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While conducting a structural analysis FEA for an aircraft wing, I had to import the CFD pressures acting on the wing, lets say at an A.o.A = 10 deg, at 80 m/s and at sea level, and say the atmospheric pressure is 1 atm. Should the imported pressures from CFD analysis (on the top surface and bottom surface of the wing) be the absolute static pressures or the net pressures? (Assuming the wing surfaces have no slip condition).

If I use the absolute pressures, then the top surface would be experiencing a positive pressure meaning that the pressure is acting onto the top surface (causing it to shrink), and if I use net pressure then that would be negative meaning that the pressure is acting out of the top surface (causing it to expand).

Which one is true? I think that absolute pressures should be used because if, for example, we want to know the structural behavior of the wing top/bottom surface under 1 atm only (assuming wing is symmetric and at an A.o.A = 0 deg), then the net pressures would be zero and we wouldn't have any forces acting on the top or bottom surface of the wing.

Also, does the wing's top surface, in reality when it is flying, deform out (i.e. expands) or deforms in (i.e. shrink)? I think since the absolute pressure is still positive so it must deform in, but some sources say that it instead deforms out. (Assume a static structural condition). A such source I am attaching in the picture.

enter image description here

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  • $\begingroup$ I adjusted the wording a bit to try to make the question a little more grammatically correct. Also, it appears that the picture relates to your add-on question, so I moved it to the end. $\endgroup$
    – FreeMan
    Commented Nov 10, 2020 at 11:52
  • $\begingroup$ Wings are hollow and open to the atmosphere, so ambient pressure acts on both sides of any surface and nets to zero. $\endgroup$
    – Pilothead
    Commented Jul 7 at 19:09
  • $\begingroup$ That just adds confusion. Yes, if there is no in or out flow inside the wing we can say it has atmospheric pressure inside. SO. . .that upper skin has less pressure above and more below, so the inner pressure Pushes the skin up which then pulls up on the ribs and spar. BUT. . . the lower skin also has more pressure below than above it and pushes the ribs and spar up. Wings are hollow, but don't need to be, so it's better to leave that out for a basic unde3rstanding. $\endgroup$
    – Steve Noskowicz
    Commented Jul 9 at 7:02

2 Answers 2

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When applying a FEA loadcase, you should always keep in mind the complete picture of which forces are applied on any surface, and in what situation you consider the wing "undeformed".

If you start out with a solid wing in the vacuum of outer space, and descend into the atmosphere, then indeed the wing will compress under a hydrostatic pressure. But this is an unlikely scenario.

A much more likely scenario is that you want to compare the deformation from standing still to flying. The initial situation is already in the atmosphere, and you would typically say this is the undeformed situation. When you want to calculate the deformation when flying, you would only apply the pressure difference. This is the most common way of doing FEA analysis.

If you're interested, the following explains what happens if you do apply atmospheric pressure. Under the assumption of linearity, we're allowed to split the loadcase into differential pressure and atmospheric pressure. So the question is really what happens if you apply atmospheric pressure. The answer is: very little. Remember that atmospheric pressure also acts on any surface on the inside of the wing. The net result is that the entire wing is under hydrostatic pressure. This adds a measly 0.1MPa to the stress of the wing. Furthermore, most failure criteria for structural materials explicitly remove the hydrostatic stress. For example, in the Von Mises stress, hydrostatic pressure doesn't show up at all. So in effect, very little happens and only pressure differentials are applied in FEA.

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  • $\begingroup$ For a wing having, for example, vaccum inside of it, then according to what you said, the absolute pressures should be imported, right? $\endgroup$
    – Rameez Ul Haq
    Commented Nov 11, 2020 at 8:24
  • $\begingroup$ Exactly. Basically, using net pressures is a very convenient simplification since we typically do not care about the hydrostatic stress. You could even apply a -1atm net pressure instead of 0 absolute pressure. $\endgroup$
    – Sanchises
    Commented Nov 11, 2020 at 8:48
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TO answer the question stated.

TO BE PERFECTLY CLEAR:

ALL pressures around a wing are INWARD toward the surface.

In your terms, the upper wing skin is not 'bulging' outward. It is being pressed inward by something a little less than the ambient atmospheric pressure that is far from the wing; or if the wing was stationary. . All surfaces EXPERIENCE "positive pressure". Even a stalled wing. In normal flight, wings do not cavitate.

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