technology of metal skinned aircraft

Most of the 170,000 airplanes built during World War I were constructed of wooden frames with fabric coverings. These materials were relatively lightweight and available. Anthony Fokker, a Dutch entrepreneur working in Germany during the war, developed a welded-tube steel fuselage to take the place of wood. German manufacturers built more than 1,000 of these aircraft, which had wooden wings. Hugo Junkers, a German designer, built all-metal aircraft, first using sheet iron. He soon switched to duralumin, a high-strength aluminium alloy developed just before the war. After the war, Junkers developed several all-metal passenger transports.

The Junkers J.L. 6 represents an important step forward in technology. It was probably the first plane with the fuselage, wings, and skin all constructed of metal.

In the spring of 1920, the American pilot John M. Larsen began demonstrating an imported Junkers all-metal passenger plane designated the JL-6. It created much excitement within the American aviation community. The U.S. Postal Service bought six of the aircraft. The enthusiasm over the JL-6 caused many aviation leaders to call for the development of all-metal aircraft. The National Advisory Committee for Aeronautics (NACA) declared in its 1920 Annual Report that metal was superior to wood because "metal does not splinter, is more homogeneous, and the properties of the material are much better known and can be relied upon. Metal also can be produced in large quantities, and it is felt that in the future all large airplanes must necessarily be constructed of metal." NACA immediately began research into all-metal construction, and the U.S. Navy developed duralumin fabrication techniques at the Naval Aircraft Factory. In 1924, the first all-metal commercial airplane, called the Pullman, was produced by William Stout. Glenn Martin Aircraft also developed all-metal aircraft for the U.S. Navy in 1923 – 1924, where the only wooden structure was the engine mount.

Airplane designers also felt that metal offered other significant advantages over wood, including protection from fire, but in reality, early aircraft metals provided little protection against airplane fires. In fact, despite the enthusiasm over the JL-6, the aircraft had a faulty fuel system causing it to catch fire in flight and the thin aluminium skin between the engine and cockpit melted, allowing flames to burst through at the pilots' feet. Two airplanes were lost within months, and the Post Office quickly sold the remaining four at a huge loss.

Despite the initial great enthusiasm over all-metal construction within the U.S. aviation community and the widespread belief among designers in the superiority of metal in the early 1920s, engineers soon found that metal was not inherently superior at the time. Wood was still lightweight and easy to work with. Over the next decade, aeronautical engineers had a difficult time designing metal wings and airframes that weighed as little as wood.

The Gallaudet DB-1 was an unsuccessful attempt at a plane with an all-metal fuselage and metal framework wings. It weighed grossly overweight and never advanced past the ground test phase.

In late 1920, the Army Air Service contracted with the Gallaudet Aircraft Company for a monoplane bomber with an all-metal fuselage and metal framework wings. The prototype, designated the DB-1 and delivered in late 1921, was grossly overweight and considered a miserable failure. It was quickly retired. By 1929, nine years after the JL-6 had created so much excitement about all-metal airplanes, an aeronautical textbook estimated that metal wings still weighed 25 to 36 percent more than wood wings. By 1930, a decade after the NACA declared metal superior to wood, only five percent of the aircraft in production were of all-metal construction.

One of the big problems with metal was that it buckled when compressed, just like a piece of paper will bend when its ends are pushed together. In comparison, wood does not buckle as easily. By the 1930s, another aircraft design trend known as stressed-skin structures made this problem more acute. Before this time, aircraft achieved much of their structural strength through their internal frameworks. But in a stressed-skin structure, the covering contributed much of the structure's strength and the internal framework is reduced. This provided a streamlined external surface for the airplane, but made metal buckling failures more likely.

In order to combat the problems of compressive buckling, metal structures had to be complex, with curves and riveting and reinforcement. This dramatically increased the costs of such an aircraft. By 1929, some manufacturers were making metal wings that were as light as wooden ones, but by the end of the 1930s, all-metal airplanes were significantly more expensive than wood and fabric airplanes.

Metal also presumably was more durable than wood, which warped, splintered, and was eaten by termites. But duralumin also had severe corrosion problems. It turned brittle. Unlike iron or steel, which rusted from the outside in, duralumin weakened internally and could fail suddenly in flight. Duralumin corroded even more in salt spray and the U.S. Navy eagerly sought a solution. The Aluminium Company of America (Alcoa) and the Federal government cooperated to develop a material known as Alclad, which consisted of an aluminium alloy bonded to pure aluminium. Alclad solved many of the corrosion problems of duralumin. Soon other alloys were developed that proved effective as well and during the 1930s, all-metal airplanes became much more common.

By the mid-1930s, wood was no longer used on American multi-engine passenger aircraft and U.S. combat aircraft. But in 1938, the British airplane company, de Havilland, began work on a fast, unarmed bomber named the Mosquito. It was one of the most successful British aircraft of World War II, able to fly faster and higher than most other aircraft. More than 7,700 Mosquitoes were built. They were made of spruce, birch plywood, and balsa-wood, proving that even in the era of all-metal planes, older materials could still achieve impressive results.

The lesson of the development of all-metal airplanes is that just because engineers may think that a new material is superior, that does not mean that it will be immediately useful. It may take many years before designers and materials specialists are able to adapt a new material to a new task.