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it is clear that the overall integrated pressure on the top of the wing is less than the overall pressure at the bottom of a wing in order to produce a net nonzero lift force. Based on the attached figure below (AoA going from small to large), it seems that, for low and medium angles of attack, the pressure both at the top and bottom surfaces of the wing is lower than the free stream atmospheric pressure. The arrows' directions, from what I understand, are pressure gradients pointing toward higher pressure area, correct? When the angle of attack is sufficiently large, the pressure on the wing's bottom surface is higher than the free stream pressure (positive pressure coefficient)...Is that correct?

Whenever the local pressure is lower than atmospheric pressure, I think air would rush to that area so air would be moving from far above the wing towards the wing top surface (same for bottom wing surface when the pressure coefficient is negative). Is that what happens? Thanks. enter image description here

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  • $\begingroup$ You may find the website How it flies interesting. $\endgroup$ – Manu H Dec 29 '18 at 22:32
  • $\begingroup$ "it is clear that the overall integrated pressure on the top of the wing is less than the overall pressure at the bottom of a wing in order to produce a net nonzero lift force." This does not take into account that the wing produces downwash at the rear of the wing to press the air flowing across the wing down, and this downward pressure pushes the wing up. The flow across the wing gets less and less smooth as the AoA increases, up to the point where the wing stalls from the loss of downwash/loss of lift. $\endgroup$ – CrossRoads Dec 31 '18 at 2:02
  • $\begingroup$ @CrossRoads: When looking from the point of view of solving the Navier-Stokes equations, the downwash is the inevitable effect of the pressure distribution on the wings. So "take into account the downwash produced by the wing" is actually the same as "overall integrated pressure on top is less than bottom". You can design a wing where the overall integrated pressure is equal and you will find zero downwash or one where it is more and you will find upwash. $\endgroup$ – slebetman Apr 12 at 6:46
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Last question first. Yes, air does try to "rush" from high to low pressure, but the movement of the wing creates a condition where the net movement of the air relative to the wing will be downwards and backwards, and lower pressure remains above the wing. This is why airspeed is so important.

Understanding the arrows in the diagrams is a bit like reading music or another language. They are simply pointing from lower to higher pressures. Isobars on the weather map show the same thing as pressure gradients.

Also important is to know that you are looking at a fully symmetrical airfoil. There will be equally opposing lower pressure areas top and bottom at 0 angle of attack. This is why many airfoils have a flat or even concave bottom to help create higher pressure underneath the wing. But even a fully symmetrical airfoil will have a pressure differential as AoA increases.

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Yes indeed, there is also some suction (so lower than atmospheric static pressure) on the lower side, because there, the air flow is "pushed away" as well. In the first image, you can see that both pressure distributions are equal, so this symmetric airfoil doesn't produce any lift at zero angle of attack (AOA). Only when you introduce an AOA, the air "sees a more convex curvature" (so even higher pressure) on the upper side and the opposite on the lower side, which then creates some lift. The third image is quite interesting: there you actually get higher than atmospheric pressure near the nose and again lower pressure further back. This all has to do with how the streamlines bend around the airfoil:

enter image description here

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    $\begingroup$ You get higher than atmospheric pressure in all three images, at the stagnation point. $\endgroup$ – Peter Kämpf Dec 31 '18 at 0:49
  • $\begingroup$ Thank you. Previously, I thought that the pressure on the bottom surface was larger than atmospheric pressure for any AoA. In reality, the pressure at the bottom surface is higher than the one at the top surface, even when both pressures are lower that ambient pressure far from the wing. If the wing was stationary and the air moved, the pressure in the airstream far from the wing would be lower than the pressure of air that is at rest. We call "static" pressure the pressure when it is measured in a flow in the direction perpendicular to the direction of airflow. That seems misleading... $\endgroup$ – Brett Cooper Dec 31 '18 at 14:55
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    $\begingroup$ Exactly, it is the pressure difference which creates lift. But be careful, it doesn't matter whether the wing or the air is stationary, as long as the relative velocity is the same. Also, the definition of static pressure is not related to how it is measured. It is simply the "actual pressure of the fluid, which is associated not with its motion but with its state" (see Wikipedia). It is usually measured with static pressure taps which are indeed perpendicular to the surface, but this is just to prevent a measurement error due to air entering the tap and slowing down. $\endgroup$ – Daniel Dec 31 '18 at 16:42
  • $\begingroup$ Thanks Daniel. The word "static" has led me to believe it would be referred to the pressure exerted by air not moving. If a fluid moves to the right at speed v, the pressure measured by an vertical area placed in the fluid would be P+dynamic pressure. But if we measure pressure in a direction normal to the fluid (using the pressure taps) the pressure would be different and smaller... $\endgroup$ – Brett Cooper Dec 31 '18 at 18:21

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