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Why does the aileron deflected upwards, thereby reducing lift, not also reduce drag on that side?

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    $\begingroup$ One significant component of adverse is yaw is due to roll rate not aileron deflection-- there is sometimes a tendency for the nose to yaw toward the rising wingtip, even if the roll is caused by spoilerons rather than ailerons. However the adverse yaw component due to aileron deflection can also be very significant, and can be seen in some demonstrations that actually involve zero roll rate. As to why-- well... $\endgroup$ Feb 17 '20 at 23:05
  • $\begingroup$ The wing that goes up trades kinetic energy for potential energy and the other wing does the opposite. $\endgroup$
    – copper.hat
    Feb 18 '20 at 19:16
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Indeed, when an aileron moves upward, it locally generates less lift and less drag.

Assume we are talking about the aileron on the right wing. The reduced lift drops that wing, rolling the aircraft right. But the reduced drag accelerates that wing, yawing the aircraft left.

On the left side the opposite happens, but it has the same effect. The aileron moves down, increasing lift to roll the aircraft right. But the added lift increases drag, decelerating the left wing to yaw the aircraft left.

That is the adverse yaw effect.

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  • $\begingroup$ When the left aileron goes down it creates extra lift, and drag, but the right aileron travels up and also creates drag. Why is there more drag on the left aileron than the right one? $\endgroup$
    – Zebrafish
    May 11 at 5:38
  • $\begingroup$ @Zebrafish the one going up reduces lift, so it reduces drag. Only when it moves up so much that it produces negative lift the drag will increase again, but still less drag than the opposite side $\endgroup$
    – DeltaLima
    May 11 at 6:23
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The aileron going up does produce drag as well - once it is producing a sizeable amount of downforce. At rest, it is aligned with the back end of the wing, and in a wing with no camber at zero Angle of Attack the aileron produces symmetrical drag. But wings usually do have camber and/or AoA ≄ 0, so at rest the aileron points trailing edge downwards and starts with positive lift plus the associated drag. When deflected upwards, lift and drag first reduce until the aileron is flush with the airstream, while the other aileron produces more lift and drag.

enter image description here

Adverse yaw has two causes:

  1. The "hooking" effect described above, and depicted in the figure from my old uni book. It shows the other way to explain the adverse yaw drag: the wing producing more lift also produces more induced drag.
  2. The roll rate resulting from aileron deflection, which increases Angle of Attack of the downwards travelling wing. This tilts the local lift vectors of rising vs. falling wing, and the horizontal component produces an additional adverse yaw effect. As depicted below: $\bar{\Delta c}_rn$ points upwards at one wing, and downwards at the opposite wing, at different tilt..

enter image description here

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  • $\begingroup$ Where is the lower picture from? It's kind of confusing... or I'm just tired... $\endgroup$
    – Jpe61
    Feb 18 '20 at 10:20
  • $\begingroup$ And regardless of who's confused, you should credit the original author - even if it is yourself. $\endgroup$
    – AShelly
    Feb 18 '20 at 18:42
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    $\begingroup$ The pictures are from a university course lecture handout by prof. Gerlach, released in 1981 and as far as I know still used today. Not available on the open market and in Dutch. As you can see, I’ve held on to it and still use it as a reference. $\endgroup$
    – Koyovis
    Feb 18 '20 at 23:17
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    $\begingroup$ Now I see where English translations have been pasted onto the diagram. $\endgroup$
    – Jasen
    Feb 19 '20 at 8:52
  • $\begingroup$ I've read about 6 explanations of adverse yaw and my reaction each time was "Yeah the upward travelling aileron creates drag as well". I feel all the explanations are just handwaving it away with a stock explanation. Though I don't if this explanation is correct, it definitely makes sense to me unlike the others I've read. $\endgroup$
    – Zebrafish
    May 11 at 5:40
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The principal drag in question is induced drag, that is the drag necessarily induced by creating lift.

The aileron that goes down increases lift on that wing, and thus also induces more induced drag; conversely, the aileron that goes up reduces lift on that wing and thus induces less induced drag. Hence, the aircraft will tend to yaw towards the aileron that goes down.

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  • $\begingroup$ So once the roll rate becomes constant rather than increasing-- implying that both wings must be making the same amount of lift-- the adverse yaw goes away? This answer would seem to suggest so, but we don't see this to be the case in reality. $\endgroup$ Feb 18 '20 at 16:16
  • $\begingroup$ @quietflyer If the roll rate is constant how do both wings make the same amount of lift? I think your point is right though, that once the roll rate is constant the major cause of adverse yaw is the other explanation. It's the rotation of the actual wing that causes it, not the aileron deflection itself. $\endgroup$
    – Zebrafish
    May 11 at 6:07

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