Before leaving the subject of ailerons, mention should be made of a behavior known as aileron reversal. As the name suggests, it is a behavior whereby the action of an aileron, as related to its deflection, is opposite to what one would normally expect. This anomalous behavior is primarily associated with airplanes that have thin, flexible, sweptback wings that operate at high speeds. As a result of the aerodynamic moment, such a wing will tend to twist as the aileron is deflected. For example, an up aileron, which is intended to drop the wing, may instead twist the wing nose up sufficiently to cause a net increase in the wing lift.
To pursue this behavior, consider Figure 8.34, which illustrates the situation with a simplified model. A symmetrical airfoil section is shown mounted on a torsional spring with a spring constant of ke. When the flap is deflected through a positive angle 8, an aerodynamic moment is produced that causes the airfoil to rotate through an angle a (shown positively). This results in an opposing moment generated by the spring that, at some a, balances the aerodynamic moment. Thus,
kff(x cjc Cm& 8
C, = (C, a + Ch 8)
Figure 8.34 A wing section mounted on a torsional spring to illustrate aileron reversal.
Since CMs is negative, it is obvious that, for a given ke, there is a q above which the flap deflection will produce a decrease in the lift.
Aileron reversal can be avoided by the obvious means of limiting the operational q or by stiffening the wing; either method can impose operational penalties on the airplane. A less obvious method is used to lock out conventional outboard ailerons at high speeds and employ spoiler control instead.