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Comment by opwieurposiu | Hacker News Reader

A wing is a device that pumps air downward, which in turn pushes the wing upward, by newton's third law. For a large plane, the wing will be pumping many tons of air per second.

Start with a cube of still air, with zero mean velocity. Fly a plane through it, and that cube will have a mean downward velocity.

http://www.aviation-history.com/theory/lift.htm

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"Pump" is doing a lot of work in that explanation. It's a very subtle thing to explain the pressure differential of a non moving airfoil, and marry it up with such real-world conditions like flying aircraft upside down, trailing edge vortices, and the like. You need both Newton and Bernoulli here, and Euler when viscosity is irrelevant (most airfoils except on the boundary layer) -- the Kutta-Joukowski theorem demands circulation (= vortices) for lift to happen. And that's why aircraft have to wait in line at the run way, since there's huge trailing edge vortex sheets shed on take off.

Fluid dynamics is really damn complex. Wikipedia is actually not that bad.

> It's a very subtle thing to explain the pressure differential of a non moving airfoil

I'm not sure that's a real thing that happens in real life or theoretical physics.

If the airfoil isn't moving, is anything happening? Or are you talking about a non-moving airfoil with air moving around it? It's impossible to tell the difference between a non-moving airfoil with air moving around it and a moving airfoil with still air around it, given that they're equivalent provided you define the reference frame correctly.

Yeah I used the wrong term: an airfoil that isn't changing its shape or being used like a bird wing, where there's more going on than just in a fixed-wing aircraft.

But yes, the relative motion is what matters.

True...and if people thought about it they would see this is true. Air must flow along the top of a wing and then be sent downward. The bottom of the wing of course "rides" along air like a boat on water. These two things cause air to go down and the plane to go up. Every pilot knows if you hit the relative air at a critical angle the air on top of the wing will stop "sticking" and seperate from the wing...this is called a stall...and you won't have flight anymore. Also...frost on the top side of the wing will cause the air not to stick to the wing...and you can't fly in that case either. So we see easily it is not some magical pressure differential that is "sucking" the plane up.

Wait a second.

Are you saying if I have a cube suspended in space, and I put e.g. a drone inside it that just flies around in a circle inside the cube, the cube will start to move down? What then? Will the drone crash into the ceiling of the cube because it's independent from the cube itself?

My brain disagrees with this, but I'm a hacker, not a physicist.

https://www.newscientist.com/article/dn26807-if-birds-in-a-t...

I think the parent poster means a "cube of air" as in a region of air, not something bounded on all 6 sides by glass walls.

But to your question, if it was a glass cube, I'm pretty sure the drone would hover in the middle of the cube while gravity pulls it down exactly like if the drone was turned off, based on conservation of momentum. Sure the drone is pushing the air down, but then the air hits the bottom, bounces up to the top, and then gets pushed by the drone again. It's (very literally) a closed system, so nothing happening inside is really noticeable outside, and vice-versa (steady state, if you smack the box the drone will notice).

the interior of the cube in your example is a closed system, so ultimately there is no net force in any direction. if the cube is large, the drone might create a slight pressure differential accelerating the cube downwards, but it would eventually be canceled out when the drone hits the top of the cube (I'm assuming zero g, so the drone immediately starts accelerating when it turns on its props). if the cube is sufficiently small, the drone might immediately create too much turbulence to generate sustained thrust in any particular direction.

If you just track that cube of air and it's not somehow fixed ... yes, because a downward force is acting on it (with corresponding flow), to keep the drone up.

A better analogy one of my favorite kids' shows (I think it was the German Wissen macht Ah) made was putting a heavy boat into a pot of water on a scale - of course the scale reflects the weight of the boat, it has to go somewhere.

While that's correct, that doesn't answer at all the question of how the wing pushes the air downwards.

It's like answering "how does a lightbulb work" by saying you put current in and it produces photons.

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