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National Advisory Committee for Aeronautics, Report - The Design of Airplane Wing Ribs

The purpose of the investigation reported here was to
obtain information for use in the design of truss and
plywood forms, particularly with reference to wing ribs.
Tests were made on many designs of wing ribs, comparing
diferent types in carious sizes. ’Many tests were also
made on parallel-chord specimens of truss and plywood
forms in place of the actual ribs and on parts of wing
ribs, such as truss diagonals and sections of cap strips.

It was found that for ribs of any size or proportions,
when they were designed to obtain a well-balanced con—
struction and were carefully manufactured, distinct types
are of carious efiiciencies; the efficiency is based on the
strength per unit of weight. filith ideal construction the
truss comes first; second, a lightened and reinforced ply—
wood type; third. a full plywood web type with stifleners;
fourth, a plywood web with lightening holes and no rein—
forcing; a-nd fifth, a full web with no stiffeners. if a
type falls out of this order, the probable reason is either
that it is poorly designed or that it was designed with
some special consideration for manufacturing details and
is therefore not so strong for its weight as it can be made.

Each type has its place in airplane design because
manufacturing difi‘iculties set up practical limits for the
various types. For example, shallow trusses can not be
manufactured and assembled without great difi‘iculty.
Neither can a reinforced plywood truss be substituted for
a full plywood type when to obtain maximum efliciency
an excessirely thin plywood must be used.

In all types of ribs the heavier are the stronger per
unit of weight. Reductions in the weight of wing ribs
are accompanied eren in eflicient designs by a much.
greater proportional reduction in strength.

Obtaining maxi-mum eficiency in truss designs would
require all diagonals to be of cruciform cross section and
all members to be proportioned according to their indi-
vidual stresses.