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National Advisory Committee for Aeronautics, Technical Notes - Torsion Test of a Monocoque Box

A monocbque box of aluminum alloy was subjected to
torques applied at the ends.- The twist; the strain in_#__
the stringers, plate, and corner posts; and the buckling
load were measured. The twist was found to be 20 to 50 _ "‘
"percent less than that giv.en by Bredt' s theory for the
torsion of a thinswalled box without reinforcements and
the sh'ear stress in the shear web, abOut 18 percent
greaterl._'

In order to obtain claser agreement between theoret-
ical and experimental results. an analysis was _de_veloped
for the twisting of a monocoque box reinforced by string—f' . _
ere, corner pestS, and bulkheads. The measured twist " '"
agreed within 10 percent with this analysis. The measured _
and theoretical values for the strains and buckling load
agreed within the error of observation.

As a part of an investigation for the NASA en mOno—
coque boxes, there was described in reference 1 results 'uj;
of compression tests. Torsion tests of the same s_pe_ci_men _ .'
are reported in this paper. The torsion tests are de- "_ m
signed to give information on the following points:

1. The agreement between the measured twist in_anJu‘
reinforced box and the theoretical twist for
a simplified box without reinforcements given'
by R. Bredt (reference 2, p. 270)

2.- The effect of buckling on the torsional stiff—
ness
2 NASA Technical Note.No. 878

5. The magnitude of the shearing stresses and of
the induced stresses due to bending

4. The variation in twist of the hex with distance
from the ends -

5. The effect of the longitudinals and bulkheads
on the stiffness of the bOx

The dimensions of the monocoo ue—box specimen are
given in figure 1. The box was fabricated from 248— T ___
aluminum alloy; 0. O75~ inch sheet was used for the shear—
web sides, and 0.026— inch sheet reinforced by Z—stringers,
spaced 4 inches on centers, was used for the top and the
bottom sides of the box. The stringers Were fastened to
the sheet by l/8—inch brazier—head rivets, spaced 7/8 inch
on centers. There were four intermediate bulkheads and
antiroll members, spaced at 19 inches.