Page 565 - Aircraft Stuctures for Engineering Student

546 Elementary aeroelasticity

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Fig. 13.4 Effect of wing sweep on wing divergence speed.

showed that wings with moderate or large sweepback cannot diverge. The opposite of
course is true for swept-forward wings where bending deflections have a destabilizing
effect and divergence speeds are extremely low. The determination of lift distributions
and divergence speeds for swept-forward wings is presented in Ref. 3.

The flexibility of the major aerodynamic surfaces (wings, vertical and horizontal tails)
adversely affects the effectiveness of the corresponding control surfaces (ailerons,
rudder and elevators). For example, the downward deflection of an aileron causes
a nose down twisting of the wing which consequently reduces the aileron incidence.
Thus, the wing twist tends to reduce the increase in lift produced by the aileron deflec-
tion, and thereby the rolling moment to a value less than that for a rigid wing. The
aerodynamic twisting moment on the wing due to aileron deflection increases as
the square of the speed but the elastic restoring moment is constant since it depends
on the torsional stiffness of the wing structure. Therefore, ailerons become markedly
less effective as the speed increases until, at a particular speed, the aileron reversal
speed, aileron deflection does not produce any rolling moment at all. At higher
speeds reversed aileron movements are necessary in that a positive increment of
wing lift requires an upward aileron deflection and vice versa.
Similar, less critical, problems arise in the loss of effectiveness and reversal of the
rudder and elevator controls. They are complicated by the additional deformations
of the fuselage and tailplane-fuselage attachment points, which may be as important
as the deformations of the tailplane itself. We shall concentrate in this section on the
problem of aileron effectiveness and reversal.

13.2.1 Aileron effectiveness and reversal (two-dimensional case)

We shall illustrate the problem by investigating, as in Section 13.1, the case of a wing-
aileron combination in a two-dimensional flow. In Fig. 13.5 an aileron deflection <
produces changes AL and AM, in the wing lift, L, and wing pitching moment M,;