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.

Over the next 100 years, aircraft pioneers and manufacturers would shift from wood and fabric to metal and composites. The enduring engineering goal has been to find lighter, stronger materials that lend themselves to easy machining, assembly and repair. For now, metal is dominating, supplanting wood and fabric. 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.

Plywood

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 semimonocoque 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.

Metal

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, multiengine 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.

Composites

As the latest and potentially most significant aircraft material since aluminum alloys were created in the 1920's, composites could be the material of the future. 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.

Sources:

A Century of Progress in Aircraft Materials—Part 1 Wood and Fabric
Stephen J. Mraz
Machine Design, November 6, 2003
www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=56409&strSite=MDSite&Screen=AEROSPACE&catId=379

A Century of Progress in Aircraft Materials—Part 2 Metals
Stephen J. Mraz
Machine Design, November 6, 2003
www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=56410&strSite=MDSite&Screen=AEROSPACE&catId=379

A Century of Progress in Aircraft Materials—Part 3 Composites
Stephen J. Mraz
Machine Design, November 6, 2003
www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=56411&strSite=MDSite&Screen=AEROSPACE&catId=379