Adverse yaw

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, causing the aircraft to roll to the right. As the right wing descends, its lift vector, which is perpendicular to the relative motion, tilts forward and therefore has a forward component. Conversely, as the left wing moves up, its lift vector tilts back and therefore has an aft force component. The fore/aft lift force components on the right and left wings constitute the adverse yaw moment. There is often an additional adverse yaw contribution from a profile drag imbalance between the right and left, although the lift-tilting mechanism typically dominates. The net effect is a tendency to yaw the aircraft to the left, in the wrong direction for the turn.

Minimizing the effect of Adverse yaw

Adverse yaw is countered by using the aircraft's rudder to perform a coordinated turn, however an aircraft designer can reduce the amount of correction required by careful design of the aileron. Three 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 minimizing drag is important (e.g. in a glider)

Differential ailerons

Because downwards deflection of an aileron typically causes more profile drag than an upwards deflection, a simple way of mitigating adverse yaw would be to rely solely on the upward deflection of the opposite aileron 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. The De Havilland Tiger Moth biplane uses this method of roll control to avoid adverse yaw problems.

Roll spoilers

On large aircraft where rudder use is inappropriate at high speeds, roll spoilers can be used to minimise adverse yaw. To function as a lateral control, the spoiler is raised on the down-going wing (up aileron) and remains retracted on the other wing. The raised spoiler increases the drag, and so the yaw is in the same direction as the roll. [Oxford Aviation Academy (2007), JAA ATPL 13: Principles of Flight, Transair]

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