# How does aileron reversal work?

I want to know how aileron reversal takes place when control reversal velocity is higher than divergence speed and operating speed is in between them (for conventional wing)

• I don't understand your question. You have a condition in your question that states "the operating speed is in between [the control reversal velocity and the divergence speed], with the divergence speed being the lower bound. If you operate an aircraft above the divergence speed, the wing will have a structural failure. Mar 19, 2014 at 16:02
• @DeltaLima: Isn't the divergence speed the speed at which the drag of the wing starts to rise steeply due to transonic effects? Jan 21, 2019 at 4:42

As I understand it, aileron reversal can occur in two regimes of flight, high speed and near stall.

Near stall, the wing is very near it's critical angle of attack. When the pilot inputs let's say a left turn, the right aileron will deflect downward. This downward deflection increases the AOA of the outboard portion of the right wing such that the lift generated by that outboard portion is lower (drag higher as well) than the lift generated on the opposite wing.

You can kind-of visualize this by thinking of a wing operating on the backside of the lift curve.

Aileron reversal at high speed, (again, as I understand it) can occur when the wing isn't stiff enough tortionally to prevent twisting about the aircraft's lateral axis when a control input is made. Some old warplanes had this characteristic at high speeds.

For example, the pilot makes a control input to the left. The right aileron deflects downward, and instead of raising the overall AOA of the wing, it lowers it because the aileron acts like a trim tab, twisting the wing's leading edge downward.

Your question is asking: If the control reversal velocity is calculated to be higher than the divergence velocity, (When the wing breaks apart), how then can control reversal take place in flight?

The answer would be when it occurs near a stall.

• The divergence speed isn't the speed where the wing breaks apart, it's the speed where drag begins to sharply increase due to transonic mach effects. Sep 27, 2019 at 23:30
• @Sean divergence speed in aeroelasticity (or aeroelastic divergence speed) is the speed above which the wings twist off. Divergence can have different meanings depending on the context. Jan 28, 2020 at 11:43

Control reversal speed is way lower than divergence speed. It may actually be lower than operating speed for some aircraft, which is why they have inboard ailerons/flaperons and/or use spoilers (that don't suffer control reversal) for roll control at high speeds.

Control reversal (at high speed; there is also control reversal at stall, which is different) occurs because the force generated by ailerons twists the wing sufficiently so that the aileron starts to act more like a trim tab. For that the wing has to flex, but it does not have to flutter at all.

Spoilers act closer to the wing mid-chord, so they don't twist it much and thus don't suffer reversal. In fact they twist the wing a bit, because pre-stall lift acts more forward than post-stall lift, but this twists the wing forward and thus further reduces the lift, so still no control reversal.

At high speeds a large downward aileron deflection can cause the laminar flow to separate from the top of the wing and become turbulent. At this point the wing essentially ceases to produce lift, whereas the other wing has an upward aileron deflection and continues to produce lift. This rolls the aircraft in the opposite direction than intended. I have experienced this in a C152 aerobat while recovering from a steep dive, there is some buffeting and a slow roll opposite to the control input.

I have also heard that at trans-sonic speeds control inputs can precipitate shock waves on parts of the airframe that can cause control reversal without laminar flow separation. But I have not experienced this in a C152 (yet).

• I sincerely hope you never experience trans-sonic speeds in a C152. Jul 28, 2015 at 17:28

There is another effect that would have been the cause of the tail of the space shuttle to have a reverse effect above about mach 1.5, where the front of the tail is blunt, designed to deflect hot air during re entry. See diagram.