If the racecar wing pushes against the atmosphere above it (it doesn't, but whatever) and doesn't need something solid beyond the atmosphere, then why does a plane need to push against the earth 5 miles below? Simple, it doesn't.

Also, if your sheet of verticity or whatever it is can dissipate some distance behond the plane, why can't some other movement dissipate and break up before it hits the earth?

How does that paper towel holder roll towards the airstream from the hairdrier anyway? Magic? Superconductive elctro-magnetic levitation? You really don't need wind tunnels and lasers to observe this.

And, a plane's density doesn't change when it flies (except for fuel burning off, but I think we can agree to ignore that.) It goes up and down by varying lift on the wings either through flap changes, velocity, or whatever. A balloon goes up withouth generating lift because it is less dense. This happens EVEN THOUGH THE ATMOSPHERE IS APPLYING EXACTLY THE SAME PRESSURE ON ALL SIDES OF THE BALLOON AT THE SAME TIME. So, density and lift are not really related. Take the lift off a plane's wings and it drops. Lift does not change the density of the plane.

Finally, if lift is due to the rollover, vortex, whatever, then are your saying there is no upward force on the center of the wing, only around the tips. The vortex doesn't generate the lift, it is a result of the higher pressure air under the wing trying to equalize with the low pressure air on top of the wing. This causes the air to want to force it's way around the wing to equalize, and the result is the vortex you see. Additional vortices are created by air spilling over the tail, etc. Then there is more turbulence from the wake of the air just backfilling the hole left by the plane. An aircraft carrier has a pretty big wake behind it, but no one would argue that it is generating lift. The water is simply filling the void left by the passage of the ship. This results in turbulence felt far away from the object itself, but not forever.

 

Because gravity only pulls down, not up. When you push up on the atmosphere, you are pushing against its weight. When you push down on the atmosphere you are adding to its weight.

I'll try one other explanation:

What makes air pressure? Most people know this, it is the weight of the atmosphere above you that makes the air pressure. That's why when you are in the mountains the pressure is less, because there is less atmosphere above you.

OK, so when a plane is flying in the atmosphere, it is being supported by the atmosphere. Its weight is being added to the atmosphere. Since there is more weight above you, then you would expect the pressure to be a little bit more, right? Does this make sense?

So hopefully you can see that the weight of the airplane adds to the pressure of the atmosphere below it. Because that pressure ultimate pushes against the earth's surface, this is how the weight of the airplane is ultimately carried by the atmosphere back to the surface of the earth. But very, very spread out.

 

No, it's not. Even though the gravity term is mostly dropped from the air pressure equations because it is so small, it still exists.

 

Buoyant.

 

mistressmotorsports, a racecar wing DOES push against the air above it. Which can easily seen by the HUGE rooster tails that F1 cars produce in rain conditions. And the reason that the Earth's atmosphere doesn't dissipate is simple - gravity holds it from escaping. With an airplane, there isn't that force of gravity to push UP on the atmosphere so obviously there must be something else - it just happens to be the fact that the air cannot pass through the Earth, and in fact pushes against it. (At about 14.7 pounds per square inch.)

Similarly, a table pushes UP against a wine glass sitting on it. (Smart table - it pushes with the exact amount of force required to support the glass's weight.)

 

1) I'm still not convinced. Streamlines just in front of airfoil are all parallel, streamlines behind airfoil are parallel (and the same direction as original), thus no downward velocity. There is also no question that the pressure on top of the airfoil is lower than that below. Bernoulli is playing a role here...and not just in this idealized case. The pressure on any wing will be lower than ambient under normal conditions. Are you saying that's caused by something other than the Bernoulli effect? Bernoulli is not sufficient to explain the amount of lift generated by any given wing, granted. But I don't think simple momentum transfer is either, is it? It has to be a combination of the two. Maybe there are 3-D effects here I'm not considering.

2) Agreed. But the force on the wing is resisted only by the air, assuming of course you are not really close to the ground. The potential energy of the pressure gradient around the wing is entirely dissipated by viscous forces in the atmosphere. I don't know how much distance it takes to dissipate all the energy, but I don't think it's anywhere near the cruising height of an airliner.

3) I should have worded my initial response differently. I agree with everything you said. For boats I was thinking about density difference causing displacement of water causing buoyant pressure, but I can't wrap my brain around it coherently right now.

 

This is where we get into interesting but futile territory. We all agree that lower static pressure on the top of the wing creates lift. We all agree that higher velocity above the wing creates lower static pressure.

But if you look at the wing from the side, higher velocity above is exactly the same thing as adding a circulation to the air around the wing. This is the same as saying a velocity of "4" above the wing and "2" below it can be broken out as a velocity of "3" on both sides plus a circulation of "1" (ie. "+1" above and "-1" below). Do you follow me?

It happens to be much easier to do the calculations when you only look at the differences, and so we say that the lift is proportional to the circulation. That doesn't mean we think air is actually flowing forward under the wing, just that it is flowing backwards less quickly.

That circulation is the vorticity, and it ends up being visible in the wingtip vortices.

It is not true that the vortices are physically caused by the air curling over the wingtip. (There is some of that, but it's not the main factor.) In fact, anywhere there is a spanwise change in the lift distribution, a small vortex forms behind the wing. Of course at the tip of the wing, there is a big change in the lift distribution (from some to none!). All of these vortices merge together into one large vortex (per side, rotating in opposite directions).

What you are not understanding is that "turbulence" does not cause these vortices. Turbulence is random, but those vortices are very much non-random. They are a direct result of the circulation around the wing, which is the direct cause of lift.

 

In my high school chemistry class the teacher told us that if we didn't believe that water - or liquids in general - were very slow in reacting to changes in pressure we should go find a swimming pool, do a belly-flop off the diving board, and see how fast the water reacts to the sudden change in pressure.

And if you think that liquids react slowly to pressure changes, try draining the pool and then doing a belly-flop: the concrete - or solids in general - will react even more slowly than liquids do.

 

Based on my understanding, the statement in bold is not correct. Streamlines behind the airfoil point down.

 

This is always a raging debate between people who don't really understand what is going on (no offense intended).

If you look at the problem from the airplane's point of view, it is the exact pressure distribution on the wing (and the rest of the airplane skin) that adds up to the lift. Whatever happens anywhere else in the atmosphere is besides the point.

But from the atmosphere's point of view, it is the net downwards momentum added to the atmosphere that cancels out the weight of the airplane. The exact details don't matter -- if you draw a box around the airplane, the net momentum downwards of the air going out of the box from the air going into the box is all that matters.

What the people who argue endlessly about these explanations don't seem to understand is that the physics of the atmosphere connects the two explanations. You CAN NOT have one happen without also having the other one happen. So to argue about which one is happening is the height of foolishness.