Control Surface Effectiveness and Reversal
To discuss the effectiveness and loss of a control surface, we allow a rigid rotation δ of the flap set by a flight control system (or similar) following steady flight conditions. A downward (clockwise) deflection of the control surface increases the lift over the wing section initiating the intended motion of the aeroplane. For instance, deploying an aileron control surface is meant to initiate a rolling motion. However, generally by deflecting the control surface an additional nose-down aerodynamic pitching moment is created tending to twist the wing in a direction reducing the lift over the wing section. In the case of the aileron, the rolling moment is consequently reduced by the reduced lift, which reduces the effectiveness of the aileron. As above in the discussion for the torsional divergence, the elastic stiffness is independent of the dynamic
pressure while aerodynamic loads are not. Thus, there may be a critical dynamic pressure when the deployment of the control surface does not initiate any motion of the aircraft. In addition, for dynamic pressure exceeding the critical value, the control surface effect is reversed. The phenomenon of control surface reversal is mainly encountered for aileron as these are usually situated near the wing tip where the wing stiffness is relatively low. While normally it does not cause structural failure of the aircraft, its effects are undesirable.