Nuts & Bolts

Building Basics MODERN COMPOSITE COMPONENTS, such as wing skins, are often created in massive molds. And the challenge in molding parts has always been keeping each part's construction consistent so that each one is identical, not only in dimension, but also in weight and mass distribution. The mold helps make the part accurate as far as the dimensions are concerned, but how do you make the parts that weigh the same? This question has vexed designers and builders for decades, including the Loughead brothers, Jack Northrop, and Tony Stadleman as they designed and built the Loughead S-l, and later the Lockheed Vega. Wood is nature's composite material, and it was easy for these builders and designers to determine how big the wood (or in modern times, the

CAUL (OPTIONAL) BLEEDER PLIES

RESIN 6 MICRO SPHERE MIXTURE

96

DECEMBER 2000

Bag It

mold and held it in place with a cover. When they inflated the air bag, it exerted a constant pressure over the entire Molding a composite structure interior surface of the fuselage-to-be, and the pressure H.G. FRAUTSCHY forced the glue and wood together tightly, squeezing out composite fabric) pieces had to be, any excess glue. but getting just the right amount of Composite structural manufacturglue or resin mixed in with it was an- ers widely use an adaptation of this other matter. Too little, and the piece process today. Instead of pressurizing would have substandard strength; a bag against a mold (still a viable too much, and the airplane would process), they evacuate the air from a weigh more than it had to. bag that is securely taped around the For the Loughead brothers (and perimeter of the part. modern builders), the answer surToday's manufacturers make a ferounds us all—air, or more accu- male mold with an outside perimeter rately, air pressure. For the S-l and large enough to adhere a sealant tape Vega, the Lougheads lined a massive to the mold. After applying a coating concrete female mold for the fuse- or a release film in the mold so the lage with a wood/adhesive matrix. finished part doesn't stick to it, they Then they lowered a flexible lay in a peel ply (whose texture deGoodyear rubber air bag into the termines the part's surface finish) and the laminate fabric and resin composite. Over this they add another layer of release film, which may or may not be porous, a breather/bleeder ply, and, finally, the "bag" itself. "Bag" is a bit misleading because it doesn't completely envelope the part being produced in the same material; it is sealed around the edges of the top film of plastic baggingtype material. For some applications the airtight film is made from nylon or Kapton film, particularly if the vacuum-bag-

ging process is combined with an autoclave to control the resin's cure rate. After installing and sealing an air fitting to the bag, a vacuum pump sucks the trapped air, and perhaps a bit of excess resin, out of the bag. Inside the bag the breather/bleeder ply allows the air to be evenly evacuated from the entire piece. The vacuum, combined with the surrounding atmosphere pressure, forces the composite material tightly together against the mold surface, allowing the manufacturer to tightly control the amount of resin used as the part is made. While more expensive than laying up a part by hand, vacuum bagging produces accurately made components. At first glance, the process may seem daunting to homebuilders, but never fear, for you're a resourceful bunch. As Ben Owen wrote in his May 1988 "Craftsman's Corner" in Sport Aviation, builder Larry Fitzgerald of Bristol, Wisconsin, created an inexpensive alternative vacuum-bagging procedure. You can read Larry's process by going to the Homebuilder's HQ section of the EAA website at www.eaa.org and clicking first on the Building

Polystyrene foam melts in the presence of gasoline, but it's excellent for combining with epoxy-type resins. To add to the sometimes confusing mix of foams, each is available in a variety of densities, ranging from 2 to 16 pounds per cubic foot. The higher density (weight) foams resist compression better, but at a cost of greater weight.

Fill'er Up

Builders use other materials to complete the fiberglass/resin structure. When combined with epoxy resin, flocked cotton fiber becomes "Flox." Because it's reasonably light, Flox is a good material for reinforcing a sharp corner on a foam/fiberglass part. Builders can use milled glass fibers in a similar fashion, but the glass fibers' small size, sharpness, and stiffness can easily penetrate the skin if mishandled. Glass bubbles, or "microspheres," are another common filler. When combined with resin, these microscopic glass bubbles create a lightweight filler material you can shape by sanding or filing. When combined with epoxy, it makes an excellent adheader, then Composite, and then Vac- hesive matrix to join foam blocks. "Cab-O-Sil" is another term often uum Bagging Fiberglass Components. heard around composite workshops. Builders use this filler material to Foam T Popular since the early 1970s, thicken epoxies before curing, makmoldless composite construction ing it an excellent choice for resin methods rely heavily on a variety of that must be used on sloping or verfoam plastics. Foams created out of tical surfaces. For example, the winpolystyrene, polyvinyl chloride dows on the GlaStar are installed (PVC), and polyurethane are the most using epoxy thickened with Cab-Ocommon, and they are often supplied Sil and colored with black powdered as a slab that builders cut with a hot dye. Depending on the amount of wire and shape with hand and power f i l l e r added, the thickened epoxy tools. Wanting: Do not cut polyurethane won't run or sag compared to norand vinyl-based foams with a hot wire mal viscosity epoxy. Other fillers used in the final finbecause this releases poisonous cyanide gases. In any case, the irritable vapors ishing of structures include sprayable resulting from any hot wire foam cut- fillers, used to fill minor pinholes ting should be avoided. and surface imperfections. Different foams are used in particular applications. Polyurethane foam Final Finish The final finish of composites can works well for the walls of a fuel cell because it resists petroleum products. range from a simple gel-coat applied

to the inside of a female mold to which you lay up the laminate to super slick, multiple coat polyurethane paints applied to sanded and primed fiberglass/resin structures. Finishing systems by Poly-Fiber and others are designed to fill in minor surface imperfections, prime the smooth surface, and protect the structure from sunlight. Composite finishes do have a common theme—white is the predominate color. Some composite structures are susceptible to softening at higher temperatures. Because darker colors absorb the sun's heat, the exposure to sunlight can elevate temperatures inside the structure to more than 200°F. There's plenty more to read and learn about building composite aircraft. I'd suggest starting with Ron Alexander's two-part EAA Sport Aviation article, Building Composite Aircraft, in October and November 1997, and you can find the entire series of composite construction articles in the Homebuilder's HQ on the EAA website at www.rart.or,?. £*£ Homebuilder's HQ, in the members-only section of the EAA website at www.eaa.org. Composite Construction For Homebuilt Aircraft, Jack Lambie, Second Edition, revised in 2000, $19.95—21-2849 Advisory Circular 43.13-1, Acceptable Methods, Techniques and Practices—Aircraft Inspection and Repair. $18.95—F00191

Aircraft Spruce & Specialty Company catalog— 800-824-1930

References For more information on using composites, check these sources, some of which are available on the EAA website at www.eaa.org or by calling 800564-6322. Sport Aviation

97