6
$\begingroup$

On standard fixed-wing aircraft that have both ailerons and elevators, can ailerons function as elevators for pitching movement if the elevators fail? Technically it should be possible if both ailerons can rotate in the same direction simultaneously.

If so, what are the key differences and problems if the center of gravity of the aircraft was forward of the ailerons or behind the ailerons?

To my understanding, the farther back the control surface (which causes the pitching) is to the center of gravity, the bigger the pitching moment which results in a more stable configuration.

Improve this question
$\endgroup$
1
  • $\begingroup$ I suppose your question exclude elevons. $\endgroup$
    – Manu H
    Oct 30, 2015 at 17:07

2 Answers 2

Reset to default
10
$\begingroup$

If the ailerons could be moved in the same way on both wings, the linkage between both is broken and they would float up near their maximum negative (trailing edge up) deflection angle. Even a hydraulic aileron booster would not help, since here still an interlink between both ailerons exists to keep actuation forces down. Only in a fly-by-wire aircraft with fully powered aileron actuators could the control system be programmed to allow symmetric deflection.

The effectivity of such a means of pitch control is very low, and only wing sweep can help to make it useable. Any pitch control input will change lift on the outer wing, and pitching up would mean a loss of lift and result in a plunging motion of the aircraft. If that happens on landing, when the pilot uses to pull the stick to raise the nose gradually, the resulting ground contact will be rather early and bumpy. Pilots of flying wings know this; they are very careful with their pitch control during a flare.

When an ASW-20 glider had crashed a couple of years ago because the pilot had forgotten to connect the elevator linkage, Gerhard Waibel flew to the glider club and demonstrated how to fly the ASW-20 with flaps for pitch control alone. The downside is a much reduced speed range and a high landing speed, because you need to reduce camber in order to pitch up, but it works. Since the flap span is higher than the aileron span, I would strongly recommend to rely on the flaps for pitch control when your elevator stops working if flaps are installed. They also do not need any modification to work as pitch controls since they are rigged to move symmetrically.

If the engines are mounted low or high, their thrust can also be used for pitch control. This was used on several occasions to bring an airliner down after all hydraulic circuits had failed.

EDIT: Mike Dunlavey insists I should mention flaperons. He is right, they are possible with mechanical linkage only, but you will find them only on wings which have flaps anyway. All modern gliders with flaps will also move the ailerons in sync with the flaps.

But they are the worst choice for pitch control. For one, they are on the part of the wing with the smallest chord. Their lift change will go with the smallest pitch moment change of all controls except the rudder, so it would be much better to use the flaps for pitch control. That the ailerons do move with the flaps will have little effect. Besides, well designed flaperons will have less deflection change for a given flap change so the spanwise lift distribution will stay close to the optimum. This will make them even less effective than the flaps.

I choose not to mention them to make the answer concise. But I'm afraid short answers are not appreciated anymore.

Improve this answer
$\endgroup$
11
  • $\begingroup$ Maybe I'm wrong, but I think your first sentence isn't completely correct. "Drooping ailerons" are not so common in non-fbw aircraft but the symmetrical/asymmetrical movement can be achieved with only mechanical links (I've seen it in a Lego model :) ). For instance this DHC-2 has both the ailerons deflected downward to provide additional lift. ;) $\endgroup$
    – Marco Sanfilippo
    Oct 30, 2015 at 15:53
  • $\begingroup$ Ever heard of flaperons? $\endgroup$
    – Mike Dunlavey
    Oct 30, 2015 at 16:09
  • 1
    $\begingroup$ @MarcoSanfilippo I doesn't look like the ailerons are deflected much, if at all in that photo. Note that the neutral position of ailerons is almost always somewhat down because they're installed on the trailing edge of a wing, which points downwards to generate lift. $\endgroup$
    – reirab
    Oct 30, 2015 at 16:16
  • $\begingroup$ @MikeDunlavey Flaperons are an example of the fly-by-wire exception that he mentioned. $\endgroup$
    – reirab
    Oct 30, 2015 at 16:18
  • $\begingroup$ @reirab: No they're not. Take a look at that link, in particular the kit planes that use flaperons. It's just a matter of using aggregate-motion linkage from the control stick and flap lever. $\endgroup$
    – Mike Dunlavey
    Oct 30, 2015 at 16:46
1
$\begingroup$

I am a 100-hour pilot, so I know the basics.

Every civilian airplane requires decalage, which is an up-turning tendency. Then if the plane is properly loaded, the center of gravity is forward of the center of lift. If there were no gravity, the plane would simply travel in a circular loop upwards. The weight of the nose is what prevents that.

Why? For speed stability. Both lift and the up-turning tendency are proportional to speed squared. So if they go slower, the nose wins, carrying them down, which makes them go faster. If they go faster, the wings win, carrying them up, which makes them go slower.

So, if the elevators become frozen, and you want to nose up, just give it the gas and go faster. Either that or lower the flaps, to get more lift. Or you could move some weight to the rear.

EDIT: This image is to show how to make a flaperon with purely mechanical linkage: enter image description here

Improve this answer
$\endgroup$
2
  • 2
    $\begingroup$ I don't think that's quite what would happen without gravity. Since gravity is what holds the atmosphere in place, it would be gone and wings and control surfaces will not to work very well in vacuum. :) On the other hand, there would be no drag, so fuel efficiency would be quite good, but with caveat that you need rocket engines instead of air-breathing engines. $\endgroup$
    – reirab
    Oct 30, 2015 at 14:42
  • $\begingroup$ @reirab: Use your imagination :) Did you ever read Arthur C. Clarke "Rendevous with Rama"? Large cylinder in space, full of air. Or do it under water, with neutral bouyancy. Or just turn the plane on its side, with a wire supporting its weight. $\endgroup$
    – Mike Dunlavey
    Oct 30, 2015 at 16:05

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .