Can anyone explain to me the physical symptoms of, and aerodynamics behind, high speed control surface reversal ?
Mad (Flt) Scientist
22nd Oct 2006, 16:45
Well, there are a couple of different scenarios, depending upon the design characteristics of the aircraft. First of all, I'm going to consider that you mean aileron reversal, which is probably the most common type.
Two major factors affect the likelihood of aileron reversal: the stiffness of the wing, and whether the control surface is powered or manual. The reversal can then potenitally take the form of a force reversal, a purely aerodynamic reversal or an aeroelastic reversal.
First of all, assume a powered aileron, where the actuator(s) are considered irreversible. The aileron goes where the pilot wants, and it stays there. Aileron reversal, in this case, would occur if despite having trailing edge DOWN aileron, the net change in lift on the wing is negative i.e. what would normally be expected for trailing edge UP. There are two ways this can happen:
1. At high enough speeds, the position of the aileron may induce a high speed (shock) stall on the local wing; this will reduce the local lift. (This can also happen near the stall at low speeds, as the high gets locally 'overloaded' aerodynamically). This I'd call a "purely aerodynamic reversal"
2. The movement of the CP aftwards locally, due to the increased camber at the TE, causes the wing to twist TE up, LE down. If the wing is not very stiff this twist could induce a local negative change in AoA which is more powerful aerodynamically than the aileron; the net result is a reduction in lift, and what I referred to as an "aeroelastic reversal".
If you now assume unpowered ailerons, such that the feel force is a function of the loading of the surface, the first case (where the surface stalls, in effect) can also cause a drastic change in the feel forces, and so this would be a condition for a "force reversal" also.