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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 122

PLYWOOD STRESSED-SKIN CONSTRUCTION                              123
Despite earlier experiments with wood and metal monocoques, by early
1923 the monocoque fuselage had clearly fallen into disfavor among Mc-
Cook Field engineers, who preferred the easier maintenance permitted by
the fabric-covered welded steel-tube fuselage (see chapter four). When Cur-
tiss developed the new PW-8 pursuit for the army in 1923, it abandoned the
wood monocoque that had proven so successful on its racing planes, and
settled instead on a welded steel-tube fuselage.24
There was nothing irrational in the army's preference for the higher drag
but lower maintenance of the boxy steel-tube fuselage. The aerodynamic
advantages of the monocoque fuselage remained limited for the relatively
low-speed airplanes of the time. Most airplanes of the early 1920s suffered
from high levels of parasitic drag. The reduction in drag provided by a
monocoque fuselage was small compared to the remaining drag due to
struts, bracing wires, landing gear, open cockpits, and especially the ex-
posed cylinders of air-cooled engines. In addition, these other drag-produc-
ing elements directly interfered with the smooth airflow over the fuselage,
making it impossible to realize the theoretical improvement in drag as mea-
sured on the fuselage alone. Only when various sources of drag were re-
duced together would the advantages of the monocoque fuselage become
Concern over the durability of plywood also influenced decisions in favor
of the welded steel-tube fuselage in the 1920s. For example, in the mid-
1920s, Boeing engineers rejected the plywood monocoque fuselage in favor
of welded steel tubing for a new mail plane, based on their belief that the
wood monocoque was insufficiently rugged for hard service. Experience
with the Lockheed Vega, however, including its use for Arctic exploration,
suggests that the plywood monocoque could withstand very rough handling
indeed. The Boeing decision does not appear to have been based on any
actual operating experience with plywood monocoques.26
For similar reasons, stressed plywood wings also remained the exception
in the 1920s. The army's objection to permanent fuselage coverings applied
equally to wings. In addition, the increased stiffness produced by the ply-
wood skin was of greater advantage to fully cantilevered monoplanes than
to the more common biplanes and braced monoplanes. External bracing
added considerable stiffness to the wings, lessening the benefits of a stressed
skin. The advantage of stressed coverings only became apparent in the late
1920s with the growing interest in high-speed, fully cantilevered mono-
planes. By this time, however, stressed-skin development was conceived
entirely in terms of metal.
As with metal structures, the difficulty of making stress calculations also
inhibited the adoption of plywood airplanes. Stress calculations were much
more difficult to make for stressed-skin than for framework structures, due
to problems in predicting buckling failures. Without a theoretical basis for

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