When a pilot rotates the control column to the left:
As we know, any increase in lift is accompanied with an increase in induced drag. So the right aileron experiences more drag than the left aileron, resulting in yaw in the opposite direction of the roll.
This is known as adverse aileron yaw.
To prevent this yaw from occurring, the pilot must apply appropriate rudder in the direction of the roll. A properly coordinated entry to a turn should involve no yaw – only roll.
The adverse aileron yaw is the result of uneven drag being created by the two ailerons. Aircraft designers often incorporate features that help to even out the aileron drag somewhat and reduce the adverse aileron yaw. Two of these features are Differential Ailerons and Frise Ailerons.
Differential Ailerons are an arrangement where the down-going aileron deflects a smaller amount than the up-going aileron. The larger deflection on the up-going aileron generates additional form drag, helping to even out the total drag created by both ailerons.
With the drag generated by both ailerons being closer to even, the adverse aileron yaw is reduced.
Frise ailerons are designed with a wedge that protrudes into the airflow only on the side that requires additional drag.
The aileron is hinged at the top and has a wedge at the bottom such that the wedge is up inside the wing on the downward deflected aileron but the wedge protrudes into the airflow on the upward deflected aileron, creating additional drag.
Some aircraft have frise ailerons that have differential deflection.