Page 560 - Aircraft Stuctures for Engineering Student

13.1 load distribution and divergence 541

In this chapter we shall concentrate on the purely structural aspects of aeroelasticity;
its effect on aircraft static and dynamic stability is treated in books devoted primarily
to aircraft stability and control''2.

rioad di stribution and divergence

Redistribution of aerodynamic loads and divergence are closely related aeroelastic
phenomena; we shall therefore consider them simultaneously. It is essential in the
design of structural components that the aerodynamic load distribution on the com-
ponent is known. Wing distortion, for example, may produce significant changes in
lift distribution from that calculated on the assumption of a rigid wing, especially
in instances of high wing loadings such as those experienced in manoeuvres and
gusts. To estimate actual lift distributions the aerodynamicist requires to know the
incidence of the wing at all stations along its span. Obviously this is affected by
any twisting of the wing which may be present.
Let us consider the case of a simple straight wing with the centre of twist (or flexural
centre, see Chapters 9 and 10) behind the aerodynamic centre (see Fig. 13.1). The
moment of the lift vector about the centre of twist causes an increase in wing incidence
which produces a further increase in lift, leading to another increase in incidence and
so on. At speeds below a critical value, called the divergence speed, the increments in
lift converge to a condition of stable equilibrium in which the torsional moment of the
aerodynamic forces about the centre of twist is balanced by the torsional rigidity of
the wing. The calculation of lift distribution then proceeds from a knowledge of the
distribution of twist along the wing. For a straight wing the redistribution of lift
usually causes an outward spanwise movement of the centre of pressure, resulting
in greater bending moments at the wing root. In the case of a swept wing a reduction
in streamwise incidence of the outboard sections due to bending deflections causes a
movement of the centre of pressure towards the wing root.
All aerodynamic surfaces of the aircraft suffer similar load redistribution due to
distortion.

13.1 .I Wing torsional divergence (two-dimensional case)

The most common divergence problem is the torsional divergence of a wing. It is
useful, initially, to consider the case of a wing of area S without ailerons and in a

Lift
A
Wing twist, Centre of twist

I
Aerodynamic centre
Fig. 13.1 Increase of wing incidence due to wing twist.