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When taking off or landing, both LEFs and trailing edge flaps (TEFs) for fighter aircraft are in the down position to provide more lift. However, when flying around at higher speeds (let's say 500-600 knots), the left LEF going down and the right LEF going up causes the aircraft to roll left.

My personal theory is that the dominant form of drag (induced) at slow speeds means that the increased camber provides more lift although slightly decreasing the angle of attack (which is good if you are close to the stall angle of the wing). However, at higher airspeeds the slight decrease in angle of attack caused by the downward LEF and also the larger parasitic drag causes the decrease in lift of the wing.

Does this theory hold up? Or is there more to it that I'm not considering?

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  • $\begingroup$ Drag is drag, get rid of it as soon as it is not needed. Even in my 'litte' Cessna Cardinal, with 180 HP, flaps are retracted as soon the benefit of slow speed lift is overcome by sufficient lift being generated by higher climb/cruise speed. That's only 10 degrees on takeoff, just to help get over obstructions. After they are just drag. Back down again for landing for more lift at lower speeds, all the way to 30 degrees for touchdown. $\endgroup$ – CrossRoads Apr 5 '18 at 18:23
  • $\begingroup$ 1. I don't know of ANY fighter aircraft with Leading-Edge Flaps. Slats yes, but not Flaps. $\endgroup$ – RAC Apr 6 '18 at 8:25
  • $\begingroup$ 2. >>the left LEF going down and the right LEF going up causes the aircraft to roll left<< This NEVER happens. Leading edge devices are always moved symmetrically. $\endgroup$ – RAC Apr 6 '18 at 8:26
  • $\begingroup$ @RAC F-16, F-18, to name a couple. Perhaps I should've worded the question differently. Say, for example, the left LEF of an F-16 gets stuck in the down position... The aircraft will have a left roll tendency for the rest of flight because the left wing is now producing less lift. Why, then, do we put LEFs in the down position when coming in for a landing? Is it to increase camber to allow the aircraft to maintain the same wing angle of attack at slower speeds without stalling? $\endgroup$ – Tom D Apr 7 '18 at 16:25
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Leading edge faps also known as slats don't really create lift. The effect on lift coefficient is minor (less than 10% in general) but they help increase the stall angle of attack. As a summary, flaps creates lift, by increasing surface area and profil camber whereas slats postpone the stall and allow for higher AOA by reducing pressure drop around the leading edge. This increases the theoretical maximal lift by increasing the stall AOA but doesn't change much the lift coefficient.

Slats are ment to work at high AOA by design, at higher speed your AOA is very small, the slat is thus not working properly, disrupting the flow on the suction side of the wind and reducing actual AOA, thus reducing lift of that particular wing while simultaneously increasing drag.

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The only reason I can think of is angle of attack. At low speed, angle of attack is higher, so even the downward drooping section ends up with a positive AoA, creating lift. At high speed, it is lower, causing the downward drooping section to have negative AoA, and thus create negative lift.

Now, why do we have this device in the first place then? (At least partly) Because a gradual curve, rather than a flat plate, results less drag and safer stall behavior.

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