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Why do flaps retract? I know, it might seem obvious at first, because of drag, but instead of retracting the flaps, can't they just make the flaps flexible enough to get into a reduced drag position, and act like an extended wing?

Maybe there is a reason we should look into this technology, since it could really help out in terms of climb rate. Or, was this already looked at, but engineers decided something was wrong about it, and threw the idea away?

And by the way most of the answers involved automatic retraction of the flaps, even though I never said anything about making the flaps being automatic. Just a disclaimer.

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  • $\begingroup$ On an airliner flying at 150 knots, how would your flap system distinguish between take off and landing configuration? How would you expect a pilot flying in gusty conditions at low level to handle a wing that changes its configuration in response to every gust? $\endgroup$ Apr 9 '20 at 14:28
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    $\begingroup$ You need different flap settings for different conditions and scenarios. CACC has mentioned a couple. Consider just a small GA aircraft’s (like a C172) different needs. You need no flaps for normal takeoff, and 10° for soft and/or short field takeoff. You may need 20° in gusts and heavy crosswind landings. Yet you need 30° for a normal landing, and 40° for soft and/or short field landings. Some pilots like to retract the flaps on rollout to increase braking efficiency. Others prefer to wait until fully exiting the runway. And, how would you resolve asymmetric flaps if you can not retract them. $\endgroup$
    – Dean F.
    Apr 9 '20 at 15:18
  • $\begingroup$ The mechanism you describe cannot take into account the change in aspect ratio of some type of flaps that extend backward (e.g. fowler flaps) $\endgroup$
    – Manu H
    Apr 9 '20 at 16:36
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    $\begingroup$ Why the downvote? What is so bad about an out-of-the-box question that would deserve a downvote? $\endgroup$ Apr 9 '20 at 18:05
  • $\begingroup$ @PeterKämpf thanks. $\endgroup$ Apr 10 '20 at 16:41
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Flaps retract in order to reduce wing area. This has several advantages when flying fast:

  • The higher wing loading (weight per lift-producing area) reduces gust loads. When hit by a vertical gust, the angle of attack increases suddenly, and so does lift. If this happens at high speed (more precisely: At high dynamic pressure), the lift increase might overstress the wing. A reduced area will only produce so much extra lift before the wing stalls, which limits the extra lift from gusts.

  • Less exposed area also means less friction drag. Your flexible flap proposal would still leave both sides of the flap exposed, adding more surface for friction drag. Put another way, the higher wing loading shifts the lift coefficient up which generally improves L/D at high speed.

  • Retraction closes the gaps between wing and flap sections. While those gaps are important to maximize the lift coefficient for the lowest possible landing speed, they increase drag at all lift coefficients.

The retraction mechanism is heavy and increases maintenance cost, but it is worth it.

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  • $\begingroup$ Very detailed yet understandable answer. Thanks for explaining it so well. $\endgroup$ Apr 10 '20 at 16:49
  • $\begingroup$ A high-bandwidth, flexible flap with a high-bandwidth flow sensor, e.g., gust sniffer, could be used to mitigate gust loads through a closed-loop gust load alleviation. If the flap were further conformal then it could mitigate the friction drag. This is the ultimate objective of the AFRL's effort to develop a compliant trailing-edge flap. $\endgroup$
    – afcdesign
    Apr 10 '20 at 20:50
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Believe it or not, "flexible" control surfaces are a secret to competition throwing gliders, where less pitch up is needed for the throw (to keep it from looping at high speeds) and more for staticly stable gliding once it slows down. These uncontrolled gliders, generally made of paper, have their elevator trim as a surface that flattens at high speed and springs up when aerodynamic forces are lower.

On manned aircraft, there are several different flap settings a pilot may wish to use for extra lift and/or extra drag. These positions must be rigidly held to control the aircraft. Having them move without pilot command would be unpredictable, there for, unsafe.

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Flaps are used to control the amount of lift and drag required during flight. e.g. during take off and landing required lift is much higher than maintaining a constant altitude at cruising speed (the high drag during landing also helps to reduce speed), so the flaps are extended during take off and landing.

If the flap is flexible so that it will adjust automatically to the minimum drag position then the pilot will not have control over the lift generated. Even though there are other ways of controlling the lift and drag, using flaps is one of the fastest way, which gives much better control to the pilot.

Flaps are necessary to control the airplane efficiently during take off, landing, turbulence, bad weather and several other situations.

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    $\begingroup$ You may develop your second paragraph. Why is it better? what are the pro/cons of other solutions? $\endgroup$
    – Manu H
    Apr 9 '20 at 16:39
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Plain flaps are as you suggest, just sections of the trailing edge that can be turned down. That helps slow the plane for landing and gives a little extra lift, but for a plane to fly slowly and safely you need as much wing area as possible.

So next up the scale are split flaps, which are just the underside section of the trailing edge. When these lower, the upper part remains so wing area is not reduced.

More effective still are flaps which extend backwards, actually increasing the wing area, as well as downwards. Leaving these flaps extended and just raising them in level flight is not viable, as they create excessive drag and are subject to excessive aerodynamic forces when the plane tries to accelerate to cruising speed.

Flexible flaps are also not a viable solution, because the air at the trailing edge is always turbulent to at least some degree and movable surfaces in this position are prone to flutter, in which they vibrate rapidly up and down. This leads both to undesirable handling effects and, ultimately, structural failure. Flaps have to be held rigidly in place when retracted.

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  • $\begingroup$ Thanks for sharing other kinds of flap designs with me. The whole reason I asked this question in the first place was to make something a little bit outside of the box. I'm happy you're doing the same. $\endgroup$ Apr 10 '20 at 16:55

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