Composites--the Lightweight Champs of Aircraft Industry
What do an ancient Egyptian hut, a concrete highway overpass, the globe-girdling Voyager, the human-powered Eagle and the experimental X-29 fighter have in common?
They are all made, at least in part, of composites, unusually strong materials that outperform more conventional building materials.
The Egyptians found that embedding straw in their bricks imparted extra strength that allowed them to build brick homes that better withstood weather and aging processes.
In 1850, a French gardener, Joseph Monier, discovered that embedding steel rods or wire mesh in concrete allowed the material to support heavier loads and resist cracking. Today, reinforced concrete is ubiquitous, from sidewalks to highway overpasses to skyscrapers.
Fibers in Plastics
In the 1950s, engineers embedded glass fibers into plastics to make strong, light panels for some sports cars and rust-resistant boats.
Today, aerospace engineers are embedding high-tech fibers of carbon or boron into lightweight plastics to produce composite materials that are stronger than steel but lighter than aluminum. Neither Voyager nor Eagle could have flown without them. Neither could the Pentagon’s futuristic X-29, with its forward-swept wings.
But experimental aircraft are not the only ones that use composites.
Commercial airliners now contain as much as 15% composites, primarily in secondary structures such as seats, doors, wall panels and interior fittings, and landing gear doors. And the next generation of airliners now on the drawing boards could contain 25%, said John Wheeler, a spokesman for the Boeing Co.
Military aircraft already contain 25% composites and experimental models now flying have as much as 40%, according to aeronautical engineer Pat Sforza of the Polytechnic University of New York. Helicopters have as much as 60% composites and one light aircraft, the Beech Starship, has a body that is 100% composites.
The proposed space station and other facilities erected in orbit will also be constructed almost entirely of composites, according to Tom Longmire, vice president of carbon fibers and advanced composites at Amoco Performance Products.
Of course, composites have many uses that are more down to earth. Since the early 1970s, for instance, graphite composites have been used in fishing poles, bicycles, sailboats, archery equipment, motorcycles, kayaks, and even some racing car shells--”anywhere where weight-saving is more important than cost,” Longmire said.
Composites also are used in heavy industry--for gears, bearings, turbine blades and other machinery parts that must be exceptionally resistant to wear.
But by far the largest use of composites is in the aerospace industry, where their light weight and high strength make them ideal building materials. Typically, graphite/epoxy composites weigh between 25% and 30% less than aluminum.
They may also add an extra measure of safety, experts said, because they are not weakened by holes that must be drilled for rivets or, in military aircraft, caused by bullets.
But composites do have their drawbacks. One is cost, which often can be more than 10 times that of aluminum. The use of composites also requires the introduction of new production techniques because composite parts are put together in a completely different fashion than their aluminum counterparts.
In 1987, 40 million pounds of composites, 75% of total production, will be used in aerospace, according to Glenn C. Kuebler, product development manager for graphite materials at Hercules Inc., the largest producer of the materials. Their use in the aerospace industry is growing by 20% to 25% per year, he said.
The remainder of the market is split between sports and industrial uses.
In a sense, the introduction of composites has brought the aircraft industry full circle. When Wilbur and Orville Wright built their Dayton Flier in 1903, they used lightweight bamboo for the framework and covered it with cloth fabric. The fabric itself was painted with a shellac-like substance to seal its pores and make it more rigid.
Today’s composites are not unlike that “doped” cloth, but the new fabric and plastics are much, much stronger and many layers are used instead of just one.
Fabric over wood or steel frames was the state of the art in aircraft production until around 1930, when manufacturers began to use aluminum for its much greater strength. “Aluminum makes very good planes and we know a lot about it,” Sforza said.
Manufacturers began using composites during the 1970s. And although such use grew slowly in commercial aviation, many predict that their use will increase rapidly. “I think the aluminum airplane will go the way of the fabric airplane,” said Voyager designer Burt Rutan, vice president of Scaled Composites Inc. of Mojave, Calif.
