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Why is it that trailing edge flaps are only used at lower speeds?

Can they be used at higher speeds too?

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Normally flaps are only used to increase drag and lift for low speed operation. There is no benefit to extending them at higher speeds, where they just reduce efficiency, and they generally aren't designed structurally for high speed operation, with a few exceptions.

Some fighters in WW2 had a "maneuvering flap" setting that could be deployed at speed temporarily to allow tighter turns. The P-38 Lightning had this feature, operated by a switch on the control column.

In rare cases a pilot may want to extend flaps slightly to effectively change the wing incidence to reduce the deck angle in cruise. You could do this with the DeHavilland Beaver. If you wanted to, you could cruise with the flaps extended about 5 degrees, when flying with heavy loads, to reduce the nose attitude. If you're in a Beaver on floats that is near max gross weight, the drag penalty from a few degrees of flap (which includes aileron droop) is not that great. Not a common practice though.

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  • $\begingroup$ Hi, how do flaps reduce efficiency at higher speeds though please? $\endgroup$
    – JohnM
    Commented Mar 25, 2023 at 21:54
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    $\begingroup$ They increase lift by increasing camber, but the camber increase raises drag more than than the increase in lift, reducing the maximum ratio of lift to drag. It also shifts the center of pressure aft so that trim drag is higher. Lots of gliders have flaps that can be lowered a lot for landing, a little bit for thermalling near stall, and reflexed UP slightly for high speed dashes. The reflex up reduces camber and moves the CP forward, having an effect a bit like reducing the chord increasing L/D. You rarely see reflex used on power planes. $\endgroup$
    – John K
    Commented Mar 26, 2023 at 0:31
  • $\begingroup$ Hi, but how does camber increase lift and drag? I get that it increases drag ( as it’s like putting more area in front of the flow) but I don’t see how it raises lift? $\endgroup$
    – JohnM
    Commented Mar 26, 2023 at 10:52
  • $\begingroup$ @JohnM: because of that. $\endgroup$
    – sophit
    Commented Mar 26, 2023 at 11:34
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Lowering the flaps puts stress on the flaps and their attachments to the wings, and also increases the loading on the wing. Extending flaps at too high an airspeed can damage the flaps, their attachments, or the wings.

Beyond a certain airspeed, the stress can even cause the flaps to separate from the wing. Naturally, this will make it much harder to control the airplane and should be avoided.

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