Introduction to Aircraft Aeroelasticity and Loads

9

Static Aeroelasticity – Effect of Wing Flexibility on Control Effectiveness

It was shown in Chapter 5 how the use of control surfaces changed the effective camber of an aerofoil and that this could be used to change the lift. Thus, control surfaces (e.g. ailerons, rudder) are used to manoeuvre an aircraft in flight and their sizing is an important issue when the aircraft is designed. It is important to know how sensitive an aircraft is to application of the control surfaces and what loads are generated. This is of particular significance for military aircraft where the need to manoeuvre rapidly is essential, but is also of course important in terms of the performance of commercial aircraft.

This chapter will consider the effect that aeroelastic deflections of the flexible wing have on the aerodynamic influence, or effectiveness, of the control surfaces in comparison to the rigid wing. It will be shown that as the speed increases the effectiveness reduces until at some critical speed – the reversal speed – there is no response to application of the control surface. At speeds greater than the reversal speed, the action of the controls reverses, a phenomenon known as control reversal. Although not necessarily disastrous, it is unacceptable that at speeds near to the reversal speed, the aircraft responds either very slowly or not at all to application of the controls, and that the opposite response to that demanded occurs beyond the reversal speed.

There are two basic ways that the ...