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Ok so thinking about aircraft like the 787, which have some quite drastic wing flex (upper image), I was wondering what the loads are that are placed on the skin on the the upper and lower surfaces of the wing (not considering torsion from control surfaces etc...) - considering tension and compression only.

So naturally, one would assume that the upper skin panels experiences compression, and the lower skin panels, tension due to the different radii of rotation they both follow. However, does, for example the lower surface of the upper wing skin experience tension, not compression, due to the material thickness (I'm thinking about the principle in the lower image).

enter image description here

enter image description here

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For the purposes of this thought experiment, you can assume that wing deforms as a whole structure, or more precisely a sandwich structure. So you have a thin skin of composite material and a foam (or air) in between.

Regarding the loads that the wing experiences, is mainly drag and lift, which you can assume that are distributed loads (almost uniform, or triangular).

In that case, the strain distribution can be seen in the following image (see below $\varepsilon_c$. The only difference is that the lift has the opposite direction to the load in the image. You can see that the top distribution is entirely in tension and the bottom skin is totally in compression.

So the strain distribution is continuous, i.e. there are no abrupt changes (even though there is no material.

enter image description here

Additionally you can see that the stress distribution is not continuous.

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