Reading about the 787 it says it has

an active gust alleviation system similar to the system used on the B-2 bomber. [1]

The gust alleviation patent says:

The gust alleviation system picks off the stick displacement signal and the aircraft normal acceleration sensor signal and processes this information. The outputs of the gust alleviation system are added back respectively to the existing flap command signal and the aircraft's original stability augmentation loop. [2]

As a layman I am not sure I fully understand this, so my question is, what is the gust alleviation system?


A gust alleviation system is a control system fitted to some Fly-by-wire (FBW) aircraft that reduces the effect of gust loads on the aircraft by deflecting control surfaces such as ailerons, rudder and elevators.

The system works by measuring the upward acceleration of the aircraft and comparing that with the acceleration commanded by the (auto)pilot. A feedback loop adds a correction signal to the signals controlling the deflection of the control surfaces in order to counteract the accelerations cause by wind gusts.

In its simplest form the accelerations are sensed near the center of gravity of the aircraft. More advanced implementations work with multiple sensors in the body and in the wings. These gust alleviation systems do not only attempt to annul the effect of the gusts on the aircraft's normal (upward) acceleration, they also reduce the wing bending moments. This is turn reduces metal fatigue.

In the A380 the load alleviation system is nicknamed "valse des aileron" (Waltz of the Ailerons) because of the dancing movements of the aileron:

  • 1
    $\begingroup$ So their movement is controlled? I've always thought that they are just swept by the wind :D $\endgroup$ – yo' Dec 23 '14 at 17:51
  • $\begingroup$ A320-100 used spoilers for gust alleviation, but the newer A320-200 variant and the rest of the A320-family don't. $\endgroup$ – Jan Hudec Dec 23 '14 at 23:02

Gusts change the dynamic pressure of the airflow over the wing. Vertical gusts even change the angle of attack. Both will change the lift the wing creates, and especially vertical gusts add stresses for the structure and discomfort for the passengers.

By changing aileron deflection, the wing's lift can also be manipulated. If the aileron deflection is just enough to compensate the added aerodynamic load due to a gust, both the wing and the passengers will benefit. By coupling the readings of accelerometers near the wing tip, the FCS can adjust the aileron deflection and ideally keep the wing bending constant.

Since the ailerons are at the outer 30% of span, they have a big influence on the root bending moment, but affect only a small part of the lift the wing can create. Therefore, while the bending moment can be nicely held constant, the lift will still vary, but less than without gust alleviation.

The part referring to the stick in the quoted patent refers to a mechanical control system with a naturally stable aircraft. If an aircraft with such a control system flies into a vertical gust, it will pitch up or down, which in turn will shift the equilibrium stick position. However, this will happen with some delay (the fuselage tip will "see" the gust first, and the tail will be affected by it some tenths of a second later), so the stick movement is a poor source for a feedback loop. I wonder why the patent mentions it - maybe the patentee wanted to cover all input sources.

  • $\begingroup$ @JanHudec: Could be. This leaves the question why they want to alleviate gusts with elevator commands when the aileron is at the source of lift creation. $\endgroup$ – Peter Kämpf Dec 23 '14 at 23:08
  • $\begingroup$ I don't think it uses stick position for calculating the alleviation, but it might choose different solution depending on what command it is augmenting. $\endgroup$ – Jan Hudec Dec 23 '14 at 23:09
  • $\begingroup$ Do they want to use only elevator? I'd expect they augment primarily ailerons, but they also augment elevator to compensate any pitching moments the gust created or something. $\endgroup$ – Jan Hudec Dec 23 '14 at 23:10
  • $\begingroup$ @JanHudec: They say "flaps" in the patent. Given how slow the wing flaps move, this is really the worst way to do it. The patent is from 1990, and plenty more had been filed earlier. I guess Grumman needed to circumvent all other patents when they wrote this. $\endgroup$ – Peter Kämpf Dec 23 '14 at 23:12

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