534 STATIC STABILITY AND CONTROL Adverse Yaw
A pilot initiates a turn from straight flight by rolling the airplane in the desired direction of the turn while at the same time applying a little rudder in order to start the airplane yawing in the desired direction. If done properly, a so-called coordinated turn is achieved where the resultant of the gravitational and inertial forces remains perpendicular to the wing. The pilot may feel a little heavier in a coordinated turn but, at least, forces tending to throw him or her to one side or the other are not experienced. A full pitcher of beer will not slosh in a coordinated turn. (Note that FAA regulations forbid the consumption of alcohol by a pilot eight hours prior to flying.)
A pair of aileron surfaces, which are simply plain flaps that travel symmetrically when deflected, will usually produce a motion known as adverse yaw. At an angle of attack, a flap deflected downward will produce a drag increment greater than that produced by the same deflection upward. Thus, for example, suppose the right aileron moves up and the left one moves down to initiate a turn to the right. The higher drag on the left aileron will produce a yawing moment that tends to yaw the airplane to the left, opposite to what was desired. This is known as adverse yaw due to ailerons, a characteristic that can make an airplane uncomfortable to fly.
To alleviate adverse yaw, the mechanical linkages are sometimes designed so that, for a given control movement, the upward movement on one aileron is greater than the downward movement on the opposite aileron. For example, on the Cherokee 180, the aileron moves up 30° but down only 15°.
Another means of alleviating the adverse yaw is in the design of the aileron. Figure 8.31 illustrates a possible way of accomplishing this. The configuration shown in Figure 8.31, known as a Frise aileron, has the hinge point below the aileron surface. As the aileron is raised, the nose projects down into the flow, thereby increasing the drag on the up aileron.