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This is a 777 flaperon. It looks rather tiny, compared to the flap.

There have been air disasters where the airplane rolled uncontrollably for one reason or the other. It seems to me that a larger roll control authority could have helped.

Admittedly, flaps are slow, and speed-limited, so less suitable as control surfaces. But at least, could they not be used for trim?

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  • $\begingroup$ Which disasters do you mean? I can't think of any where larger aileron authority would actually help. $\endgroup$
    – Jan Hudec
    Commented Sep 26, 2020 at 19:18

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This flaperon is actually the high speed aileron. Using the outer ailerons at high speed will twist the wing, which in turn reduces their effectiveness. For every aileron there exists a speed where it becomes ineffective and above that speed its function will even be reversed. See the flexible wing as a flap in its own and the aileron as its Flettner tab of sorts.

By using those high speed ailerons, Boeing can build the wing lighter with less torsional stiffness. In addition, it creates a gap between the Fowler flaps at the spanwise location of the engine exhaust, so the Fowler flaps will not reach into the very turbulent exhaust stream. The 737 uses an exhaust gate for the same purpose.

Using flaps for trim means you need to be able to finely adjust the position of each flap individually. This fine adjustment is better left to the smaller control surfaces or the even smaller trim tabs.

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  • $\begingroup$ ah, aileron reversal. btw, surely we need more than a fine adjustment after losing part of our wing or something? $\endgroup$ Commented Sep 26, 2020 at 10:32
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    $\begingroup$ What he means is if the wing is torsionally flexible enough, the aileron in effect turns into a wing warping servo tab. Imagine the Wright Flyer using a little servo tab surface at the wing trailing edge at the tip to warp the wing for roll control instead of cables connected to the pilot. Tab goes down, and instead of rolling airplane left, it warps wing LE down by lifting the TE, and the airplane rolls right. $\endgroup$
    – John K
    Commented Sep 26, 2020 at 12:46
  • $\begingroup$ @JohnK I knww. I mentioned it above $\endgroup$ Commented Sep 26, 2020 at 15:13
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    $\begingroup$ I've never seen it and I can't imagine the rolling moment you could generate would be worth the trouble. You would have to have much larger surfaces in order to be able to combine pitch and roll inputs and still have the same overall pitch authority and the weight and complexity issues quickly kill any business case for trying it. $\endgroup$
    – John K
    Commented Sep 26, 2020 at 16:32
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    $\begingroup$ @Abdullah: Losing part of a wing is hardly a common occurence. Why would one design for that, rather than designing wings not to fail? $\endgroup$
    – jamesqf
    Commented Sep 26, 2020 at 17:20
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Only simple flaps can be flaperons. Fowler and double-slotted flaps generally used on large aircraft cannot move upward, and are designed to extend slowly, which is more appropriate for their complexity and fairly large effect. But that's opposite of what you need in aileron, which must react quickly and using bigger deflection up than down is desirable to reduce the adverse yaw.

Normally in large aircraft roll control is augmented by using spoilers. These have the advantage that they create proverse rather than adverse yaw (the spoiler is raised on the inside wing and adds drag there) and don't suffer reversal at high speed as they don't torsion the wing. B777 is somewhat unusual in having a flaperon at all.

There have been air disasters where the airplane rolled uncontrollably for one reason or the other.

I can't remember any other reason than impeding stall, and in impeding stall the ailerons are reversed because deflecting the aileron pushes the wing deeper into stall and makes it go down rather than up. In impeding stall, the appropriate way to keep wings level is using rudder and yaw-roll coupling, which the pilots know.

Oh, and then I remember the Embraer E190 incident at Alverca (i.e. they managed to get it under control and land safely) where the control cables were connected crossed during maintenance, but in that case they were saved by the ailerons not having that much authority and roll control also employing spoilers which were still connected correctly and cancelled the ailerons at larger deflection.

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  • $\begingroup$ The DC-7's left wing to the left side of the number one engine was mangled by the impact and was no longer capable of producing substantial lift. The engine had been severely damaged as well, and the combined loss of lift and propulsion left the crippled airliner in a rapidly descending left spiral from which recovery was impossible. et al $\endgroup$ Commented Sep 27, 2020 at 5:33
  • $\begingroup$ @Abdullah What flight is your quote above from? $\endgroup$
    – TomMcW
    Commented Sep 27, 2020 at 23:36
  • $\begingroup$ @TomMcW this $\endgroup$ Commented Sep 28, 2020 at 5:56
  • $\begingroup$ @Abdullah, it's unlikely that larger aileron authority would help in any of such accidents. Loss of part of the wing is too much for an airliner, and such collision would also sever the hydraulic lines, so the remaining aileron would not be controllable anyway. $\endgroup$
    – Jan Hudec
    Commented Sep 29, 2020 at 8:56

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