For many years, aircraft designers could propose theoretical designs that they could not build because the materials needed to construct them did not exist. (The term "unobtainium" is sometimes used to identify materials that are desired but not yet available.) For instance, large spaceplanes like the Space Shuttle would have proven extremely difficult, if not impossible, to build without heat-resistant ceramic tiles to protect them during reentry. And high-speed forward-swept-wing airplanes like Grumman's experimental X-29 or the Russian Sukhoi S-27 Berkut would not have been possible without the development of composite materials to keep their wings from bending out of shape.
Composites are the most important materials to be adapted for aviation since the use of aluminum in the 1920s. Composites are materials that are combinations of two or more organic or inorganic components. One material serves as a "matrix," which is the material that holds everything together, while the other material serves as a reinforcement, in the form of fibers embedded in the matrix. Until recently, the most common matrix materials were "thermosetting" materials such as epoxy, bismaleimide, or polyimide. The reinforcing materials can be glass fiber, boron fiber, carbon fiber, or other more exotic mixtures.
Fiberglass is the most common composite material, and consists of glass fibers embedded in a resin matrix. Fiberglass was first used widely in the 1950s for boats and automobiles, and today most cars have fiberglass bumpers covering a steel frame. Fiberglass was first used in the Boeing 707 passenger jet in the 1950s, where it comprised about two percent of the structure. By the 1960s, other composite materials became available, in particular boron fiber and graphite, embedded in epoxy resins. The U.S. Air Force and U.S. Navy began research into using these materials for aircraft control surfaces like ailerons and rudders. The first major military production use of boron fiber was for the horizontal stabilizers on the Navy's F-14 Tomcat interceptor. By 1981, the British Aerospace-McDonnell Douglas AV-8B Harrier flew with over 25 percent of its structure made of composite materials.
Making composite structures is more complex than manufacturing most metal structures. To make a composite structure, the composite material, in tape or fabric form, is laid out and put in a mold under heat and pressure. The resin matrix material flows and when the heat is removed, it solidifies. It can be formed into various shapes. In some cases, the fibers are wound tightly to increase strength. One useful feature of composites is that they can be layered, with the fibers in each layer running in a different direction. This allows materials engineers to design structures that behave in certain ways. For instance, they can design a structure that will bend in one direction, but not another. The designers of the Grumman X-29 experimental plane used this attribute of composite materials to design forward-swept wings that did not bend up at the tips like metal wings of the same shape would have bent in flight.
The greatest value of composite materials is that they can be both lightweight and strong. The heavier an aircraft weighs, the more fuel it burns, so reducing weight is important to aeronautical engineers.
Despite their strength and low weight, composites have not been a miracle solution for aircraft structures. Composites are hard to inspect for flaws. Some of them absorb moisture. Most importantly, they can be expensive, primarily because they are labor intensive and often require complex and expensive fabrication machines. Aluminum, by contrast, is easy to manufacture and repair. Anyone who has ever gotten into a minor car accident has learned that dented metal can be hammered back into shape, but a crunched fiberglass bumper has to be completely replaced. The same is true for many composite materials used in aviation.
Modern airliners use significant amounts of composites to achieve lighter weight. About ten percent of the structural weight of the Boeing 777, for instance, is composite material. Modern military aircraft, such as the F-22, use composites for at least a third of their structures, and some experts have predicted that future military aircraft will be more than two-thirds composite materials. But for now, military aircraft use substantially greater percentages of composite materials than commercial passenger aircraft primarily because of the different ways that commercial and military aircraft are maintained.
Aluminum is a very tolerant material and can take a great deal of punishment before it fails. It can be dented or punctured and still hold together. Composites are not like this. If they are damaged, they require immediate repair, which is difficult and expensive. An airplane made entirely from aluminum can be repaired almost anywhere. This is not the case for composite materials, particularly as they use different and more exotic materials. Because of this, composites will probably always be used more in military aircraft, which are constantly being maintained, than in commercial aircraft, which have to require less maintenance.
Thermoplastics are a relatively new material that is replacing thermosets as the matrix material for composites. They hold much promise for aviation applications. One of their big advantages is that they are easy to produce. They are also more durable and tougher than thermosets, particularly for light impacts, such as when a wrench dropped on a wing accidentally. The wrench could easily crack a thermoset material but would bounce off a thermoplastic composite material.
In addition to composites, other advanced materials are under development for aviation. During the 1980s, many aircraft designers became enthusiastic about ceramics, which seemed particularly promising for lightweight jet engines, because they could tolerate hotter temperatures than conventional metals. But their brittleness and difficulty to manufacture were major drawbacks, and research on ceramics for many aviation applications decreased by the 1990s.
Aluminum still remains a remarkably useful material for aircraft structures and metallurgists have worked hard to develop better aluminum alloys (a mixture of aluminum and other materials). In particular, aluminum-lithium is the most successful of these alloys. It is approximately ten percent lighter than standard aluminum. Beginning in the later 1990s it was used for the Space Shuttle's large External Tank in order to reduce weight and enable the shuttle to carry more payload. Its adoption by commercial aircraft manufacturers has been slower, however, due to the expense of lithium and the greater difficulty of using aluminum-lithium (in particular, it requires much care during welding). But it is likely that aluminum-lithium will eventually become a widely used material for both commercial and military aircraft.
--Dwayne A. Day
Sources and further reading:
Christensen, R.M. Mechanics of Composite Materials. New York, John Wiley & Sons, 1979.
"Fiber Composite Materials." Papers Presented at a Seminar of the American Society for Metals, October 17 and 18, 1964, Metals Park, Ohio: American Society for Metals, 1965.
Hancox, N.L. Fibre Composite Hybrid Materials, New York: Macmillan Publishing Co., 1981.
Hoskin, B.C., and Baker, A.A., eds. Composite Materials for Aircraft Structures, New York: American Institute of Aeronautics and Astronautics, Inc., 1984.
Kroschwitz, Jacqueline. High Performance Polymers and Composites, New York: John Wiley & Sons, 1991.
Noton, Bryan R. Engineering Applications of Composites. New York, Academic Press, 1974.
Tien, John K., and Caulfield, Thomas, eds. Superalloys, Supercomposites and Superceramics. New York: Academic Press, 1989.
Tsai, Stephen W. Introduction to Composite Materials, Westport, Conn.: Technomic Publishing Company, 1980.
Weeton, John W., Peters, Dean M., and Thomas, Karyn L., eds. Engineers' Guide to Composite Materials, Metals Park, Ohio: American Society for Metals, 1987.
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