Adverse yaw is a yaw moment on an aircraft which results from an aileron deflection and a roll rate, such as when entering or exiting a turn. It is called "adverse" because it acts opposite to the yaw moment needed to execute the desired turn. Adverse yaw has three mechanisms, listed below in decreasing order of importance. Assuming a roll rate to the right, as in the diagram below, these three mechanisms are explained as follows:
1) By definition, lift is perpendicular to the oncoming flow. Hence, as the left wing moves up, its lift vector tilts back; 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.
2) 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 father below.
3) Initiating the right roll rate 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 this mechanism disappears once a steady roll rate is established (and the left/right lift imbalance disappears), while mechanisms 1) and 2) persist.
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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 aircraft. Some methods are common:
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.
A strong directional stability is the first way to reduce adverse yaw.[1] That means important vertical tail moment (area and lever arm about gravity center).
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.
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.[2] That prevents important differential drag forces.
On large aircraft where rudder use is inappropriate at high speeds or ailerons are too small at low speeds, roll spoilers can be used to minimize 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.[3]
Collection of balanced-aileron test data, F.M. Rogallo, Naca WR-L 419