Adverse yaw

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in profile drag between the upward and downward deflected ailerons, the difference in lift and thus induced drag between left and right wings, as well as an opposite rotation of each wing's lift vector about the pitch axis due to the rolling trajectory of the aircraft. The effect can be greatly minimized with ailerons deliberately designed to create drag when deflected upward and/or mechanisms which automatically apply some amount of coordinated rudder. As the major causes of adverse yaw vary with lift, any fixed-ratio mechanism will fail to fully solve the problem across all flight conditions and thus any manually operated aircraft will require some amount of rudder input from the pilot in order to maintain coordinated flight.

Adverse yaw is a secondary effect of the inclination of the lift vectors on the wing due to its rolling velocity and of the application of the ailerons. Some pilot training manuals focus mainly on the additional drag caused by the downward-deflected aileron and make only brief or indirect mentions of roll effects. In fact the rolling of the wings usually causes a greater effect than the ailerons. Assuming a roll rate to the right, as in the diagram, the causes are explained as follows:

By definition, lift is perpendicular to the oncoming flow. As the left wing moves up, its effective angle of attack is decreased, so its lift vector tilts back. Conversely, as the right wing descends, its lift vector tilts forward. The result is an adverse yaw moment to the left, opposite to the intended right turn.

The downward aileron deflection on the left increases the airfoil camber, which will typically increase the profile drag. Conversely, the upward aileron deflection on the right will decrease the camber and profile drag. The profile drag imbalance adds to the adverse yaw. The exception is on a Frise aileron, described further below.

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