Aileron Reversal


Well-Known Member
Hey Everybody,
I was wondering if anybody can shine some light on the reversal of aileron during high speed.

Hey Everybody,
I was wondering if anybody can shine some light on the reversal of aileron during high speed.


Long, flexible, swept wings tend to twist when the ailerons are deflected. When the aileron goes down, the wing tip rotates forward, actually lowering the local AoA, producing a roll in the opposite direction from desired.

Spoiler control can eliminate the problem, or another set of ailerons set further inboard.
the above is correct. the aileron basically becomes a control tab, twisting the wing, and the resulting roll is opposite of what is expected. the wing/aileron work similar to a dc-9 aileron/tab.

this is common around 400 kias.

and as stated above, it is rather common, at least in the older jet transports, for the outboard ailerons to be disabled in high speed and the inboard ailerons and/or roll spoilers be used for roll control.

there are other things that can happen at these speeds with the ailerons. depending on wing design, the ailerons may simply become immovable do to shock wave formation on them. they may also cause the wing tip area to mach stall when the aileron is deflected downward. in this case, in lieu of an increase in lift, there is a loss. the upward deflection aileron should still function, but roll control would be degraded.

it was common for the military to demonstrate these phenomena to student pilots in the 1950's and 1960's by exceeding maximum speed limitations.
As stated, locking out the outboard ailerons solves the problem for the most part. Use of spoilers for roll control is another design feature to offset it.

Interesting, disappointing, and unfortunately not altogether surprising, that there are people flying jets that do not learn this stuff. Most civilian, and many military pilots, have a surprisingly poor understanding of the aerodynamics involved in flying large swept wing transports. This encompasses both low altitude and high altitude regimes.
Or, if'n you're NASA and have an early-model F-18 with thinner wing panels than are in production models, you call it an Active Aeroelastic Wing Research Aircraft.

The idea was that the aircraft uses the leading and trailing edge devices to twist the wing (spanwise) to provide lateral control.
Interestingly, the last commercial I had was on a UAL 747-400 in a new rear-facing business class seat.

I was next to the window, the CA I was flying with that trip happened to be in the seat next to me. The view was perfect to see almost all of the wing from the seat without having to move around to see it all in action.

I was watching the wing with the leading edge flaps moving and the ailerons and all that. It was amazing what happened with that wing tip.

When the aileron would deflect downward, it was almost unbelievable how the wing section from the inboard part of that aileron outwards acted.

First, the wing would bend up, causing the trailing edge to be higher than the leading edge. The inboard ailerons are less effective, so the inboard part of the wing was slower to move. Then as the rest of the airplane caught up with the roll command, the wing returned to a "normal" look.

It was just crazy to see. You sit through a theoretical class, run the equations, look at all kinds of diagrams, and cross sections, but sitting and watching it under normal operations is quite an eye opener.