Into the Jet Age
Materials used in aircraft construction changed little from the early 1950s to the late 1970s. Aluminum alloyed with zinc metals, first introduced in 1943, grew steadily in sophistication, leading to the introduction of a new line of even lighter-weight aluminum-lithium alloys in 1957.
Composite structure remained mostly a novelty item in aerospace construction. Progress continued to be made with developing composites, but demand was driven mainly by unique performance requirements, such as for high-speed atmospheric flight or exo-atmospheric travel.
A few exceptions emerged in the general-aviation market. The Federal Aviation Agency (FAA) certified the Taylorcraft Model 20 in 1955, which was based on a steel substructure but incorporated fiberglass for the skins and cowlings. Even more progress was made by Piper Aircraft, which launched the PA-29 "plastic plane” project a few years later. The PA-29 was essentially a commercial X-plane, experimenting with materials that could replace aluminum alloy for light aircraft. The PA-29’s all-fiberglass structure demonstrated the potential strength properties of composite material. Piper’s engineers reported that the wing survived to 200 percent of ultimate load in static tests; the fuselage cracked at 180 percent because of a weakened bolt hole near the cockpit. Piper concluded that it "is not only possible but also quite practical to build primary aircraft structures of fiberglass reinforced plastic.”
Commercial airliners built in the early 1950s relied almost exclusively upon aluminum and steel for structures. Boeing selected 2024 aluminum alloy for the fuselage skin and lower wing cover of the four – engine 707. It was not until Boeing started designing the 747 jumbo airliner in 1966 that it paid serious attention to composites. Composites were used on the 747’s rudder and elevators. Fiberglass, however, was in even greater demand on the 747, used as the structure for variable – camber leading-edge flaps.
In 1972, NASA started a program with Boeing to redesign the 737’s aluminum spoilers with skins made of graphite-epoxy composite and an aluminum honeycomb core, while the rest of the spoiler structure—the hinges and spar—remained unchanged. Each of the four spoilers on the 737 measures roughly 24 inches wide by 52 inches long. The composite
material comprised about 35 percent of the weight of the new structure of each spoiler, which measured about 13 pounds, or 17 percent less than an all-metal structure. The composite spoilers initiated flight operations on 27 737s owned by the airlines Aloha, Lufthansa, New Zealand National, Piedmont, PSA, and VASP. Five years later, Boeing reported no problems with durability and projected a long service life for the components.
The impact of the 1973 oil embargo finally forced airlines to start reexamining their fuel-burn rates. After annual fuel price increases of 5 percent before the embargo, the gas bill for airlines jumped by 10 cents to 28 cents per gallon almost overnight. Most immediately, airframers looked to the potential of the recently developed high-bypass turbofan engine, as typified by the General Electric TF39/CF6 engine family, to gain rapid improvements in fuel efficiency for airliners. But against the backdrop of the oil embargo, the potential of composites to drive another revolution in airframe efficiency could not be ignored. Graphite-epoxy composite weighed 25 percent less than comparable aluminum structure, potentially boosting fuel efficiency by 15 percent.
The stage was set for launching the most significant change in aircraft structural technology since the rapid transition to aluminum in the early 1930s. However, it would be no easy transition. In the early 1970s, composite design for airframes was still in its infancy, despite its many advances in military service. Recalling this period, a Boeing executive would later remember the words of caution from one of his mentors in 1975: "One of Boeing most senior employees said, when composites were first introduced in 1975, that he had lived through the transition from spruce and fabric to aluminum. It took three airplane generations before the younger designers were able to put aluminum to its best use, and he thought that we would have to be very clever to avoid that with composites.” The anonymous commentary would prove eerily prescient. From 1975, Boeing would advance through two generations of aircraft—beginning with the 757/767 and progressing with the 777 and
Next Generation 737—before mastering the manufacturing and design requirements to mass-produce an all-composite fuselage barrel, one of the key design features of the 787, launched in 2003.
By the early 1970s, the transition to composites was a commercial imperative, but it took projects and studies launched by NASA and the military to start building momentum. Unlike the transition from spruce to metal structures four decades before, the industry’s leading aircraft makers now postured conservatively. The maturing air travel industry presented manufacturers with a new set of regulatory and legal barriers to embracing innovative ideas. In this new era, passengers would not be the unwitting guinea pigs as engineers worked out the problems of a new construction material. Conservatism in design would especially apply to load-bearing primary structures. "Today’s climate of government regulatory nervousness and aircraft/airline industry liability concerns demand that any new structural material system be equally reliable,” Boeing executive G. L. Brower commented in 1978.