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I was reading this when it occurred to me that - unless I'm misunderstanding it - it depends on the assumption that the fluid is moving.

http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf

Sure, the fluid is moving relative to the wing, but that's not what the explanation is premised on. He talks about streamlines and particles with velocity, which they don't (negligible) in my office here.

Now, if I imagine a wing passing in front of my face as I sit at my desk, the wing is just separating still particles and then they come back together.

If there was a puff of smoke in front of me and a wing passed through, the smoke would mostly still all be in front of me, diffused more.

I'm an ordinary idiot off the street, so I assume I'm wrong and embarrassing myself by asking. But why?

I find the explanation unsatisfactory, its just not what happens, gas molecules are laying still and a solid body is moving through them, pushing some up and some down.

Originally, erroneously posted here:

https://math.stackexchange.com/questions/3387491/how-wings-work-and-still-air

Luke Puplett
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5 Answers5

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Velocity is all relative to the frame of reference.

To explain aircraft dynamics, it usually makes more sense to focus on the reference frame of the aircraft. When a plane is flying through the air, from it's point of view, you could say it is stationary, and the air is coming towards it with a velocity opposite of the velocity that the plane is moving relative to the stationary air.

So air flowing past a stationary wing in a wind tunnel at 500 kilometers per hour has the same dynamics as a wing flying through stationary air at 500 kilometers per hour (if the wings are the same size).

JMac
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  • When I think about why my hand wants to shoot up or down when I swish it through water, depending on the angle, it's explained with same theory whether the water is moving or my hand is moving: water molecules are pushed down, hand goes up. Surely the math should work both ways. – Luke Puplett Oct 09 '19 at 23:55
  • @LukePuplett It does, that's what my answer was saying... If the wing is moving, from the wings perspective, stationary air still has a streamline, because that air is constantly moving relative to the wing. The same applies in reverse for the smoke streamlines in a wing test. The wing isn't moving but the air is moving the same speed relative to the wing, in the observers perspective (who is stationary like the wing) you get streamlines again. It's all about the fluid having a relative movement to the object. – JMac Oct 10 '19 at 01:32
  • The paper's math is premised on moving particles, they have a high value v but in reality this is nonsense, they have no velocity in still air. This practical observation was all it took for me to realise it was nonsense, and I found that the real reason wings have lift is because of my example of swishing my hand through water, the wing/hand simply pushes mass down. Its interesting we can so easily get stuck in the theory and forget reality :) – Luke Puplett Oct 10 '19 at 13:22
  • Useful to be aware of our flaws https://www.psychologytoday.com/us/blog/ulterior-motives/201510/why-does-misinformation-continue-affect-thinking – Luke Puplett Oct 10 '19 at 13:28
  • @LukePuplett No, you seemed to have completely missed their point and my point. The air particles do have a high value of $v$ from the perspective of a wing. Velocity is all relative. A plane flying through still air has some velocity relative to that air, and therefore, the air has some velocity relative to the wing. Velocity is all relative to your reference frame, and a wing moving with a constant velocity is an inertial reference frame where newtons laws can equally apply. – JMac Oct 10 '19 at 13:32
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A still wing in still air provides no lift. One must move relative to the other, whether it is a fixed wing in a wind tunnel or a wing moving through air that is either still or moving at a different speed.

Adrian Howard
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Wings will still work like normal in still air. It's just that to my understanding if the fluid is moving against the wing it will make the wing generate more lift because the air velocity would add on to the wing lift because the air is moving faster over the wing. so if the fluid is moving with the wing than it would subtract from the wings lift force because it would e moving slower over the wing I think.

T. Fisher
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  • I mentally model this as if the fluid is moving perfectly with the wing, then the "whole system" is moving along together and the wing is not "displacing" any particles, so no effect occurs. – Luke Puplett Oct 09 '19 at 23:31
  • Does that mean the air is moving as fast as the wing thus no air would move over the wing in the system? – T. Fisher Oct 09 '19 at 23:34
  • Yeah, though I prefer to think that the air is still and the wing moves through it, as this is what's actually happening. Like your hand through water. – Luke Puplett Oct 09 '19 at 23:36
  • well you have befuddled me. Because if the fluid is moving perfectly with the wing like you said then the wing wouldn't generate lift. but if the fluid is not moving perfectly with the wing meaning in the same direction and velocity and acceleration and ect than it would generate lift. – T. Fisher Oct 09 '19 at 23:39
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Okay I have found the answer and its on this site.

What really allows airplanes to fly?

It's as I thought, air gets pushed down, like my hand in water. I can sleep now :)

(I've no idea what that paper I linked to was going on about, then)

Luke Puplett
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Wings generate lift according to air speed, i.e. the relative speed of the wing to air. This is what an aircraft measures with its instruments (it can only measure relative values). If the airspeed is too low the plane will stall, i.e. not generate enough lift to stay up. The air speed relative to ground is only relevent when the plane is taking off or landing in order when one wants to be going into the wind in order to minimise runway length needed without stalling.

The same goes for all wings, eg sails on a boat.

Paul Childs
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