Roll Control

The angle of bank of the airplane is controlled by the ailerons. The primary function of these controls is to produce a rolling moment, although they frequently introduce a yawing moment as well. The effectiveness of the ailerons in producing rolling and yawing moments is described by the two control derivatives dCJd8a and ЭC„/d8a. The aileron angle 8a is defined as the mean value of the angular displacements of the two ailerons. It is positive when the right aileron movement is downward (see Fig. 3.20). The derivative dC/d8a is normally negative, right aileron down producing a roll to the left.

For simple flap-type ailerons, the increase in lift on the right side and the de­crease on the left side produce a drag differential that gives a positive (nose-right) yawing moment. Since the normal reason for moving the right aileron down is to ini­tiate a turn to the left, then the yawing moment is seen to be in just the wrong direc­tion. It is therefore called aileron adverse yaw. On high-aspect-ratio airplanes this tendency may introduce decided difficulties in lateral control. Means for avoiding this particular difficulty include the use of spoilers and Frise ailerons. Spoilers are il­lustrated in Fig. 3.21. They achieve the desired result by reducing the lift and increas­ing the drag on the side where the spoiler is raised. Thus the rolling and yawing mo­ments developed are mutually complementary with respect to turning. Frise ailerons diminish the adverse yaw or eliminate it entirely by increasing the drag on the side of the upgoing aileron. This is achieved by the shaping of the aileron nose and the choice of hinge location. When deflected upward, the gap between the control sur­face and the wing is increased, and the relatively sharp nose protrudes into the stream. Both these geometrical factors produce a drag increase.

The deflection of the ailerons leads to still additional yawing moments once the airplane starts to roll. These are caused by the altered flow about the wing and tail. These effects are discussed in Sec. 5.7 (СПр), and are illustrated in Figs. 5.12 and 5.15. P

A final remark about aileron controls is in order. They are functionally distinct from the other two controls in that they are rate controls. If the airplane is restricted only to rotation about the x axis, then the application of a constant aileron angle re­sults in a steady rate of roll. The elevator and rudder, on the other hand, are displace­ment controls. When the airplane is constrained to the relevant single axis degree of freedom, a constant deflection of these controls produces a constant angular displace­ment of the airplane. It appears that both rate and displacement controls are accept­able to pilots.