The Voyager, which completed its 9 1/2-day around-the-world flight on Dec. 23, needed to be light in weight to be able to carry enough fuel, yet strong enough to sustain at least some buffeting from the weather.
Rutan thus used composites to construct all components of the plane, including the ribs and spars that gave the fuselage and wings strength and the thin skin that covered wings and fuselage. All told, the plane weighed only 1,858 pounds.
Voyager’s strength was demonstrated when the tips of the fuel-laden wings scraped the ground during takeoff, shearing off the plane’s small winglets. The flight continued unimpaired.
Similarly, the pedal-powered Eagle also had to be exceptionally light or pilot Glenn Tremml could not have flown it 37.2 miles on Jan. 22 as it set a world record for human-powered flight.
The Eagle weighed only 92 pounds, and its designers at the Massachusetts Institute of Technology achieved that weight by using graphite/epoxy spars for structural strength. Where strength was less important, such as in shaping the wings, they used Styrofoam, which is much lighter. The entire craft was then covered with a thin film of plastic similar to transparent plastic food wrap.
Weight saving is important for commercial airliners also, Sforza noted. “If you save 200 pounds, which doesn’t seem like much,” he said, “you can add one passenger to each flight. Since the planes are designed to last 15 years and fly 10 hours a day, that represents a great amount of revenue to an airline.”
Indeed, considering the potential advantages of composites, “It is surprising that they haven’t penetrated this market to the extent you would think they might have,” Sforza said.
“We don’t have the degree of confidence in composites that we do in aluminum,” said Wheeler, whose employer, Boeing, is the largest builder of commercial airliners. “We have always been a very conservative design company.”
“Technologies have to be very well proven before they go into a commercial airplane because they have to fly eight to 14 hours per day,” said Elaine Bendel, a spokeswoman for the Douglas Aircraft Co. division of McDonnell Douglas Corp.
Although most commercial airliners contain about 7% composites by weight, the Boeing 767 has more than a ton of composites, accounting for a considerable part of its weight. “Every moving external part of the airplane is a composite structure--wing flaps, spoilers, leading edge slats, rudder, elevator,” Wheeler said.
With sponsorship from the National Aeronautic and Space Administration, Boeing has also installed composite tails on five 737s that are now being used by commercial airlines. The composite structures weigh 22% less than aluminum stabilizers, and save 116 pounds per plane.
“This is not an experimental program and there is no risk to safety,” Wheeler said. “We simply want to . . . see how they hold up to wear and tear, weathering, and temperature extremes.”
Future Uses
The next generation of passenger jets could have as much as 25% of their structures made from composites. The Boeing 7J7, a 150-passenger twin-engine jet that will replace the 727, “most likely” will use composites for the entire tail assembly, the tail cone, the engine nacelles and the passenger floor, Wheeler said.
Douglas will also incorporate more composites in its new MD-91 and MD-92 passenger planes, both of which will be updated versions of the twin-engined MD-80 now in use, according to company spokeswoman Elaine Bendel.
The Douglas planes, she said, will be the first commercial aircraft to use prop-fan engines, jet engines that turn huge propellers that look something like Cuisinart blades.
The prop-fans are expected to give at least a 15% improvement in fuel economy compared to the jet engines they will replace, but they are also somewhat noisier, particularly in the nearby sections of the cabin. Much of the tail section of the plane will probably be made of composites, Bendel said, to help muffle the noise as well as to save weight.
The main drawback to the use of composites, experts agree, is their cost. While aluminum now sells for about $2 to $3 per pound, the graphite fiber/epoxy resin composites most commonly used in aircraft now cost $25 to $40 per pound. Lighter, more exotic materials can cost as much as $500 per pound.
But that cost disadvantage can be overcome. If a complex part has to be machined from aluminum, according to aerospace engineer William Richter of Lockheed Corp., there is a lot of labor and waste involved. Moreover, a complex aluminum part might be assembled from as many as 1,000 different pieces, including nuts, bolts, washers, rivets, and so forth.
