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

6. Neglected alternative i: plywood stressed-skin construction,   pp. [114]-134

Page 126

noted that the continued use of wood might aid the transition to monoco-
que structures, but such observations were extremely rare. Typical discus-
sions of stressed-skin construction simply ignored wood. For example, a
1932 article on "The Monocoque Fuselage" stated simply that "comparison
[of metal] with wood construction is not warranted," without giving any
justification.34 Because aviation engineers viewed monocoque design as an
aspect of all-metal construction, most of them ignored the potential benefits
of wood monocoques.
NACA Wood Research and the Durability of Glues
Another factor that discouraged the development of plywood stressed-skin
structures, and of wooden airplanes in general, was distrust of glued joints.
Gluing provided by far the strongest joints between wood parts, far stronger
than screws or nails, and so was indispensable to wooden airplane struc-
tures. Critics of wood construction correctly identified glued joints as the
place where deterioration began in wooden airplanes. Researchers in wood
structures had long recognized the need to improve the durability of glues,
yet the widespread belief in the inevitable triumph of metal discouraged
the aviation community from pursuing such research. Instead, aviation re-
search remained focused on problems of metal construction, despite the
widespread use of wood in airplane structures into the 1930s. This neglect
of wood research is strikingly demonstrated by the NACAs lukewarm inter-
est in the durability of glues, especially when contrasted with the NACAS
vigorous response to intercrystalline corrosion in aluminum alloys. These
radically different responses clearly bear the imprint of the progress ideol-
ogy of metal.
By 1920 airplane builders had made considerable progress with wood-
working glues, especially with glues for making plywood. Before World
War I, the structural use of plywood was hindered by the lack of water-
resistant glues. Traditional hide and bone glues produced strong joints but
remained water soluble, making their use unwise in structures exposed to
the weather. By World War I, however, manufacturers were gluing veneers
with blood-albumin and casein glues, both of which became insoluble after
setting. Albumin glues required heat to set, whereas casein glues could be
applied cold. Widespread use of these glues in the United States was a direct
result of military demand for water-resistant plywood during World War I.
During the war, the Forest Products Laboratory (FPL) developed tests for
the water resistance of plywood glues. These tests included boiling a ply-
wood sample in water for twenty-four hours and soaking a sample in cold
water for two weeks. By 1918 McCook Field engineers had identified over
a dozen commercial plywood glues that could survive these tough tests.35

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