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I know that there are dozens of questions about lift generation here but after reading them I still don't understand everything. My question is: why does air accelerate over the (upper side) wing? I know that it accelerates because there is pressure differential etc. but why wouldn't the air just flow over both sides of the wing with equal speed?

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  • $\begingroup$ This is probably a dupe, but I'll bite anyway. $\endgroup$ – Abdullah Jul 1 at 11:32
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    $\begingroup$ It may also have answers on Physics SE $\endgroup$ – Abdullah Jul 1 at 11:51
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The Abdullah's answer is correct to it's level of approximation, but I'd like to expand on why the pressure is reduced on the leeward side.

As the air encounters the leading edge, it is pushed out of the way. Due to first law of motion, it would like to continue moving outward from the wing and avoid the area just above the wing. But besides inertia, air also has viscosity, which prevents sharp changes in air velocity, so the incoming air trying to pass high above the wing drags the air near the wing along. But when the air just above the wing is pulled out aft, there is shortage of air just over the wing, which means low pressure.

This low pressure causes the air bend around the wing, and since it pulls air from all sides, accelerate as it enters the low pressure region over the leading edge and decelerate again as it leaves it over the trailing edge.

Without viscosity (e.g. in liquid helium) the pressure would not be reduced, because the oncoming fluid would just continue straight over the highest point and the area over the receding part of the upper surface would be filled with stagnant fluid moving with the wing. In fact that's exactly what happens in stall – as the curvature increases (due to higher angle of attack), at some point the viscosity is no longer enough to keep the air moving, the area over the wing gets filled with air just whirling around and not moving away, so there is no longer shortage of air, the sucking of oncoming air stops and the generated lift rapidly decreases ­– just the lower surface still generates some.

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  • $\begingroup$ The common sinktop aspirator helps explain this one. Moving viscous fluid (water) pulls some air along with it, creating lower pressure in the air tube. Same thing with the wing. Important to see the wing in motion and proper AOA are needed for lift. Still wonder about liquid helium at a much higher speed, but, oh well, no practical application (yet). $\endgroup$ – Robert DiGiovanni Jul 2 at 0:27
  • $\begingroup$ hmm, liquid helium. interesting +1. lack of viscosity does also mean zero friction. $\endgroup$ – Abdullah Jul 2 at 4:58
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Air accelerates over the upper side. The reason is simple: enter image description here As the wing - or anything else in air - moves, it creates high pressure at the front and low pressure at the back. The air flowing around the wing gets sucked into this low pressure region, and the suction accelerates it. (It will slow down again at the end of the low pressure zone) But the downturned trailing edge means that only the air coming over the upper surface can access this suction. The air flowing over the bottom has no idea of the suction zone above.

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  • $\begingroup$ The "conventional wisdom" for the layman claims that the accelerated air moving over the top of the wing is what creates the low pressure, not that the low pressure zone is what accelerates the air. I've been reading answers here for years and I'm still a bit lost on that... $\endgroup$ – FreeMan Jul 1 at 13:42
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    $\begingroup$ @FreeMan It's a chicken or egg question. They go hand in hand or none at all (for subsonic). $\endgroup$ – JZYL Jul 1 at 14:26
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    $\begingroup$ @FreeMan, it makes a bit more sense to say the lowered pressure causes the increase in velocity, though it's really the pressure field that causes both, and the pressure field looks like it does because of inertia and viscosity. $\endgroup$ – Jan Hudec Jul 1 at 19:21
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    $\begingroup$ Note that in ‘ideal fluid’, the pressure behind a moving object is just as increased as ahead of it, so called pressure recovery. It is only due to inertia and viscosity that in real fluids the pressure recovery is imperfect and leaves the pressure behind lower than ahead. $\endgroup$ – Jan Hudec Jul 1 at 19:57
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From a layman’s point of view:

As far as lift generation, it is more precise to say that the lower static pressure generated perpendicular to the chord line of the wing is created by an increase in velocity of the air molecules instead of saying that the acceleration is created by the lower pressure. The acceleration of air molecules is greatest on the side (Upper or lower) of the wing with the greatest curvature. Basic aircraft wings are usually curved more on the upper-side of the wing than it is on the lower-side. This curvature creates a greater distance to travel in the same amount of time. Hence, the molecules are accelerated.

Think of it like a marching band or a column of soldiers marching. When soldiers or musicians have to march, they do so at the same speed. Both the cadence and the length of everyone’s steps are identical. When soldiers or musicians have to turn their column to march around an object, the individuals at the outside of the turn will move at a faster pace than the ones on the inside by lengthening their strides. If the turn is very sharp (like a 90° turn), the individuals on the inside of the turn will slow their pace by shortening the their strides. They do this because their is now a difference in the distance they have to travel in the same amount of time. Because there is a greater distance on the outside of the turn, there are fewer individuals per square feet of space on the outside of the turn than the amount of individuals on the inside. But since the column is not stopping, the file of individuals will continue to build up on the inside of the turn while waiting on the individuals on the outside of the turn to catch up. The column viewed from above will look like an accordion or a spring bent in the middle: a lot of material on the inside of the bend; very little material on the outside of the bend.

Let’s transfer this analogy to air molecules that are stationary relative to an airmass. The wing is actually moving through the airmass instead of the airmass moving over the wing. Either way, the side of the wing that makes the molecules move the furthest will temporarily have a lower static pressure and a higher velocity as the same mass of air on the other side of the wing. The lower static pressure creates a vacuum along the curved side of the wing. The molecules that were adjacent to one another at the wings leading edge, before encountering the wing, do not have to necessarily meet at the trailing edge. The average static pressure difference along the wing is enough to create this vacuum. The dynamic pressure of the air mass build up on the other side of the wing will also contribute to lift and increase with angle of attack.

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    $\begingroup$ 1. The air flowing over the wing reaches the trailing edge long before the air flowing below it that encountered the leading edge at the same time. That is, the speed difference is much bigger than could be explained by different path length. $\endgroup$ – Jan Hudec Jul 1 at 19:08
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    $\begingroup$ 2. Flat plate generates lift exactly the same way as typical wing, except it forms a separation bubble behind the leading edge almost immediately, which then leads to early stall. Obviously the upper and lower surfaces of a flat plate have the same length in the flight direction. $\endgroup$ – Jan Hudec Jul 1 at 19:09
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    $\begingroup$ 3. Thin cambered wings like most birds and early aircraft have also have the same or almost same length of the upper and lower surface in the flight direction, yet the curvature means the effect on upper and lower surface are exactly opposite, lowering the pressure and increasing speed on top, but increasing pressure and lowering speed on the bottom. $\endgroup$ – Jan Hudec Jul 1 at 19:14
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    $\begingroup$ It does not really make sense to say whether the lowered pressure causes speed increase or the other way around, they are simply tied together by conservation of energy. Inertia and viscosity is closest to what ‘causes’ them as far as it makes sense to say that – physical laws work both ways. $\endgroup$ – Jan Hudec Jul 1 at 19:19
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    $\begingroup$ No, Bernoulli's principle is not the “reason” the system works. It is part of the reason, but the reason is only the complete set of Navier-Stokes equations (which of course includes conservation of energy). The reason Bernoulli's principle explains nothing is that it is just one equation with three free variables. Well, you need two more equations to solve anything – and note that there is no other equation that would fix velocity here, they all fix the pressure (field – you need to calculate it at all points). $\endgroup$ – Jan Hudec Jul 1 at 19:46

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