4

In airplanes like the F-22, both the main wings and the tailerons provide (at least at subsonic speed) positive lift. My question is this: when performing a roll using all control surfaces asymmetrically, specially the tailerons, it seems that the longitudinal moment around the center of gravity wouldn't be zero, since one of the tailerons goes down, so the aircraft should tend to pitch (up) adversely. Is this true?

I guess the surfaces don't move completely asymmetrically but in a differential fashion, so the up going taileron moves further up to compensate for the negative lift of the down going taileron, or maybe the forces distribution is such that this is negligible. Also I'm disregarding important effects like drag.

So, does this adverse pitch may really happen?

Ralph J
  • 51,356
  • 17
  • 157
  • 249
Gabriel
  • 41
  • 2
  • If differential displacement causes a pitching reaction that is not what is being called for by the pilot at a given time, the computers will simply adjust the differential positions to compensate. It's all going to be going on continuously in the background. – John K Dec 29 '21 at 01:31
  • 1
    I understand that the forces probably aren't perfectly symmetrical, but I'm curious why you wouldn't assume that they would mostly cancel each other out? My presumption is that equal and opposite deflection would simply result in a nice tight spiral, with little to no pitching moment. What makes you think it would pitch up? – Michael Hall Dec 29 '21 at 03:53
  • Well, in stable flight, both tailerons generate positive lift. Jut by one taileron going neutral (cero AOA) an thus generating no lift, the longitudinal moment around the center of gravity wouldn't be cero. If that surface goes even further down generating negative lift, the unbalance would increase even more. So the up going taileron needs both to compensate this and generate the necessary additional lift to maintain longitudinal stability. Pure asymmetric movement can't achieve this, I guess. But well, as everybody has said, computers will take care of this. – Gabriel Dec 29 '21 at 17:39
  • But positive lift or negative lift, if the roll is initiated when the pitching moment is zero, then they are both moving opposite each other from a neutral position, right? And if one needed to travel further than the other to avoid a pitching moment it seems engineers could compensate mechanically too. – Michael Hall Dec 30 '21 at 04:52
  • Yeah, but my point was that neutral position is not cero angle of attack, but a positive one, so there is a "lift bias" (may I say it like that?) that has to be compensated. Anyway, like you and Koyovis have mentioned, any compensation can be mecanically enforced too, pretty cool indeed. By the way, happy new year! – Gabriel Jan 02 '22 at 21:45

1 Answers1

1

enter image description here

Pic above is a still @ 1:37 of this video, of a model F-22, and shows the ailerons and the all-flying tail deflecting to command a roll. Both ailerons and tailerons can be commanded such that there is no resulting pitching moment: negative lift of the one end is exactly the positive lift of the other end, cancelling each other out. This is not necessarily at symmetrical deflections, no problem for the flight control computer of the real aircraft.

From the wiki:

Flight control surfaces include leading-edge flaps, flaperons, ailerons, rudders on the canted vertical stabilizers, and all-moving horizontal tails (stabilators); for speed brake function, the ailerons deflect up, flaperons down, and rudders outwards to increase drag.

So all flight control surfaces can be commanded individually, and a feedback loop from the motion sensors will position the matrix of surfaces such that only a roll rate results. Also compensating for:

  • Adverse yaw, which is not a very dominant effect in short wing span fighters.
  • Inertial coupling, where the nose starts to point in increasing circles at sustained high roll rates.

The all flying tail surfaces can superimpose a symmetric (pitch axis) deflection upon any asymmetric (roll axis) deflection, compensating for any unwanted pitch accelerations. Even without a flight control computer the pitch and roll can be uncoupled mechanically, like in the 4-DoF helicopter mixers.

enter image description here

Koyovis
  • 61,680
  • 11
  • 169
  • 289
  • Well, maybe my question was more like "can this effect happen?", but since these computer-driven machines are another kind of beasts there has to be a lot of things the computers compensate for like the two things you mentioned. Thanks everybody! – Gabriel Dec 29 '21 at 17:43
  • That's really cool. I really like the power of electronics, but being able to achieve such things mecanically is just so pleasant. Happy new year! – Gabriel Jan 02 '22 at 21:47