Page 239 - Aircraft Stuctures for Engineering Student

7.3 Function of structural components 223

In Chapter 8 we shall examine in detail the calculation of ground and air loads for a
variety of cases.

7.3 Function of structural components

The basic functions of an aircraft’s structure are to transmit and resist the applied
loads; to provide an aerodynamic shape and to protect passengers, payload, systems
etc. from the environmental conditions encountered in flight. These requirements, in
most aircraft, result in thin shell structures where the outer surface or skin of the
shell is usually supported by longitudinal stiffening members and transverse frames
to enable it to resist bending, compressive and torsional loads without buckling.
Such structures are known as semi-monocoque, while thin shells which rely entirely
on their skins for their capacity to resist loads are referred to as monocoquc’.
First, we shall consider wing sections which, while performing the same function,
can differ widely in their structural complexity, as can be seen by comparing Figs
7.7 and 7.8. In Fig. 7.7, the wing of the small, light passenger aircraft, the De Havil-
land Canada Twin Otter, comprises a relatively simple arrangement of two spars,
ribs, stringers and skin, while the wing of the Harrier in Fig. 7.8 consists of numerous
spars, ribs and skin. However, no matter how complex the internal structural arrange-
ment the different components perform the same kind of function. The shape of
the cross-section is governed by aerodynamic considerations and clearly must be
maintained for all combinations of load; this is one of the functions of the ribs.
They also act with the skin in resisting the distributed aerodynamic pressure loads;
they distribute concentrated loads (e.g. undercarriage and additional wing store
loads) into the structure and redistribute stress around discontinuities, such as under-
carriage wells, inspection panels and fuel tanks, in the wing surface. Ribs increase the
column buckling stress of the longitudinal stiffeners by providing end restraint and
establishing their column length; in a similar manner they increase the plate buckling
stress of the skin panels. The dimensions of ribs are governed by their spanwise
position in the wing and by the loads they are required to support. In the outer
portions of the wing, where the cross-section may be relatively small if the wing
is tapered and the loads are light, ribs act primarily as formers for the aerofoil
shape. A light structure is sufficient for this purpose whereas at sections closer to
the wing root, where the ribs are required to absorb and transmit large concentrated
applied loads, such as those from the undercarriage, engine thrust and fuselage
attachment point reactions, a much more rugged construction is necessary. Between
these two extremes are ribs which support hinge reactions from ailerons, flaps and
ot‘her control surfaces, plus the many internal loads from fuel, armament and systems
installations.
The primary function of the wing skin is to form an impermeable surface for
supporting the aerodynamic pressure distribution from which the lifting capability
of the wing is derived. These aerodynamic forces are transmitted in turn to the ribs
and stringers by the skin through plate and membrane action. Resistance to shear
and torsional loads is supplied by shear stresses developed in the skin and spar
webs, while axial and bending loads are reacted by the combined action of skin and
stringers.