Definition

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Adverse yaw is a secondary effect of the application of the ailerons in aircraft. Its cause and effect can be explained as follows:

When the control column of an aircraft is moved to the right, the right aileron is deflected upwards, and the left aileron is deflected downwards. As a result the lift of the right wing is decreased and that of the left increased producing a roll to the right. However the deflection also causes increased drag of both ailerons, moreso on the lowered left side. The effect of this is to yaw the aircraft to the left, in the wrong direction for the turn, (away from the lowered wing).

It is this effect which is known as Adverse yaw, and the force which causes it is known as Aileron drag.

Minimising the effect of Adverse yaw

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Adverse yaw is countered by using the aircraft's rudder to perform a co-ordinated turn, however an aircraft designer can reduce the amount of correction required by careful design of the aileron. Two methods are common:

Frise ailerons

Frise ailerons are designed so that when up aileron is applied, some of the forward edge of the aileron will protrude downward into the airflow, causing increased drag on this (down-going) wing. This will counter the drag produced by the other aileron, thus reducing adverse yaw.

Unfortunately, as well as reducing adverse yaw, Frise ailerons will increase the overall drag of the aircraft, and therefore they are less popular in aircraft where minimising drag is important (e.g. in a glider)

Differential ailerons

As it is the downwards deflection of an aileron that causes aileron drag, a simple way of eliminating adverse yaw would be to rely solely on the upward deflection of the opposite wing to cause the aircraft to roll. However, this would lead to a slow roll rate - and therefore a better solution is to make a compromise between adverse yaw and roll rate. This is what occurs in Differential ailerons.

As can be seen from the diagram, the down-going aileron moves through a smaller angle than the up-going aileron, reducing the amount of aileron drag, and thus reducing the effect of adverse yaw.

Aerodynamics