Control reversal

Control reversal is an adverse affect on the controllability of aircraft. To the pilot it appears that the controls have reversed themselves; in order to roll to the left, for instance, they have to push the control stick to the right, opposite of the normal direction.

There are several causes for this problem: pilot error, effects of high speed flight, incorrectly connected controls, and various coupling forces on the aircraft.

Pilot error is the most common cause of control reversal. In unusual attitudes it is not uncommon for the pilot to become disoriented and start feeding in incorrect control movements in order to regain level flight. This is particularly common when using helmet mounted display systems, which introduce graphics that remain steady in the pilot's view, notably when using a particular form of attitude display known as an inside-out display.

Incorrectly connected controls is another common cause of this problem. It is a recurring problem after maintenance on aircraft, notably homebuilt designs that are being flown for the first time after some minor work. However it is not entirely uncommon on commercial aircraft, and has been the cause of several near-accidents.

Another version of the problem occurs when the amount of airflow over the wing becomes great enough that the force generated by the ailerons is enough to twist the wing itself. For instance when the aileron is deflected upwards in order to make that wing move down, the wing twists in the opposite direction. The net result is that the airflow is directed down instead of up and the wing moves upward, opposite of what was expected. This form of control reversal is often lumped in with a number of "high speed" effects as compressibility.

Due to the unusually high speeds for the time that it could be dived at, this problem of aileron reversal became apparent on the Supermarine Spitfire when it was wished to increase the lateral manouverabilty (rate of roll) by increasing the aileron area. The aircraft had a wing designed originally for an aileron reversal speed of 580 mph and any attempt to increase the aileron area would have resulted in the wing twisting when the larger ailerons were applied at high speed, the aircraft then rolling in the opposite direction to that intended by the pilot. The problem of increasing the rate of roll was temporarily alleviated with the introduction of "clipped" wing tips (to reduce the aerodynamic load on the tip area, allowing larger ailerons to be used) until a new, stiffer wing could be incorporated. This new wing was introduced in the Mark XXI and had a theoretical aileron reversal speed of 825 mph.

Finally the Wright Brothers suffered yet another form of control reversal, one normally referred to as adverse yaw. In their pre-Flyer gliders they continued to encounter a problem where the glider would start to turn one direction, then suddenly reverse direction and spin into the ground. They eventually cured the problem by adding a moving rudder system, now found on all aircraft.

The root cause of the problem was dynamic. Warping the wing did what was expected in terms of lift, thereby rolling the plane, but also had an unexpectedly large effect on drag. The result was that the upward-moving wing was dragged backwards, yawing the glider. If this yaw was violent enough the additional speed on the lower wing as it was driven forward would make it generate more lift, and reverse the direction of the roll.

External links

• A320 Incident (http://www.cs.york.ac.uk/hise/safety-critical-archive/2001/0362.html) - incorrect maintenance led to reversal of the roll control on the pilot's controls

References:

• Jeffrey Quill OBE, AFC, FRAeS Spitfire - A Test Pilot’s Story - Arrow Books 1983-89 - ISBN 0-09-937020-4