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ENGINEERING ENTHUSIASM                                             33
duralumin. Beginning in 1916, the navy urged the Aluminum Company of
America (Alcoa) to develop a duralumin-type alloy for a rigid airship. Later
that year, Alcoa received duralumin samples from a German zeppelin that
had crashed in France. After the United States entered the war, Alcoa gained
access to German patents seized by the U.S. government. The secrets of pro-
ducing high-quality duralumin remained elusive, however, and Alcoa found
it difficult to move from experimental to quantity production. Through the
end of the war, duralumin remained an experimental metal available only in
limited quantities.26
The Empire company received its first delivery of duralumin from Alcoa
in February 1918. Initial tests suggested that duralumin wing spars would
be competitive with spruce. That spring, the Empire company built two
more sets of wings for the Curtiss JN-4, this time with duralumin spars,
steel ribs, and fabric covering. These wings were delivered to the army's
aircraft engineering center at McCook Field near Dayton, where they re-
ceived strength and flight tests.27
The strength or "static" tests were relatively simple, but nevertheless es-
sential for designing new airplanes. To test a small component like a rib or
spar, the engineers fixed the test item in a jig and hung weights that simu-
lated the loads expected in flight. More weights were added until the part
failed. A metal component was considered equal in performance to a wood
part when it sustained an equal or greater load but weighed no more than
the wood part. If the metal part supported a larger load but weighed more,
the results were ambiguous.28 Static tests of complete wings followed a sim-
ilar procedure. The wings were attached upside down in a jig, and then
loaded with sand bags to simulate the distribution of air pressures on the
wing in flight. The engineers increased the load until the wing collapsed
(figure 2.5). The maximum load sustained without failure determined the
load factor, a measure of airplane strength that consisted of the maximum
load divided by the normal load in steady flight. As with ribs and spars, the
metal wing was superior to the wood wing only if it sustained at least as
great a load but weighed less.29
The Empire company's first metal wings were not competitive with wood.
These wings, built in spring 1918, included the set with duralumin spars
and steel ribs, as well as two all-steel designs. The army performed static
tests on each design at McCook Field. It also tested a set of standard wood
JN-4D wings for comparison. The wings with duralumin spars were signifi-
cantly weaker than the wood wings, supporting 17 percent less load than
wood wings of nearly equal weight. The steel wing also failed to match the
strength of the wood wing, while weighing significantly more. Late in 1918,
McCook Field received and tested another all-steel wing from the Empire
company This wing used alloy steel, heat treated after fabrication. It
weighed 425 pounds, 7 percent more than the wood wing, but it supported