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Why don't ailerons affect pitch, or do they?

I'm told spoilers can affect pitch if not placed in the right spot.

On a classic standard wing configuration, why don't ailerons affect pitch as they are at the trailing edge and would create a moment around the spar which I'm told it usually located at 25% chord in ultralights.

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    $\begingroup$ Are you interested in a specific wing configuration (you can imagine that elevon on delta wings does not react the same way as aileron with droop function such as the one used on the A320)? $\endgroup$
    – Manu H
    May 11 '20 at 8:20
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Two reasons:

  1. Because they don't have a very long moment arm.

  2. Because their movement works in opposite directions to each other, any pitching moment one side might cause is cancelled out by a pitching moment going the other way from the aileron on the other wing.

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    $\begingroup$ The Kolb Ultrastar ultralight suffered a little bit from the single strut wing, who's spar was a 5" tube, twisting away from the aileron movement, which had the effect of negating part of the roll input. Left stick twisted the right wing LE down and the left wing LE up. $\endgroup$
    – John K
    May 10 '20 at 22:42
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    $\begingroup$ So that's the ailerons having a local pitching moment on each wing, but they would (at least partially) cancel each other out and not affect the net pitching moment on the whole aircraft. This sounds like it would be bad for the lifespan of the spar though - do you know if it lead to metal fatigue? $\endgroup$ May 10 '20 at 23:45
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    $\begingroup$ No I don't believe so; it was just considered a quirk of the design and was only apparent with very large control inputs. The torsional stress on a 5" .052" wall spar extrusion was insignificant. It was just a case of the wing being a little bit springy in the torsional axis. $\endgroup$
    – John K
    May 11 '20 at 1:25
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This isn't specific to ultralights, but more generic to airplanes.

They could, but ailerons don't have the torque to change the direction of the aircraft like the elevators would. The center of pitch change would be a line that goes through the aircraft similar to the position of the wings. The elevators being further away from the center of the aircraft can push more. Designers purposely only have the elevators move for this reason.

The ailerons move in opposite directions when the move to induce roll. The center of roll would be roughly the center of the fuselage from head to tail. The ailerons are positioned away from this point to increase their torque.

I hope this helps.

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When de Havilland developed the first Cirrus-engined Moth in the mid 1920s, they included differential movement of the ailerons. The common problem they faced was was that when turning and inadvertently approaching a stall, moving the inner aileron down to pull out of the turn would actually encourage tip stall at that moment. So the aileron being lowered moved by only a small amount, while that being raised moved more authoritatively. Overall this did create a small pitch-up moment but it was minimal and did not adversely affect handling. de Havilland used it extensively afterwards, I don't know about other designers.

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