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I've noticed that the wings of the 787 and A380 tend to flex a lot. Does wing flex help an aircraft in any way?

enter image description here

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    $\begingroup$ Not flexing would be a bad thing. The flexing absorbs a lot of the loads on the wing. Look at the picture above. Now imagine that the wings were rigid. Imagine the internal structure required to keep them rigid and how the loads would be spread through the structure. $\endgroup$ – Simon May 14 '15 at 7:41
  • $\begingroup$ See also this question $\endgroup$ – Peter Kämpf May 14 '15 at 7:44
  • $\begingroup$ I'm surprised none of the answers so far discuss any cyclical fatigue. When I see flexing wings, the first thing i think of is how much more bending can they take before they snap like a paperclip? $\endgroup$ – DrZ214 Jan 18 '16 at 1:36
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The wings of the Boeing 787 are so flexible because its carbon fiber material can be stretched more, and the high aspect ratio of 11 will magnify this effect. In flight the consequences are:

  • Less shaking due to gusts, because the wing will dampen load changes more effectively.
  • Delayed aileron response, because the lift change due to aileron deflection will first bend the wing before it starts to roll the aircraft.

On the ground one consequence is:

  • The wing might have less tip clearance, because less in-built dihedral is needed - the rest is supplied by the wing's elasticity in flight.

In general, wing flex is like the suspension in a car. It costs a little bit of performance but gives a much smoother ride.

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    $\begingroup$ @Dave: We are not comparing blocks of the same size, but structures made to carry the same load. And if you look at christinedemerchant.com/carboncharacteristics.html, carbon fibre composite is ~20 times stronger than aluminium. While it only has ~2.5 times higher elastic modulus. The strength numbers are by weight while the elastic modulus is by cross-section, so it has to be corrected for difference in density and the best aluminium alloys used are probably better than the figures listed, but still indicates that the difference in strength is bigger than difference in rigidity. $\endgroup$ – Jan Hudec May 14 '15 at 16:59
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    $\begingroup$ @Dave: So a structure built to carry the same load can be lighter by more than the difference in elastic moduli and as a result will flex more. That is what “can be stretched more” means. $\endgroup$ – Jan Hudec May 14 '15 at 17:02
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    $\begingroup$ Carbon fiber does bend a lot more for the same strength. A rule for thumb for carbon fiber is that when it is rigid enough that you feel safe (like a bicycle for example) it is safe. $\endgroup$ – Phil May 14 '15 at 17:23
  • $\begingroup$ My apologies I have always been under the impression that it did not bend that much (at least from my experience with it) I will remove my comment to reflect my error. $\endgroup$ – Dave May 14 '15 at 18:08
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I will try to add over @PeterKampf answer. Flexibility is finally another parameter, when you make your design allowing your airplane to be flexible, you are introducing a new variable. As in all optimization problems adding new (smart) parameters allows you to create a more optimized design.

Just taking the example provided by @PeterKampf , if the airplane is less shaking due to gusts the requirement over the aerodynamics/structure of gust response is more easily achieved. That makes relaxation in some areas of the structure making it lighter. So, altohugh there is a potential cost in aerodynamic performance, it might end in savings in fuel as the airplane will be lighter for the same gust response.

There is also an effect that is not seen in the picture, what you is the flexion produced by the lift, but also the lift is producing cambering, which can be also exploited by the wing to have a more optimized design.

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  • $\begingroup$ In case of the 787, its flexibility was not a free design parameter, but the consequence of the material, the low airfoil thickness for flight at Mach 0.85 and the high aspect ratio. And to my knowledge cambering only manifests itself in hang glider wings - here the cover is flexible enough to be pulled out by aerodynamic suction. $\endgroup$ – Peter Kämpf May 14 '15 at 19:48
  • $\begingroup$ Flexibility is a parameter.... and the decision is to use that material and reduce thickness of the airfoil. Is the other way arpund. $\endgroup$ – Trebia Project. May 15 '15 at 17:27
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    $\begingroup$ Do you honestly think that cabonfiber was selected because it produces more flexible wings? Or was it selected because its lower mass, and the higher flexibility was accepted? $\endgroup$ – Peter Kämpf May 15 '15 at 19:25
  • $\begingroup$ We are saying the same thing here... traditional design was avoiding flexibility, in order to accept flexibility a benefit in weight was obtained in exchange. My point is that we add another parameter or removing one constrain. Making the airplane more flexible. We are discussing if allowing flexibility from aerodynamic point of view was the enabler or a new material put pressure on aerodynamic for accepting the material. Is chicken-egg question. It is both $\endgroup$ – Trebia Project. May 15 '15 at 19:35
  • $\begingroup$ @PeterKämpf they could have changed the parameters of the wing panels and struts to make them more rigid, but didn't to prevent overstressing the material. $\endgroup$ – jwenting May 16 '15 at 5:10
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Wing flex is also to increase the aerodynamics of the plane. Wing flexes produce more lift because of their flexibility which actually allows more lift to generate. Wing flex acts as flaps but isn't, wing flexes can also support thousands of pounds and lift them with ease because of the lift it generates. This is why the Boeing 787 doesn't need flaps extended to the required curve for takeoff due to the curvature and flex of the wing.

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    $\begingroup$ welcome to aviation.se. may I ask you if you have any source for what you wrote? thank you! $\endgroup$ – Federico Sep 24 '18 at 16:56
  • $\begingroup$ I have no source, it is just pure knowledge on plane aerodynamics and how it helps an aircraft produce lift $\endgroup$ – Fat Boeing 787-OOF WING FLEX Sep 24 '18 at 17:25
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    $\begingroup$ Would you then back up this knowledge with an explanation on how exactly a spanwise curvature performs the same task as a flaps extension? $\endgroup$ – Sanchises Sep 24 '18 at 17:41
  • $\begingroup$ Unfortunately I can’t find any evidence that your statement is true and is counter intuitive to everything I know about aerodynamics. $\endgroup$ – Carlo Felicione Sep 24 '18 at 17:43

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