# Why will the bending of air over an airfoil make a force? [duplicate]

As seen here, the air will bend downwards when it travels over the upper surface. Nothing new here.

The reason it does this is because if the air was not to bend, it would create a vacuum.

The confusing part is why this bending will actually make lift. I understand why the air bends downward over the upper surface, but I don't understand why that would make lift.

A good way to think of it is that air never pulls anything. Only pushes. If that is the case, you could say the air traveling over the upper surface actually gets pushed downwards towards the wing. If you consider it that way, why will there be lift created from the air being bent downward? The air molecules are getting pushed towards the wing's upper surface under their own power, so to speak.

To summarize, I understand why a pressure difference between the surfaces would make lift, but because air can't pull anything I don't understand why the air bending over the upper surface would make lift.

• (This might seem like a dumb question, but I've been thinking of it on a molecular level and it doesn't really make sense to me that way) Commented Jul 14 at 4:47

Imagine yourself on a cart with very low friction wheels on a plain and smooth surface. With you on that cart you have a supply of tennis balls. Or brass ingots, doesn't matter as long as what you have with you has some mass. Now imagine what happens if you start throwing them in one direction.

The cart with you on it would start moving in the opposite direction. By accelerating some mass in one direction, you experience a reaction force in the opposite direction. Or, as Isaac Newton said: Actio equals reactio.

Basically, the pressure field around the wing accelerates an air mass downwards, and the reaction force is applied to the wing as lift. Or, as I am not tiring to repeat that lift can be explained in several ways, the wing accelerates a stream of air downwards, experiencing lift as the reaction force. Or, the more intense bouncing of air molecules against the lower side of the wing and the less intense bouncing on the upper side give them an added impulse downwards. Again, the reaction, summed up over the wing's surface, is lift.

• Yes, a thousand times yes. The flow causes a pressure field, which in turn causes "the air to bend," and causes forces and torques such as lift and drag. Start with the pressure field. Commented Jul 14 at 16:38
• Sorry for late comment, but just to clarify is the last comment on this answer correct? (The other comments can provide context) Thanks for your answer! Commented Jul 18 at 3:15
• @Wyatt It is all connected. Of course does the pressure difference over an area create a force when summed over that area. But it also causes the flow to change direction (in that case it is better to speak of a pressure gradient - a difference means some jump in value and only happens across a solid surface or a shock). But I think the comment meant something else than what you thought when you spoke of "bending" - the answer itself is very good. Commented Jul 18 at 8:58

The objective of a cambered wing is to tilt the momentum vectors of oncoming air parcels downwards. This requires the wing to apply a force to the air as it passes by and the reaction force is equal to the lift generated by the wing.

There are several different mathematical models of this process, with varying levels of accuracy and complexity, but none of them by themselves captures all the relevant physics. The best models are instead finite-element compressible flow simulation programs that run on large computers, and all the major airplane designers use them.

air never pulls anything. Only pushes.

Not quite right. On a molecular level, viscosity of moving air does pull in surrounding air.

But in the lift case, you are right. Air from underneath the wing pushes the wing up into the lower pressure above the wing (created by camber and angle of attack).

The air flow above the wing bends downwards because ambient air is also of higher pressure as compared with the localized lower pressure just above and behind the wing.

• Interesting. Regarding the first paragraph of your answer: So the viscosity will pull the other air molecules due to friction I’m guessing? Commented Jul 14 at 15:07
• @Wyatt yes, and the working fluid does not even have to be air. A common sink aspirator uses moving water (from a faucet) to create a vacuum in a closed tube. Commented Jul 14 at 15:26
• I see. So similar to how if you rub your hand along a piece of paper the paper will get pulled in the direction of movement. As for the rest of your answer: So the bending of air directly above the wing doesn’t really make the lift? Another question I asked recently here would agree with that, so that would make sense. Commented Jul 14 at 15:34