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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 51

METAL AND ITS DISCONTENTS                                             51
dence that airplane designers turned to metal to meet future requirements
for large airplanes.°
After 1930 another design trend, the increasing use of stressed-skin struc-
tures, more than offset the reduction in buckling promised by larger air-
planes. In a stressed-skin (or monocoque) structure, the covering con-
tributes a large part of the structure's strength, in contrast to framework
structures, whose strength depends primarily on a skeleton of structural
members (see chapter eight). Stressed-skin structures solved two problems
at once, providing a streamlined external surface for the airplane as well as
a load-bearing structure. At the same time, stressed-skin structures made
buckling failures more likely In a framework structure, most of the mate-
rial is concentrated in a few major members with relatively thick cross sec-
tions, whereas a stressed-skin structure spreads its material over a large
area, resulting in relatively thin cross sections. For all-metal stressed-skin
structures, preventing buckling failures became the designer's most vexing
Many engineers recognized the unreliability of comparisons based on ma-
terial properties and insisted that only complete structures provided a basis
for judgment. Edward P. Warner, a prominent aeronautical engineer, ar-
gued in 1927 "that the only possible basis of comparison is a direct balanc-
ing of the weights of complete subassemblies in metal and in wood." Com-
parisons of material properties did not give due credit to metal, claimed
Warner, since they neglected its ability to be formed into more efficient
shapes, like tubing. Unfortunately, the comparison of complete structures
was also not decisive. Warner admitted that the efficiency of a complete
structure depended as much on the skill of the designer as on the choice of
Advocates of metal had always assumed that ingenuity would solve the
buckling problem, and by the late 1920s this assumption proved justified.
By the end of the decade, several firms were building metal-winged air-
planes with weight efficiencies comparable to wooden-winged airplanes.23
However, preventing buckling led to a new problem: metal structures cost
much more to produce than equivalent wood structures.
Designers of metal airplanes relied on two main techniques to prevent
buckling, both used widely in German metal airplanes during World War I.
The first technique involved complex curved shapes, such as the corrugated
coverings favored by Junkers. Curving a flat sheet greatly increases resis-
tance to buckling perpendicular to the radius of curvature, as one can dem-
onstrate by pressing on the ends of a rolled sheet of paper. The British took
this technique to extremes with elaborate shapes for steel spars. Curved
shapes, however, only increase buckling strength in one direction. To in-
crease buckling strength in all directions, engineers attached reinforcing
"stringers" to the metal sheet, breaking it up into small panels, as visible in

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