Weight opposes lift. Weight and lift are equal when a plane flies level at constant velocity. Because excess weight requires more lift, and therefore more thrust, heavy planes are more difficult to get off the ground as compared to lighter planes. Planes with less weight require less thrust. Thus, planes are designed to be as light as possible.
The engineering goal in aerospace material science has been to find lighter, stronger materials that lend themselves to easy machining, assembly and repair. For now, metal is dominating, but many expect composites to take over by the end of the next decade. These materials are arguably stronger and lighter than metal, though they remain more costly to manufacture and more challenging to examine and repair.
The Wright Flyer
December 17, 2003 marked the 100th anniversary of powered flight. On that day, 100 years ago, the Wright brothers took to the air in a delicate wooden contraption they had built from spruce, boxwood roller-skate wheels, and waxed twine. They covered the wings with a cotton muslin fabric widely used at the time for women's underwear. And for parts that required extra strength, they used ash, a sturdy, shock-resistant wood. They also utilized some steel rod and sheet steel, along with homemade control cables, for hardware and strapping.
In the 1920s, plywood attracted the attention of aircraft designers who were used to fashioning wood and hesitant to try metal skins. Fabric was not a viable option for these designers since the material could not handle stressed-skin designs though it is light and strong enough to use in aircraft skins. Using plywood for the monocoque (a structure in which the skin absorbs all or most of the stresses to which the body is subjected) fuselage and skin, Jack Northrop designed the 1920 S-1 "sport plane" when he was an employee of the Loughhead brothers, Allan and Malcolm. (They would later use the phonetic spelling of their name—Lockheed). The plane marked one of the first uses of plywood for a monocoque fuselage and skin.
While Northrop's design was unique, featuring a bullet-like fuselage, wing flaps and folding wings, the plane was a commercial flop because of its hefty price tag of $2,500. Northrop was more successful with his second plywood plane—the Lockheed Vega—which he assembled with fellow aircraft designer Gerrard Vultee. The four-passenger Vega, which first flew in 1927, had a semi-monocoque design and a wood-skinned wing. The Lockheed brothers used the plane for their airline routes. While its modest passenger capacity prevented it from being a runaway hit, it served as an airliner for TWA and Braniff. The Lockheed Vega also took home all the speed trophies at the Cleveland Air Races in 1928 and set a number of altitude and transcontinental speed records. Many regard the Vega as the finest plywood plane.
In 1925, Ford Motor Co. purchased an aircraft company to provide planes for its airline. Ford constructed the 4-AT (Air Transport), the first metal airliner and one of the earliest all-metal planes. Nicknamed the "Tin Goose," the airplane featured three engines, a corrugated metal fuselage and a high-mounted wing that was also covered with corrugated metal. But the corrugated skin was tricky to shape and attach, and it added drag to the airplane. Even Ford's consummate skill in mass production could not make the trimotor plane profitable because the technology to construct an all-metal plane was simply not in place. However, the company did manage to promote the concept of all-metal planes, emphasizing their safety to a public that viewed flying with trepidation.
In 1935, the DC-3 made its debut and became the first all-metal, multi-engine monoplane (an airplane with only one pair of wings) to make money as an airliner. And Boeing's P-26 "Peashooter"—an all-metal, low-wing monoplane—would become the country's first metal fighter plane.
Titanium entered public consciousness in 1964 when Dick Tracy cartoons hailed it as the metal that "makes space travel possible." While that statement is not entirely accurate, the metal did claim a substantial role in military planes and spacecraft, where performance is paramount. Its high strength-to-weight ratio, corrosion, heat resistance and tendency to get stronger as it is heated caught the attention of aircraft designers. Propelled by the Cold War and the Space Race, engineers quickly came up with new alloys and developed special machining and joining methods to make use of the metal. Currently, titanium accounts for 10% of a commercial airliner's weight and a higher percentage of that of a military plane. For instance, it represents 40,000 lbs. or 20% of the weight of the 1980s-era B-1B Lancer.
As the latest and potentially most significant aircraft material since aluminum alloys were created in the 1920's, composites are widely used in the aerospace industry today. They are formed when a matrix or resins, such as epoxies and polyamides, are mixed with reinforcements, such as glass, boron and carbon fibers. While composites are strong and lightweight, they remain expensive to manufacture, machine and repair. Currently, engineers use them in airliners and military planes to reduce weight and to fulfill otherwise unattainable design goals. For example, composites account for a third of the new F-22 Raptor fighter aircraft's structure. And some analysts expect that warplanes will consist of more than two-thirds composites in the future.
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 mould 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 fibres are wound tightly to increase strength. One useful feature of composites is that they can be layered, with the fibres 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.
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.
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 aluminium 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.
Other Developments in Aerospace Material Science
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 a higher 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.