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As we know ailerons aid the aircraft to make roll movements and provide lateral stability.

But, what is the intent behind drooping them in some flight conditions, especially when deploying flaps?

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    $\begingroup$ If you mean drooping them symmetrically on both sides, it's to complement flaps and increase lift at low speed, but a few commercial aircraft have this possibility. On the MD-11 they are named "deflected ailerons", and, as explained in the video, extending them allows a 3 kt reduction in takeoff speed. $\endgroup$ – mins Jul 22 '17 at 20:14
  • $\begingroup$ interesting video ! MD-11 is my favorite one , thanks. $\endgroup$ – Mehdi shelby Jul 23 '17 at 7:43
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The purpose of aileron droop is to use the ailerons as wing flaps. Wing flaps increase the lift generated by the wings at low speed. They occupy a good part of the trailing edge of the wing, except for the outer part, where the ailerons are. By deflecting both the ailerons downwards, they change the wing camber and act like simple plain flaps. They can still act as ailerons, by normal differential deflection.

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Image source

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    $\begingroup$ so can we call them flaperons? $\endgroup$ – Mehdi shelby Jul 23 '17 at 7:38
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    $\begingroup$ @Mehdishelby flaperon, elevon and spoileron are relatively extended portmanteaus, but I personaly avoid them unless the surface has been specifically designed to carry out both functions in a similar measure, not as a secondary function alongside dedicated surfaces. You'll still be understood by most people in the industry if you do use them. $\endgroup$ – AEhere supports Monica Aug 2 '17 at 7:06
  • $\begingroup$ You mean ailerons are used as part of flaps with the fowler flaps? $\endgroup$ – lemonincider Aug 2 '17 at 10:03
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    $\begingroup$ @lemonincider No they're used as plain flaps, the first picture. Not as effective as Fowler flaps but every bit helps. $\endgroup$ – Koyovis Aug 2 '17 at 13:59
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Many different types of aircraft (certain models of Airbus/Boeing, etc) are designed so that when flaps are extended (for increased lift at slower speeds - e.g., takeoff, landing) part (or all) of the aileron assembly "droops" to add to the benefit and efficacy of the flaps.

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