Recently introduced into the production market are two composite-construction high-performance singles, the Cirrus SR20 and Lancair Columbia. And Raytheon recently flew for the first time a small corporate jet, the Premier I, which has a composite fuselage.
Composite materials generally are comprised of different types of synthetic cloth-like material mixed with a hardening liquid that serves as a bonding agent and filler. They offer some definite advantages over sheet metal and rivets. Composites are easier to form into complex shapes than sheet metal. The cloth laminate is saturated with liquid resin, placed in or fitted to a mold, then cured. Because the material is extremely flexible before it is cured, parts can be made in any shape.
Components made from composite materials can also be much lighter than metal of the same strength, and much stronger than metal of the same weight. For example, DuPont?s composite material, known as Kevlar, which is commonly used in bulletproof vests, has four times the tensile strength of aluminum of equal weight?260,000 pounds per square inch. Of course, the material used in producing Kevlar is more expensive than aluminum, but the manufacturing costs for Kevlar products can be lower once the equipment is in place.
Homebuilders have enjoyed excellent performance from composite materials. But homebuilders rarely use expensive molds to form their composite aircraft structures. Since they generally produce only one aircraft of a given type, they don?t need to worry much about whether parts on their aircraft are interchangeable with those on another aircraft of the same model.
The same can?t be said of production aircraft using composite parts. These parts are made from precision molds because the FAA requires that manufacturers maintain strict quality control so that every part produced from a given mold is completely interchangeable. Composite parts for production aircraft also must be thoroughly tested for such things as tolerance for extreme temperatures, vibration, fatigue, ultraviolet light, and lightning effects.
Lightning protection is a special concern with composite structures since they do not conduct electrical energy well. When a metallic aircraft is struck by lightning, the metal airframe efficiently conducts the electrical energy to static wicks, which dissipate it into the atmosphere. In airplanes without wicks, the electrical energy dissipates directly from the airframe to the atmosphere.
When an unprotected composite structure is hit by lightning, the electrical resistance of the material creates extreme heat, melting and evaporating the resin. (High ambient temperatures can potentially weaken composite structures, so most composite aircraft are painted white to reflect the sun and keep skin temperatures down.) Composite aircraft can be protected from lightning-strike damage by embedding metal rods, wires, screen, or foil in the composite material during the construction process. The metal in the material conducts the energy and dissipates it to the atmosphere.
Another significant difference between composite-construction homebuilts and certified aircraft is the type of resin and the process used. Most homebuilt composite structures use the cold lay-up process. The layers of cloth laminate are saturated with resin, formed to the desired shape, and then allowed to cure at room temperature. This process is very simple and inexpensive.
Certified composite products are generally cured at elevated temperatures and pressures. This technique has the advantage of producing material that is stronger, more stable at extreme temperatures, and more resistant to the damaging effects of ultraviolet light and lightning strikes. Heat-cured composites, which use different resins than cold-cure products, are also much more resistant to fuel, oil, grease, hydraulic fluid, and other fluids used in aircraft.
In one common commercial manufacturing process, called compression molding, a hollow mold is made, the mold surfaces are covered with a release wax, and the saturated laminate layers are placed in the mold. The mold is then closed so that the layers can be heated under pressure during the curing process. The airframe of the Beech Starship, the first corporate aircraft made from composite materials, is produced in an autoclave. This device basically is a pressurized oven large enough to contain the entire component. (The Starship wing is a single component, and must fit in the autoclave.) This technique produces parts that are uniform enough to be completely interchangeable.
Another process, called filament winding, is commonly used to make propeller blades. (The Premier I fuselage is also made by this process.) A continuous filament of graphite or boron coated with resin is wound around a shaft and contoured to the desired shape. The resulting parts withstand vibration extremely well but can be difficult to repair.
Another type of composite is used to make floors and bulkheads. The inner part of the material, which resembles honeycomb, can be made from aluminum, steel, fiberglass, carbon, Kevlar, or virtually any material depending on the properties required. The honeycomb core is then covered with layers of laminate, saturated with resin, and cured. The resulting structures have excellent compression strength and are very light.
The most commonly used material for composite laminate is fiberglass cloth. The two most common types of glass cloth are E glass and S glass. E glass is made of borosilicate glass filaments woven together. S glass is made of magnesia-alumina-silicate glass filaments woven together and has a very high tensile strength when cured. Because the two types of fiberglass have different types of strength, they are often combined to form a single part. This is different from the silica-based fiberglass commonly used in auto body and boat repair. That material is heavy and brittle, but inexpensive. It is sometimes used to make non-structural cowlings or fairings.
Fiberglass is by no means the only material used to make composite laminates. Kevlar can be used to create very strong, lightweight, flexible, and vibration-absorbent material. Graphite may be used to create strong, rigid composites for propeller blades. Ceramic-based composites, which can withstand temperatures of 2,500 degrees Fahrenheit, are used for aircraft firewalls, turbine engine compartment liners, and the tiles that cover the exterior of the space shuttle.
The orientation of the fibers in a composite part helps determine the strength and stiffness of the end product. Woven fabrics have strands running in two directions. The primary direction, or length, is called warp, and the direction oriented 90 degrees to the warp is called weft or fill.
Other materials are unidirectional, with virtually all of the filaments running in only one direction, or bi-directional, where the number of filaments in warp and weft are the same. Materials made of chopped filaments compressed together like particle board are called mats.
Maintenance shops must resort to different techniques to inspect composite materials because, in general, they are opaque to light, non-magnetic, and do not conduct electricity. The simplest test is a tap test. A light metal object such as a quarter is tapped along a composite part. There is a noticeably hollow sound if the part has delaminated, which means that some of the layers of cloth have separated or a resin void has created a weak area. Ultrasound, thermographs, and radiographs also may be used to test composite structures.
As the FAA, manufacturers, and aviation consumers all gain more knowledge and experience in the composite field, these materials will become more commonplace as primary structural components in general aviation aircraft.
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