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National Advisory Committee for Aeronautics, Technical Notes - Torsion in Box Wings

The scope of this paper is the analysis of the elements of
a conventional type of box wing under a torsional load. This
wing has as a primary structure two wooden box or pl; beams, their.
Thesis submitted in partial fulfillment of the requirements for
the degree of. Engineer in Hechenical Engineering Aeronautics,
Stanford University-, 1930.
Maximum moments of inertia being about axes parallel, or nearly
so, to the Wind chord. They areI connected by a plywood skin,
forming the wing covering, in such a way that—a cross section
parallel to the plane of symmetry of the airplane forms a _
rectangle or a trapezoid. Figure 1 shows a typical box—wing
crIoss section, the two spars proper being box beams, and_ the
skin covering being plywood which forms part of the airfoil
section. None of the formulas covered in this report takeI acu
count of the curvature which in practically all cases is present
in either the top skin or both tOp and bottom. It is believed
that thIiIs. factor isI so unimportant th_at 1_t would Inct beI worth
while to introduceI the complex1ty attendant uponI _its co_nsider~
ation into the relatively simple formulas obtained when the
curvature is neglected.
It is (shown in this paper that the analysis of a box wing
by rationalI methodsI results in the computation of Imuch lower
stresses in the various members of the box thanI are obtained
when present design procedure is followed As a means Icf in
simplifying the computations necessary in such_ an analysis, it
will also be demonstrated that approximate methods, involving
some reasonable assumptions,- are available. Assuming the
validity of this statement, which shall be proven subsequently,
the Value of this paper will.11e Iin the application by the _ -
designer of its methods to reduce the material_necessary to
carry a given wing load; and by sOwdoing,-he will decrease the
weight of the wing structure.