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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#41

Post by eganist »

opwieurposiu wrote: post #40

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

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.

Re: The mysteries of aerodynamic lift

#44

Post by nimish »

opwieurposiu wrote: post #40

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

"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.

Re: The mysteries of aerodynamic lift

#45

Post by _whiteCaps_ »

eganist wrote: post #41
opwieurposiu wrote: post #40

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

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...

Re: The mysteries of aerodynamic lift

#46

Post by pdonis »

kryogen1c wrote: post #11

this article is so confused and unscientific i have a hard time forming a coherent response. > although bernoulli's theorem is largely correct ... the theorem alone does not explain why this is so or why the higher velocity atop the wing brings lower pressure along with it this blurb is accompanied by an upside down plane with the caption "doesnt explain why planes can fly inverted". this is a "tide goes in, tide goe…

The main thing I see that the article leaves out is circulation. A wing with a positive angle of attack doesn't just push air down; it creates circulation.

Re: The mysteries of aerodynamic lift

#48

Post by czinck »

eganist wrote: post #41
opwieurposiu wrote: post #40

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

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.

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).

Re: The mysteries of aerodynamic lift

#49

Post by jvanderbot »

H8crilA wrote: post #39

I'm surprised people don't start with the basics on this confusing topic. The third law of Newton's mechanics tells us that for the plane to get an up force to counteract the gravity, the air must receive and equal amount of down force. Therefore what planes must be doing is deflect air masses down. A plane must be applying a downward force to air masses, with total force value of "mass * g", i.e. supply "mass * g *…

Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved. This principle seems to work just fine for me w.r.t. fixed wings.

Re: The mysteries of aerodynamic lift

#50

Post by leetcrew »

eganist wrote: post #41
opwieurposiu wrote: post #40

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

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.

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.
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