A symmetrical section develops an upward aerodynamic force at the aileron hinge point when the aileron is deflected downward. This force acts behind the elastic axis and so produces a nose-down pitching moment. Because this moment is dependent on square of velocity, its value is higher at higher speed. If the wing lacks sufficient rigidity to resist this pitching moment, i.e., if it is an elastic wing, its nose will twist downward, its angle of attack will reduce, and the resultant lift will be lower. A situation occurs at some high speed when the moment is so large that there is total loss of lift when the aileron is deflected downward, and the aircraft rolls in the reverse direction. This is called aileron reversal.
An adverse effect when an aircraft rolls in the reverse direction of the aileron input. This can be caused by the following:i
. Aerodynamic twisting of the wing caused by ailerons as speed is increased, which may reduce, neutralize, or reverse the direction of the lift. Aileron reversal results if the aileron structure is insufficiently stiff in torsion. The effect is most pronounced at nearsonic speeds.ii
. In some aircraft, very low speeds. The descending aileron increases the angle of attack of that portion of the wing, resulting in stall of the wing that should be moving upward and causing it to drop.