Do inboard high-speed ailerons help with the roll control by decreasing the moment arm as a secondary benefit? As in make it harder to exceed the roll limits when at high-speed.

I know the main benefit is to avoid aileron reversal. Also I'm aware discussing tailoring the stick forces in depth will be too broad. So I'm not after an in-depth analysis. Just if that secondary function is indeed a reason.

Comparison of high-speed aileron placement on Airbus and Boeing is shown below. In an Airbus with fly-by-wire (FBW), the stick commands a roll-rate. And all the Airbus FBW heavies—including the A380—do not feature far-inboard ailerons like the Boeings. In other words, if FBW did not exist, could Airbus have still been able to not have mid-wing ailerons?

enter image description here
Airbus (top) source; Boeing (bottom) source.


1 Answer 1


No it does not reduce the aeroforces: the hydraulic actuators that deflect the inboard aileron have as much of a force to apply as the ones at the outboard aileron. Force for deflecting a flight control surface is a function of dynamic pressure, surface area and of surface deflection.

As you state in the question, the inboard aileron reduces wing twist which could produce aileron reversal at high dynamic pressures, particularly problematic with relatively thin wings and high wing sweep angle.

The reduced moment arm affects the aircraft response upon aileron deflection. The problem with the ailerons is that a deflection equates to an end roll rate, as opposed to the elevator and rudder where a deflection results in an end angle. Many aircraft have a slow roll oscillation in cruise that the autopilot compensates for. If it must do that with an outboard aileron at M=0.85, the strong response of the outboard aileron makes it hard to not overcompensate. I have seen the inboard ailerons on a B747 do a slow deflection dance in cruise, while the aircraft roll angle stays perfectly still.

Slow roll oscillation in cruise was a problem with the F100 for a short while, the gains in the autopilot could not be set low enough to obtain a constant zero attitude. Not really a problem, only in the perception of passengers looking out of the window. It was ultimately solved using different logic but unfortunately I don't know how they did it.

  • 1
    $\begingroup$ The Fokker 100 (?) had a single aileron per wing, right? Which means a secondary inboard one would have helped? $\endgroup$
    – user14897
    Aug 21, 2017 at 15:24
  • $\begingroup$ Yes it has a single aileron per wing, they could fix the oscillation without the help of an inboard aileron, Airbus apparently can as well. $\endgroup$
    – Koyovis
    Aug 21, 2017 at 15:38
  • $\begingroup$ That's my point, Airbus use FBW, Boeing initially did not. So do the inbd ailerons of Boeings as a secondary benefit help in the control? $\endgroup$
    – user14897
    Aug 21, 2017 at 15:46
  • $\begingroup$ From what I've seen yes, but unfortunately my references are anecdotal and from observation as a passenger. $\endgroup$
    – Koyovis
    Aug 21, 2017 at 15:56
  • $\begingroup$ That's okay, I just wanted to be sure we were on the same page. It's an informative answer still. Thank you. $\endgroup$
    – user14897
    Aug 21, 2017 at 16:07

You must log in to answer this question.