Ailerons are a primary flight control surface which control movement about the longitudinal axis of an aircraft. This movement is referred to as "roll".
The ailerons are attached to the outboard trailing edge of each wing and move in the opposite direction from each other. Ailerons are connected by cables, bellcranks, pulleys, and/or push-pull tubes to a control wheel or control stick.
In some large aircraft, two ailerons are mounted on each wing. In this configuration, both ailerons on each wing are active during slow speed flight. However, at higher speed, the outboard aileron is locked and only the inboard or high speed aileron is functional.
Ailerons are made for lateral control, producing a rolling moment by increasing the lift on one wing and decreasing it on the other.
Moving the yoke or control stick to the right results in the aileron mounted on the right wing to deflect upward while, at the same time, the aileron on the left wing deflects downward. The upward deflection of the right aileron reduces the camber of the wing resulting in decreased lift on the right wing. Conversely, the downward deflection of the left aileron results in an increase in camber and a corresponding increase in lift on the left wing. The differential lift between the wings results in the aircraft rolling to the right.
Since the downward deflected aileron produces more lift as evidenced by the wing raising, it also produces more induced drag, whereas the reduced lift on the down-going wing gives a decrease in induced drag. This added drag causes the wing to slow down slightly. The difference in drag on the two wings produces a yawing moment which is opposite to the rolling moment, that is, a roll to the left produces a yawing moment to the right. This is known as adverse yaw.
From the pilot's perspective, the yaw is opposite the direction of the bank. So, application of rudder is used to counteract adverse yaw. The amount of rudder control required is greatest at low airspeeds, high angles of attack, and with large aileron deflections.
In order to reduce the effects of adverse yaw, manufacturers have engineered 4 systems: differential ailerons, frise-type ailerons, coupled ailerons and rudder, and flaperons.
With differential ailerons, one aileron is raised a greater distance than the other aileron is lowered for a given movement of the control wheel or control stick. This produces an increase in drag on the descending wing.
The aileron that is being raised pivots on an offset hinge. This projects the leading edge of the aileron into the airflow and creates drag. It helps equalize the drag created by the lowered aileron on the opposite wing and reduces adverse yaw.
Example of aircraft with frise-type ailerons is Progressive Aerodyne SeaRey.
Coupled ailerons and rudder are linked controls. This is accomplished with rudder-aileron interconnect springs, which help correct for aileron drag by automatically deflecting the rudder at the same time the ailerons are deflected.
They combine both aspects of flaps and ailerons. In addition to controlling the bank angle of an aircraft like conventional ailerons, flaperons can be lowered together to function much the same as a dedicated set of flaps. The pilot retains separate controls for ailerons and flaps. Mostly used on a large jets.
Example of flaperons could be found on Boeing 777.