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Schatzberg, Eric, 1956- / Wings of wood, wings of metal : culture and technical choice in American airplane materials, 1914-1945

2. Engineering enthusiasm: World War I and the origins of the metal airplane,   pp. [22]-43

Page 26

alloy developed in 1906 by Alfred Wilm, a German metallurgist. About
1908 Wilm granted an exclusive license for the German production of his
alloy to the Darener Metallwerke, which marketed it under the trade name
Duralumin, often shortened to dural. Duralumin consisted of aluminum
alloyed mainly with copper, with smaller amounts of magnesium and man-
ganese. The key to duralumin's strength was its heat treatment. Heat treat-
ment involves the controlled heating and cooling of a metal in the solid state
to produce desired characteristics, such as hardness or ductility. One com-
mon type of heat-treatment, quenching, is used to increase the strength and
hardness of steel by rapidly cooling the red-hot metal in water or other
liquids. Wilm was experimenting with similar processes for aluminum al-
loys, but his alloys did not behave as expected. Instead of increasing in
strength and hardness immediately upon quenching, Wilm's heat-treated
alloy gained strength through a process known as age (or precipitation)
hardening, in which the quenched alloy gradually hardens over a period of
several days. Age-hardened duralumin has the tensile strength and ductility
of mild steel, with just over one-third the weight. Like heat-treated steel,
duralumin and related alloys also lose strength when welded, making rivets
the preferred joining technique.9
In addition to switching from iron to duralumin, Junkers abandoned the
supporting cover because of its excessive weight. He then developed a new
type of all-metal wing and fuselage structure using a duralumin-tube frame-
work covered by dural sheet, which was corrugated to improve its stiffness
(figure 2.3). The corrugations ran in the direction of flight to avoid exces-
sive air resistance, which prevented the cover from carrying any of the main
bending loads of the wings, although it did contribute to the torsional
strength of the wing.10
Junkers' first success using this new structural system was the J4 armored
biplane of 1916. The army had insisted on the biplane wing, despite Junk-
ers' preference for the monoplane. The J4 was designed for ground obser-
vation and combat directly above the trenches. Such an airplane required
neither a good rate of climb nor high speed but rather ruggedness of con-
struction and resistance to ground fire. The all-metal airplane proved well-
suited to this task. The J4 became the first all-metal airplane to enter pro-
duction. In all, 227 were manufactured, although the Armistice arrived
before many of these saw combat. Junkers also built a few all-duralumin
monoplane fighters near the end of the war.1
The second major pioneer of German all-metal aircraft construction was
the engineer Claude Dornier (1884-1969). Dornier had experience in metal
aircraft structures through his work for the Luftschiffbau Zeppelin begin-
ning in 1910. This company was formed in 1908 to build rigid airships, also
know as zeppelins, for Count Ferdinand von Zeppelin. In contrast to non-
rigid airships (blimps), rigid airships use internal metal structures to se-

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