“You have to have paper work for every one of those pieces,” Richter said, “and you have to maintain inventories of all of them. A composite analog of that same part might have a 75% reduction in the number of parts. That represents a big cost reduction.”
Another potential problem, according to aerospace engineer Stephen Smith of NASA’s Ames Research Center, is that plane builders will have to change their manufacturing techniques. While aluminum parts are stamped and machined into shape and riveted and glued together, composite parts are formed in molds.
Simplifying Complex Parts
Counterbalancing the need to shift production techniques are several advantages to this casting process. For one thing, quite complex parts can be molded more easily than they can be machined. Also, the absence of external rivets reduces drag, which can give a 3% increase in fuel economy, Sforza said.
The composite materials can also be safer than aluminum parts, he said, because the holes drilled for rivets can serve as stress points that allow cracks to start.
Cost is less of a concern in military aircraft because the performance requirements are so stringent that they can often be met only with more expensive composites, Richter said.
Composites account for perhaps 25% of the structures of current military aircraft, according to Sforza. In addition to secondary structures, composites are used in the tail assemblies of all U.S. fighters, as well as in wing and body coverings of planes like the McDonnell Douglas F-18, said Dick Hadcock, deputy director of technology development at Northrop Corp.
McDonnell Douglas is now building a U.S. version of the AV-8 Harrier, a British short-takeoff-and-landing jet fighter. Unlike the British, however, McDonnell Douglas is constructing the main wing entirely from composites. That one change, Bendel said, “will double the plane’s range with the same payload or double the payload with the same range.”
Reluctance to Talk
Composites will also play an important role in the Air Force’s new Advanced Technology Fighter, now on the drawing boards at Lockheed and McDonnell Douglas. Both companies are reluctant to talk about the new planes. But “it seems likely that the range, maneuverability, speed and weight requirements can’t be met any other way,” Longmire said.
Perhaps the most dramatic application of composites is Northrop’s X-29, which literally could not fly without them. The X-29 is a “technology demonstrator” rather than a prototype fighter, and was built to explore the feasibility of using a forward-swept wing.
Earlier studies had shown that an aircraft with forward-swept wings should have better handling characteristics around the speed of sound and should be more maneuverable, yet should be able to fly more slowly before the wings stall and lose lift.
But the forward-swept wing is subject to unusual stresses, Hadcock said. Those stresses not only bend the wing upward, as in a conventional plane, but also twist it. The wing must therefore be specially reinforced to resist this twisting. If that reinforcement were done with metal, Hadcock said, the plane would be too heavy to get off the ground.
“We have also produced a very complicated shape in the wing,” Hadcock said, “which we couldn’t have done with metal without enormous costs.”
The Air Force’s mysterious new Stealth bombers and fighters, designed to avoid detection, are probably also made of composites because they do not reflect radar signals nearly as strongly as metal. None of the major aircraft companies would talk about such applications, however.
Helicopter Designs
Composites also play a major role in new helicopters, where excess weight is even more detrimental than in conventional airplanes.
Bell Helicopter Textron of Fort Worth is using graphite/epoxy for 60% of the structure of its V-22 Osprey, which is a cross between a helicopter and a propeller-driven fighter. The Osprey has large rotors mounted on the end of its wing. The rotors are tilted upward for takeoff and landing like a helicopter, but are tilted forward for normal flight.
Because of its light weight, according to spokesman Terry Arnold, the Osprey will be able to cruise at a speed of 300 m.p.h. and carry a combat payload more than 1,000 miles--”twice as fast and twice as far” as the helicopters it will replace.
Other new helicopters on the drawing boards for the Air Force and the Army will have bodies made almost entirely of composites, according to Sforza.
Composites are also making an appearance in planes for civil aviation. Beech Aircraft Corp. of Wichita last year began flying prototypes of its Starship, a twin-engined business airplane whose body is made entirely from graphite/epoxy and other composites. The 7,900-pound plane will be able to carry a 6,100-pound load, at least 15% more than comparable aircraft, company spokesman Michael Potts said.
