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

3. Metal and its discontents,   pp. [44]-63

Page 49

Behavior of an Euler strut.
As load increases, strut will
fail by elastic buckling
I Load
:           Beam under load        --
Top flange under compression
Bottom flange in tension
Top flange of loaded beam is under
compression, and subject to local
buckling like the Euler strut
Figure 3.1. Elastic buckling, showing the analogy between an Euler strut and the
compression flange of a beam in bending. Adapted from Nathan Rosenberg and
Walter G. Vincenti, The Britannia Bridge (Cambridge: MIT Press, 1978), 20.
plane materials (see chapter nine).16 These comparisons showed how great
a handicap metal had to overcome in structures limited by buckling
strength. In a 1942 textbook, engineering professor John Younger estimated
the relative weights of wings of equal buckling strength, calculated on the
basis of flat plates. According to Younger's calculations, if a plywood wing
weighed 100 pounds, an aluminum wing of equal strength would weigh 255
pounds, and a steel wing 500 pounds. These calculations show why buck-
ling was the most serious problem faced by metal airplane designers. Such
calculations provided some of the strongest arguments in favor of wood. For
these same reasons, proponents of metal usually preferred aluminum to
steel, since aluminum was only one-third as dense.7
An army project to develop metal wing spars illustrates the difficulties
that compressive buckling posed for designers of metal structures. In 1925
McCook Field requested bids for metal wing spars designed for identical
ten-ton loads; bids for thirty different spars were accepted. The Air Corps
also tested some standard wood spars for comparison (figure 3.2). The

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