Aircraft Roll Control

 AILERONS AND SPOILERS

The ailerons are the primary flight control surfaces used to provide lateral (roll) control of the aircraft; that is; they control aircraft movement about the longitudinal axis. They are usually mounted on the trailing edge of the wing near the tip. They are rigged so that when one is moving upward on one wing, the other is moving downward on the opposite wing. This movement changes the camber of the respective wings, with the down-ward-moving aileron increasing the camber and lift, and the upward-moving aileron decreasing the camber and lift.

Since the ailerons are located outboard of the roll axis, this change in camber will result in a rolling motion. Large turbine aircraft often employ two sets of ailerons, one set being approximately mid-wing or immediately outboard of the inboard flaps, and the other set being in the conventional location near the wingtips.

The outboard ailerons become active whenever the flaps are extended beyond a fixed setting (at low speeds). As the flaps are retracted, the outboard aileron control system is "locked out" and flairs with the basic wing shape. Thus, during cruise flight at comparatively high speeds, only the inboard ailerons are used for control. The outboard ailerons are active during landings and other slow-flight operations. This is primarily to reduce wing-bending moments.

The ailerons are moved by means of a control wheel or stick in the cockpit. If a pilot wants to roll the airplane to the right, he or she turns the wheel or stick to the right. After the desired degree of bank is obtained, the wheel is returned to neutral to stop the roll. During normal turns of an airplane, the movement of the ailerons is coordinated with movements of the rudder and elevators to provide a banked horizontal turn without "slip' or "skid."

- A slip, or sideslip, is a movement of an airplane partially sideways. In a turn, the slip is downward and inward toward the turn.

- A skid in a turn is a movement of the airplane sideways and outward from the turn.

ADVERSE YAW

Aileron control in an airplane is complicated somewhat by an effect called adverse yaw. An aileron that moves down at the trailing edge of a wing creates considerably more drag than the aileron on the opposite wing that moves upward the same amount. Also due to the induced turn the down-going wing will have a lesser speed than that of the up-going wing increasing the drag on the up-going wing. 

Therefore, if the ailerons were rigged to move the same distance in response to the movement of the cockpit control, the drag of the downward moving aileron would cause the airplane to turn toward the side on which the downward-moving aileron is located. Thus, a pilot wishing to make a left turn would move the control to the left, causing the right aileron to move downward, but the drag caused by the aileron would cause the airplane to turn to the right, except for strong rudder control.

To overcome adverse yaw, the ailerons of an airplane are rigged for differential movement. The differential control causes the up- moving aileron to move a greater distance than the down-moving aileron. The amount of differential is sufficient to balance the drag between the ailerons, thus eliminating the yaw effect.