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I have been playing some flight sims and have noticed that in some of them you get wing flex during hard landings. Is it realistic? Can wings flex upon a hard landing?

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    $\begingroup$ Wings flex during all phases of flight. $\endgroup$
    – Jim
    Feb 17 at 15:07
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    $\begingroup$ Not just in flight, on the ground as well. Taxing, you can see the wingtip flex up and down a couple of cm. $\endgroup$
    – WPNSGuy
    Feb 17 at 16:03
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    $\begingroup$ One thing you learn in engineering is that everything is a spring, and everything flexes to different degrees. $\endgroup$ Feb 18 at 16:46
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    $\begingroup$ So does your car's chassis. You don't get to build any transportation vehicle without studying this at length, and designing your vehicle so all normal flexure happens in the elastic range (bouncing back), and not the plastic range (de-forming) for that metal and thickness. If you want to see "plastic" forces, watch Dukes of Hazzard. Most of those Dodge Chargers did not survive those jumps. $\endgroup$ Feb 18 at 20:29
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    $\begingroup$ Fun fact: if you are seated in the right place, you can observe this from inside the plane. Which can be a little unnerving. $\endgroup$ Feb 19 at 15:01

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Yes, when the wheels touch down the wings still have a negative vertical velocity component and they sag down dynamically, and then rebound back up again to a constant value of "static" sag that reflects the weight of the wing and its stiffness.

In the C-5 heavy transport, the touchdown sag was sufficient to "eat up" the fatigue design margin in the huge wing attachment structure which transferred wing lift to the fuselage, causing premature cracking. This required a significant redesign and retrofit work on that "box structure" to allow the C-5 airframe to meet its design lifetime goals.

At one point, the Air Force considered using active thrust vectoring from the engines to "prop up" the wing during landings and thereby reduce the magnitude of flexure. One of my engineering professors worked on a dynamic model of C-5 wing flexure stresses and strains during touchdown sag to see if tipping the engine exhaust stream downwards was a viable solution to the problem.

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    $\begingroup$ Is there any good video of this happening? I feel like a video would say 1000 words. $\endgroup$
    – stevec
    Feb 19 at 3:55
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    $\begingroup$ @stevec, not that I know of. -nn $\endgroup$ Feb 19 at 3:55
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    $\begingroup$ Not sure if it's the best example, but here's one: youtube.com/watch?v=OkL5tFmk9BU $\endgroup$
    – stevec
    Feb 19 at 3:57
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    $\begingroup$ Was there a solution?? $\endgroup$
    – copper.hat
    Feb 20 at 7:36
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Yup. While flying, the wings are supporting the aircraft's full weight. After touchdown this weight transfers to the wheels, preferably smoothly and rapidly. So of course the wings will flex.

However some flight sim designers turn this effect up to 11 ;)

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  • $\begingroup$ I remember playing X-Plane 11. I don't know if it was me settings (probably) or what, but when the plane got high enough, this would happen to the point it would flap its wings similar to a bird. I could never figure out why but enjoy the game. $\endgroup$ Feb 18 at 7:36
  • $\begingroup$ It's up to the modeller in X-Plane - you can scale it down with some keyhole surgery to the aircraft's .obj files. $\endgroup$
    – Jack Deeth
    Feb 18 at 9:52
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Are you talking about bouncing up and down or "remaining sagged"?

For the first one, oftentimes upon (hard) touchdowns, there is some bouncing.

For the second one, the wings remain "sagged" while producing lift.
If there is still noticeable airspeed, the wing will still be producing lift, so there will be some "sagging" indeed.
BUT as soon as the spoilers are activated, (most) lift gets spoiled and the wings stop flexing.

It's actually very cool to see how the wings drop as soon as the spoilers are deployed.

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    $\begingroup$ Sagging is usually downward. Wings flex up while providing lift. They sag or “droop” under their own weight when not providing lift. A B-52’s wings can flex upward by as much as as 22 feet and droop 11 feet on the ramp. They even have outrigger gear near the wing tips to keep them from hitting the ground. $\endgroup$
    – Jim
    Feb 17 at 22:07
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    $\begingroup$ Please don't use codeblocks for anything that is not actual code. $\endgroup$
    – Nij
    Feb 18 at 10:18
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The sag of the wings is particularly apparent, if you sit in a window seat near the wings.

The following pictures were taken from the lower deck of an A-380 in-flight during the approach and later post-flight on the ground.

Notive, that in-flight, the root of the wing-tip is visible, while post-flight, only a portion of the wing-tip can be seen.

in flight

post flight

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Structures like aircraft wings and even bridges are quite rigid, but not totally. There is always some strain in response to stress (stretching in tension or compressing in, well, compression). Bending loads are basically just tension on one side and compression on the other, so bending loads result in, well, bending. Structures that are extremely rigid are often brittle - in other words, they tend to fracture before exhibiting much, if any strain. The visible flexing of an aircraft wing is generally a good thing, because it is a more forgiving response to high levels of stress than a sudden brittle fracture. The only downside to a lot of flex is the possibility of harmonic resonance, in which oscillations can build up in intensity and overload the structure (in wings, it is known as flutter). Aircraft wings are designed to allow for some flex to provide a forgiving response to high stresses, while remaining stiff enough to avoid flutter.

On a hard landing, the wing unload its lift abruptly, so the upward flex it had under flight loads is suddenly relieved, but the vertical velocity of the aircraft is suddenly arrested by the landing gear as it contacts the runway; the wings do have mass, and thus inertia, so want to keep moving downward even as the rest of the aircraft's vertical velocity is canceled by touchdown. Put all that together, and the wings will always appear to flex downward on landing, it's just more pronounced on a hard landing.

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If you get many chances to fly on an airliner, sometimes sit behind the wings, and enjoy looking out the window, you will quickly learn that the wings flex all the time. They are long and fairly thin, and all materials have some give. During turbulence the flexing can be quite visible. I think you can see it during landings too.

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