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Are there any aircraft that can mechanically modify the dihedral angle of the wings whilst in flight? Would this solution give any advantage?

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    $\begingroup$ Fabrizio, you call this a solution. What problem are you trying to solve? $\endgroup$ Nov 5, 2015 at 22:05
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    $\begingroup$ Every aircraft increases the dihedral angle with load factor. $\endgroup$ Nov 5, 2015 at 22:16
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    $\begingroup$ @SkipMiller I think that his second question was about the existence of such a problem. $\endgroup$ Nov 5, 2015 at 22:31
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    $\begingroup$ Birds do it, bees do it Even educated fleas do it ... so why don't airplanes? $\endgroup$ Nov 6, 2015 at 7:30
  • $\begingroup$ The final sentence might be better worded as "is this a solution to anything?" or something equivalent but less awkward. $\endgroup$ Nov 6, 2015 at 21:50

3 Answers 3

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There was one, and it did it to increase directional stability at supersonic speed. I am talking of the North American XB-70, of course. There were three benefits to this configuration:

  • Improved directional stability. Without the effect of the folded-down wingtips, the XB-70 would had lost all directional stability upwards of Mach 2. The wingtips folded down to 64.5° anhedral (or is it -64.5° dihedral?) which effectively made them into ventral fins. This answer explains how a ventral fin is especially helpful at supersonic speed, because it works in the compressed air below the aircraft.
  • Less shifting of the aerodynamic center. Since the transition to supersonic flight will shift the aerodynamic center back, folding the wing tips down reduces lift in the aft section of the wing, thus reducing the aftward shift.
  • Better capture of the compressed lower wing flow for lift creation, which is called a waverider. This increases the pressure on the lower side of the wing at high supersonic speed and allows to fly with a reduced angle of attack, thus reducing drag.

XB-70 landing

XB-70 landing with wingtips straight (picture source)

XB-70 in flight

XB-70 in flight with wingtips down (picture source)

The XB-70 is also one of only a few types where every aircraft ever built had different dihedral: AV-1, the first prototype, had 0° while AV-2, the second prototype, had 5° dihedral over the whole wing to improve yaw and roll stability. Only the dihedral of the outer panels could be adjusted in flight, however. According to this source (PDF!) they were the largest moveable aerodynamic device ever used on an airplane.

Other cases of variable dihedral were less intentional, though, and there was really no advantage to it:

F-4 with wingtips folded up

Like on many Navy aircraft, the wings of the F-4 could be folded for stowage. If the mechanism was not locked, the wings would fold up. The F-4 was even powerful enough to be flyable this way. When only one side folded up, the F-4 would crash, however.

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    $\begingroup$ could you elaborate a bit more on how the folding of the wing tips increased directional stability exactly? I don't quite understand how it would help. $\endgroup$
    – ROIMaison
    Nov 6, 2015 at 7:51
  • $\begingroup$ @ROIMaison: They folded down to 60° anhedral which effectively made them into ventral fins. $\endgroup$ Nov 6, 2015 at 8:23
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    $\begingroup$ It has been said that with a strong enough jet engine, even a brick could be made to fly. The F-4 required two. $\endgroup$
    – dotancohen
    Nov 6, 2015 at 12:21
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    $\begingroup$ @dotancohen: This is soooooo true! The aerodynamics of the F-4 were outright horrible. $\endgroup$ Nov 6, 2015 at 12:41
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    $\begingroup$ @PeterKämpf - aerodynamics? F-4's don' need no steekeeng areodynamics! You just sit yerself down in that chair right there, strap a couple of big ol' J-79's across your back, and I assure you - aerodynamics will not matter. :-) $\endgroup$ Nov 6, 2015 at 17:49
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In general, any aircraft will have its dihedral modified in flight due to the aerodynamic loads, though it is more pronounced in case of aircraft having high aspect ratio and slender wings like the Boeing 787.

Boeing 787 wing flex

Image from stackexchange.com

The North American XB-70 had a wing tip which was foldable in flight, which helped trap the shock wave under the wing between the downturned wing tips, creating compression lift, and also improved directional stability at high speeds by adding more vertical surface to the aircraft and reducing the rearward shift of aerodynamic center.

XB 70

"XB-70 final proposal" by NASA - NASA CR-115703, Volume 2, page 23.Transferred from en.wikipedia to Commons by TheDJ using CommonsHelper.. Licensed under Public Domain via Commons.


Ornithopters vary their dihedral all the time.

Ornithopter

Ornithopter built by University of Toronto School of aerospace technologies; image from philschophoto.com

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Hang-gliders are an obvious answer. For over 30 years, all higher-performance hang-gliders have had a pull-cord to tighten/loosen the sail, variously called "variable billow" or "variable geometry". A sail which billows more in the middle will naturally give you higher dihedral. The tradeoff of course is higher drag, which is why it's controllable - pilots pull on VB/VG for best performance in glides, and let it off for tight manoeuvring (e.g thermalling) or slow flying (e.g. takeoff/landing).

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  • $\begingroup$ Re "A sail which billows more in the middle will naturally give you higher dihedral." -- are you sure? Sail billow causes the trailing edge to rise as you trace from root to about halfway out to the wingtip, but then to descend again as you trace from about halfway out to the wingtip on out to the wingtip. Sort of a gull-shaped geometry, as far as the trailing edge is concerned. Viewing the wing from multiple different angles (with the sail loaded) we can see that this creates a dihedral geometry inboard and an anhedral geometry outboard. $\endgroup$ Sep 19, 2023 at 3:21
  • $\begingroup$ As long as the taper ratio is not too extreme (i.e. as long as the wing area of the outboard anhedral portions is not too tiny), it seems reasonable that the anhedral geometry of the outboard portions should dominate over the dihedral geometry of the inboard portions, by virtue of the greater moment-arm of the outboard portions from the CG. Thinking specifically of, say, the roll torque generated during a sideslip-- is it dihedral-like (acting to raise the "upwind" wingtip) or anhedal-like (acting to raise the "downwind" wingtip)? $\endgroup$ Sep 19, 2023 at 3:22
  • $\begingroup$ Some in-flight experiments I've conducted suggest that tightening the VG cord shifts the slip-roll coupling at any given airspeed to give a weaker dihedral effect or a stronger anhedral effect. $\endgroup$ Sep 19, 2023 at 3:23
  • $\begingroup$ (Re-posted some of that to fix errors) $\endgroup$ Sep 19, 2023 at 3:24

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