Page 381 - Aircraft Stuctures for Engineering Student

Stress analysis of

aircraft components

In Chapter 9 we established the basic theory for the analysis of open and closed
section thin-walled beams subjected to bending, shear and torsional loads. In addi-
tion, methods of idealizing stringer stiffened sections into sections more amenable
to analysis were presented. We now extend the analysis to actual aircraft components
including tapered beams, fuselages, wings, frames and ribs; also included are the
effects of cut-outs in wings and fuselages. Finally, an introduction is given to the
analysis of components fabricated from composite materials.
Aircraft structural components are, as we saw in Chapter 7, complex, consisting
usually of thin sheets of metal stiffened by arrangements of stringers. These structures
are highly redundant and require some degree of simplification or idealization before
they can be analysed. The analysis presented here is therefore approximate and the
degree of accuracy obtained depends on the number of simplifying assumptions
made. A further complication arises in that factors such as warping restraint,
structural and loading discontinuities and shear lag significantly affect the analysis;
we shall investigate these effects in some simple structural components in Chapter
11. Generally, a high degree of accuracy can only be obtained by using computer-
based techniques such as the finite element method (see Chapter 12). However, the
simpler, quicker and cheaper approximate methods can be used to advantage in
the preliminary stages of design when several possible structural alternatives are
being investigated; they also provide an insight into the physical behaviour of
structures which computer-based techniques do not.

Major aircraft structural components such as wings and fuselages are usually tapered
along their lengths for greater structural efficiency. Thus, wing sections are reduced
both chordwise and in depth along the wing span towards the tip and fuselage
sections aft of the passenger cabin taper to provide a more efficient aerodynamic
and structural shape.
The analysis of open and closed section beams presented in Chapter 9 assumes that
the beam sections are uniform. The effect of taper on the prediction of direct stresses
produced by bending is minimal if the taper is small and the section properties are