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.

Causes

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.^[1]:327 Some pilot training manuals focus mainly on the additional drag caused by the downward-deflected aileron^[2][3] and make only brief^[4] or indirect^[5] mentions of roll effects. In fact the rolling of the wings usually causes a greater effect than the ailerons.^[6] Assuming a roll rate to the right, as in the diagram, the causes are explained as follows:

Lift vectors

By definition, lift is perpendicular to the oncoming flow.^[1]:18 As the left wing moves up, its effective angle of attack is decreased,^[1]:361 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.

Profile drag

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.

Induced drag

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,^[1]:351 while the other mechanisms described above persist.

Minimizing the adverse yaw

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

Aileron to rudder mixing

As intended, the rudder is the most powerful and efficient means of managing yaw but mechanically coupling it to the ailerons is impractical. Electronic coupling is commonplace in fly-by-wire aircraft.

Lift coefficient

As the tilting of the left/right lift vectors is the major cause to adverse yaw, an important parameter is the magnitude of these lift vectors, or the aircraft's lift coefficient to be more specific. Flight at low lift coefficient (or high speed compared to minimum speed) produces less adverse yaw.^[1]:365

Yaw stability

A strong directional stability is the first way to reduce adverse yaw.^[7] This is influenced by the vertical tail moment (area and lever arm about gravity center).

Differential aileron deflection

Illustration of a Differential aileron

The geometry of most aileron linkages can be configured so as to bias the travel further upward than downward. By excessively deflecting the upward aileron, profile drag is increased rather than reduced and separation drag further aids in producing drag on the inside wing, producing a yaw force in the direction of the turn. Though not as efficient as rudder mixing, aileron differential is very easy to implement on almost any airplane and offers the significant advantage of reducing the tendency for the wing to stall at the tip first by limiting the downward aileron deflection and its associated effective increase in angle of attack.

Most airplanes use this method of adverse yaw mitigation — particularly noticeable on one of the first well-known aircraft to ever use them, the de Havilland Tiger Moth training biplane of the 1930s — due to the simple implementation and safety benefits.

Frise ailerons

Illustration of a Frise aileron

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 much more than applying rudder correction. Therefore they are less popular in aircraft where minimizing drag is important (e.g. in a glider).

Note: Frise ailerons are primarily designed to reduce roll control forces. Contrary to the illustration, the aileron leading edge has to be rounded to prevent flow separation and flutter at negative deflections.^[8] That prevents important differential drag forces.

Roll spoilers

On large aircraft where rudder use is inappropriate at high speeds or ailerons are too small at low speeds, roll spoilers (also called spoilerons) can be used to minimise adverse yaw or increase roll moment. 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.^[9]

References and notes

Collection of balanced-aileron test data, F.M. Rogallo, Naca WR-L 419