metal skinned aircraft
Boeing P-26A was the first all-metal monoplane fighter produced in
quantity for the U.S. Army Air Corps.
Its nickname was the "Peashooter. "
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.
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
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.
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
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.