Page 242 - Aircraft Stuctures for Engineering Student

7.4 Fabrication of structural components 225

Although the thin skin is efficient for resisting shear and tensile loads, it buckles
under comparatively low compressive loads. Rather than increase the skin thickness
and suffer a consequent weight penalty, stringers are attached to the skin and ribs,
thereby dividing the skin into small panels and increasing the buckling and failing
stresses. This stabilizing action of the stringers on the skin is, in fact, reciprocated
to some extent although the effect normal to the surface of the skin is minimal.
Stringers rely chiefly on rib attachments for preventing column action in this direc-
tion. We have noted in the previous paragraph the combined action of stringers
and skin in resisting axial and bending loads.
The role of spar webs in developing shear stresses to resist shear and torsional loads
has been mentioned previously; they perform a secondary but significant function in
stabilizing, with the skin, the spar flanges or caps which are therefore capable of
supporting large compressive loads from axial and bending effects. In turn, spar
webs exert a stabilizing influence on the skin in a similar manner to the stringers.
While the majority of the above remarks have been directed towards wing
structures, they apply, as can be seen by referring to Figs 7.7 and 7.8, to all the
aerodynamic surfaces, namely wings, horizontal and vertical tails, except in the
obvious cases of undercarriage loading, engine thrust etc.
Fuselages, while of different shape to the aerodynamic surfaces, comprise members
which perform similar functions to their counterparts in the wings and tailplane.
However, there are differences in the generation of the various types of load. Aero-
dynamic forces on the fuselage skin are relatively low; on the other hand, the fuselage
supports large concentrated loads such as wing reactions, tailplane reactions, under-
carriage reactions and it carries payloads of varying size and weight, which may cause
large inertia forces. Furthermore, aircraft designed for high altitude flight must
withstand internal pressure. The shape of the fuselage cross-section is determined
by operational requirements. For example, the most efficient sectional shape for a
pressurized fuselage is circular or a combination of circular elements. Irrespective
of shape, the basic fuselage structure is essentially a single cell thin-walled tube
comprising skin, transverse frames and stringers; transverse frames which extend
completely across the fuselage are known as bulkheads. Three different types of
fuselage are shown in Figs 7.7, 7.8 and 7.9. In Fig. 7.7 the fuselage is unpressurized
so that, in the passenger-carrying area, a more rectangular shape is employed to
maximize space. The Harrier fuselage in Fig. 7.8 contains the engine, fuel tanks etc
SO that its cross-sectional shape is, to some extent, predetermined, while in Fig. 7.9
the passenger-carrying fuselage of the British Aerospace 146 is pressurized and
therefore circular in cross-section.

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7.4 Fabrication of structural components
The introduction of all-metal, stressed skin aircraft resulted in methods and types of
fabrication which remain in use to the present day. However, improvements in engine
performance and advances in aerodynamics have led to higher maximum lift, higher
speeds and therefore to higher wing loadings so that improved techniques of fabrica-
tion are necessary, particularly in the construction of wings. The increase in wing
loading from about 350N/m2 for 1917-18 aircraft to around 4800N/m2 for