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Schatzberg, Eric, 1956- / Wings of wood, wings of metal : culture and technical choice in American airplane materials, 1914-1945
(c1999)

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


Page 124

CHAPTER SIX
calculating the strength of stressed-skin structures, designers had to rely
heavily on empirical testing. As Walter Vincenti has argued, engineers rou-
tinely develop rational design methods in the absence of theory by means of
systematic parameter variation. This process involves tests on representa-
tive components, and the reduction of test results to a series of equations or
tables applicable to a range of expected design conditions.27 In airplane de-
sign, such data depended on the materials tested, and one could not safely
use data from one material to design structures in another.2 Extrapolation
from metal to wood was especially problematic, given wood's anisotropic
properties and its very different mode of compression failure.
Designers of plywood monocoques clearly recognized the need for exten-
sive testing as early as 1920. According to Armin Elmendorf of Haskelite,
tests on monocoque and semimonocoque fuselages showed that "failure
does not take place in compression . .. until after collapse due to buckling.
The engineer must therefore design the monocoque fuselage so as to get
maximum buckling strength." Unfortunately engineers had no reliable
equations telling them how to design a fuselage skin for maximum buckling
strength, especially with regard to plywood, where stiffness varies with di-
rection. Given the limits of calculation, engineers had to rely on extensive
testing to design an optimal monocoque fuselage. The Haskelite company
did perform a series of such tests, but the company apparently abandoned
this research as interest in plywood monocoques declined.29
In the late 1920s, growing interest in stressed-skin construction led to a
resurgence in research on the strength of thin-walled structures. Plywood
found almost no place in these studies. The navy initiated this research in
1927, when it asked the Bureau of Standards to conduct a study of the
strength of fiat metal plates under edge compression, information important
for the design of flying-boat hulls. This research relied heavily on parameter
variation, and was performed entirely on metals. The bureau tested sheets of
four different metals in six thicknesses and six widths, for a total of 144
tests. McCook Field engineers were also interested in this study because of
its possible application to wing coverings, but in early 1927 they did not
consider stressed coverings important enough to justify an extensive re-
search program. Within a few months, however, the Materiel Division
changed its tune and began planning its own study of stressed-skin struc-
tures, beginning with tests of corrugated metal in compression. All the pro-
posed tests involved metal components. By early 1928, Capt. Carl Greene
and John Younger of McCook Field had begun a study to develop a metal
"skin stressed wing," with some of the research to be performed by Alcoa.
As part of this project, the army engineers first tested the torsional strength
of box beams with thin plywood sides, and then extended the tests to du-
ralumin. Plywood structures, however, were not themselves a subject of
research but merely a means to help develop an all-metal wing. The ply-


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