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National Advisory Committee for Aeronautics, Technical Notes - Relative Structural Efficiencies of Flat Balsa Core Sandwich and Stiffened Panel Construction

An analysis is made and charts are presented for the determination
of regions of efficient application of flat balsa-core sandwich and
stiffened-panel construction for a large range of design requirements.
Optimum sandwiches were found to have relatively low values of the ratio
of core thickness to face thickness.

The choice of the proper type of construction for the compression-
carrying upper skin of an airplane wing is affected by many factors such
as cost, ease of production, and structural efficiency: Although numer-
ous types of construction have been proposed, the most common in the past
was the longitudinally stiffened compression panel. Extensive studies
(references 1 to 10) have been made of the compressive strength of
stiffened panels with the result that the proportions of such panels-
can readily be chosen to give the maXimum structural efficiency that
can be obtained with this type of construction.

Efficiency studies of the type used for the stiffened panels lead
logically to the conclusion that no one type of construction is univer-
sally the most efficient. For example, in a comparison of stiffened—
panel with multiweb wing construction, Gerard showed in reference 11
that for thin wings carrying high bending moments, the multiweb wing
construction is more efficient.

One type of construction, as yet not evaluated,‘is the end-grain_
balsa—core sandwich. Simple physical reasoning does not indicate in
advance the range of loading conditions for which the sandwich is most
efficient. Consequently, some quantitative studies must be made if the
region is to be determined for which this type of construction is more
efficient.

In the present paper an analysis is made to determine the respec-
tive regions of application in which the sandwich and the stiffened panel
represent the more structurally efficient construction when used as a
compression cover for a wing structure. The effects of transverse air
load and shear stress due to torsion in the wing on the strength of the
structure are not considered.