places. Because these long bits can be easily flexed with your fingers they can be made to drill close to corners, and just about anywhere else that a 90 degree angle drill would otherwise have to be used. I also recommend the acquisition and use of a good lightweight air drill because it is less tiring to use than the average electric drill and has an easily teased trigger for precise speed control.

FOR INSTRUMENT PANEL CUT-OUTS2" DIA./2 1/4" DIA73 1/8" DI A.

METAL HOLE SAW (MANY SIZES

AVAILABLE)

FOR KEY IGNITION SWITCH HOLE - 7/8" DIA.

FIGURE 1. MAKING LARGE HOLES IN THIN METAL

SPORT AVIATION 65

find it to be more accurate for your own use. Although a punch mark indentation helps ensure that the drill will not stray off the mark, you can help,

too.

Hold the drill perpendicular to the surface, exert a little pressure,

and turn the drill chuck by hand a little before pulling the trigger. This will almost guarantee that the drill bit will stay accurately seated in the punch mark. Ignore this technique and you will risk the chance that your drill will take a walk and scoot off the marked

With several thousand rivet holes to be drilled in an average metal project, the opportunity exists to mis-drill a few holes . . . so, think twice before drilling once.

Even though you are building a metal airplane, you may have to drill holes through fiberglass fairings, cowlings, and other component parts. Drilling holes through fiberglass

hole's location. Be alert. Before you mark a rivet

location, assure yourself that there is no obstruction (or flute) in the underlying frame where you intend to drill a rivet hole. It is permissible to alter

dulls drills quickly. Expect this and count on resharpening or replacing the dulled bits before using them again In aluminum.

the hole spacing slightly to avoid such problem traps.

Before You Drill...

ery important hole. Better still, use an automatic center punch to pinpoint every rivet location. An automatic center punch is much easier to use and you may

When a line of rivet holes is to be drilled, a centerline is normally drawn first and then short cross marks are made to establish each

Use a center punch and a light hammer tap to pinpoint and slightly emboss the marked location for ev-

hole location. When this happens, the drill will leave an ugly scarred trail across that nice smooth metal surface ... what a shame. Here's a useful tip passed on from builder to builder. Lay masking tape over the route where your rivet layout and spacing is to be drawn on the aluminum skins. Instead, draw and mark the hole locations on the tape. Drill the holes through the tape - then remove the tape. Don't leave the masking tape on

more than a day or so because it becomes increasingly difficult to remove with the passage of time. But why use the tape in the first place? Well, the tape protects the metal from drill scratches and helps prevent the drill from walking.

Drill Bit Selection Choosing the correct size bit for the hole to be drilled is extremely important. Small drill sizes are very difficult to judge by eye, so use a metal drill gauge to remove all doubt. In the typical flush riveted metal project most of the rivets used will be either 3/32" (AN426-3) or 1/8" (AN426-4) in diameter. You cannot use a fractional size 3/32" drill bit for a 3/32" rivet... the hole will be too small and the rivet

simply will not go in. The same applies to the 1/8" rivet and a 1/8" hole. To accommodate these rivet sizes

you must use what are known as numbered drill bits. A #40 drill bit is used for 3/32" rivets, and a #30 drill bit is used with 1/8" rivets. Notice that the bigger numbered (#40) drill bit is for the smaller holes, and the smaller numbered (#30) drill bit is for the bigger 1/8" rivet holes. Who said anything has to be logical?

Drilling Techniques A long (12") drill bit can be flexed to work in close quarters almost as effectively as an angle drill. 66 FEBRUARY 1993

Drill aluminum, especially the thinner sheets, at a fairly high speed.

You can improve your hole drilling accuracy and quality by making it a practice to drill all your holes, initially with either a #41 or #40 drill bit. You will find that drilling all holes initially with a small bit first is quicker and helps maintain greater accuracy. Later, if necessary, different holes can be redrilled to whatever larger size is required. This is called "line drilling" and will ensure that the final hole alignment on assembly is exact. When you can redrill a small hole to a specified larger diameter, you may even be able to correct a slight hole misalignment... all the more reason to drill your holes undersized first. Drilling accurate round holes in thin gauge metal skins is difficult. Unless extra care is taken, you can end up with holes that tend to be irregular and oversized (often taking on a hexagon appearance) instead of neat and round like you want. If the metal thickness is less than half the diameter of the drill bit, the described difficulty is almost sure to occur. Improve the quality of the critical holes you have to drill in thin metal by clamping a scrap metal back-up block behind your work before you start drilling. This will allow the body of the bit to be in solid material at all times . . . before, during, and after penetration. The result - a nice round hole. When drilling holes in thicker aluminum parts, withdraw the drill bit frequently to clear the bit of chips. If you don't, the aluminum chips inside the hole will build up and bind, preventing you from withdrawing the bit. This is more likely to be a problem when drilling softer 6061-T6 bar stock than it is with the harder 2024T4 bar aluminum. When using a hand held electric, or an air drill, to drill deep holes in aluminum, take care when withdrawing the drill that you don't accidentally tilt it sideways. This would be bad because it will cause an elongation of the hole. If at all possible, do all your important drilling in a drill press. Never hold small parts in your fingers when drilling holes on a drill press. The drill bit will surely grab just as the bit exits the metal. Before you realize what is happening, the drill will lock (jam) in the metal and the part you are drilling will whip right out of your hands and spin wildly out of control.

No need to go into the gory de-

PRE-DRILL ALL RIVET HOLES IN RIBS FRAMES/BULKHEADS 3J INSTALL CLECOS AS HOLES ARE DRILLED

CLAMP

SKIN TO RIBS USE ANY KIND OF

CLAMPS

J^DRILL SKINS BY BACK-DRILLING THRU PREDRILLED RIVET HOLES.

SHOULDER OR C-CLAMP

FIGURE^.A GOOD SKIN DRILLING OPTION tails of what would happen to your hands and fingers if contacted by that spinning cleaver.

Making Large Holes in Sheet Metal Sometimes you will have to drill or make holes much larger than those needed for rivet installation.

For example, rudder control cables are usually routed through holes drilled in fuselage bulkheads. These holes must be large enough to allow the passage of the rudder control cables and their formed terminals. Because these holes may have to be 5/8" in diameter, or larger, a regular drill would do a shoddy job in the thin metal. SPORT AVIATION 67

Rivet Spacing and Edge Distances Drilling holes too close to the edges of the aluminum skins, or fittings for that matter, is not acceptable. How close is too close? The basic rule is that you do not drill rivet holes closer to the edge than two diameters of the rivet size used. For 1/8" rivets this would mean that the centerline of any hole would be at least 1/4" away from the edge of the skin (material). This is quite a critical factor as insufficient edge distance reduces the strength of the riveted joint. You should not reduce the edge distance from that An air drill drills faster, is lighter, and is less tiring to use in shown by the deawkward positions than the average electric drill. signer on the plans. _^_—__-