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 lift and drag of each wing. 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 was first experienced by the Wright brothers when they were unable to perform controlled turns in their 1901 glider which had no vertical control surface. Orville Wright later described the glider's lack of directional control.

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:

During a positive rolling motion, the left wing moves upward. If an aircraft were somehow suspended in air with no motion other than a positive roll, then from the point of view of the left wing, air will be coming from above and striking the upper surface of the wing. Thus, the left wing will experience a small amount of oncoming airflow merely from the rolling motion. This can be conceptualized as a vector originating from the left wing and pointing towards the oncoming air during the positive roll, i.e. perpendicularly upwards from the left wing's surface. If this positive-rolling aircraft were additionally moving forward in flight, then the vector pointing towards the oncoming air will be mostly forward due to forward-moving flight, but also slightly upward due to the rolling motion. This is the dashed vector coming from the left wing in the diagram.

Thus, for the left wing of a forward-moving aircraft, a positive roll causes the oncoming air to be deflected slightly upwards. Equivalently, the left wing's effective angle of attack is decreased due to the positive roll. By definition, lift is perpendicular to the oncoming flow. The upward deflection of oncoming air causes the lift vector to be deflected backward. Conversely, as the right wing descends, its vector pointing towards the oncoming air is deflected downward and its lift vector is deflected forward. The backward deflection of lift for the left wing and the forward deflection of lift for the right wing results in an adverse yaw moment to the left, opposite to the intended right turn. This adverse yaw moment is present only while the aircraft is rolling relative to the surrounding air, and disappears when the aircraft's bank angle is steady.

Initiating a roll to the right requires a briefly greater lift on the left than the right. This also causes a greater induced drag on the left than the right, which further adds to the adverse yaw, but only briefly. Once a steady roll rate is established the left/right lift imbalance dwindles, while the other mechanisms described above persist.

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. A Frise aileron reduces this imbalance drag, as described further below.

There are a number of aircraft design characteristics which can be used to reduce adverse yaw to ease the pilot workload: