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## How do airplanes really fly

 Quote by boneh3ad Yes, you refer to the wake. However, this really relates to drag, not so much to lift.
The direction of the "wake" is mostly downwards (lift), and somewhat forwards (drag). For a propeller, the "wake" is mostly aftwards (thrust), and somewhat circular (drag).

The force exerted by gravity onto the aircraft is in turn exerted by the aircraft onto the air, and eventually, that force is exerted by the air onto the earths suface (forces do not vanish), via a continous impulse (downward shifting of air) that originated at the aircraft.

The other law of physics involved with an aircraft (in level flight) is that the impulse (force x time) exterted by the aircraft onto a volume of air over some period of time results in a corresponding change in momentum of that air, and in the real world, during the interaction between aircraft and air, the downwards component of velocity is not offset by a reduction of pressure, so there's a net "exit velocity", representing the increase in mechanical energy of the air affected by an aircraft.

This NASA article includes an image of the downwash effect on a cloud:

downwash.htm

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 Quote by boneh3ad Bernoulli's principle is one way to calculate the lift on a wing in certain situations (the wing cannot be separated, for example). Given a velocity distribution over a wing, you can use Bernoulli's equation to deduce the pressures on the wing and hence the lift. It says nothing about how you find sai velocities or the best shape of a wing. Bernoulli's equation is merely a tool; it cannot explain lift completely.
Bernoulli Equation doesn't explain lift at all. It can be used as a good estimate, under certain conditions, but in order to get exact lift, you need pressure at the actual boundary. And at the actual boundary the flow velocity is precisely zero and Bernoulli Effect is also zero.

The only reason Bernoulli Equation works to estimate lift is because viscosity of the air is low, and if you are working in regime where air can be treated as incompressible, the pressure gradient near the boundary is low. So you can take air flow near the boundary, and use Bernouli Equation there to say that pressure at the boundary will be the same. At mach number << 1, you will be correct.

There is one more problem with Bernoulli in that it is difficult to implement near the critical angle of attack. As the separation layer creeps up the wing, relevance of Bernoulli Effect to pressure at the boundary decreases further. So you really want a different method of estimating lift, and that's where Kutta Condition and Kutta-Joukowski Theorem come up, as you point out.

But ultimately, it's the pressure differential on the wing surface that's the direct cause of the lift, and you cannot apply Bernoulli Principle at the boundary. You can use it as a motivation to look for an effect, but if you want to actually explain it, you can't do much better than, "Boundary conditions of the problem." Because that's the real reason for the pressure differential.

Quote by Lunar-Scooter
 Quote by russ_watters What does the thrust have to do with the lift?
Everything.
Please, explain flight of a glider, which generates no thrust.

K^2 wrote:
 Please, explain flight of a glider, which generates no thrust.
Back to flying school for you!

If a glider has no source of thrust what on earth do you think balances the force of drag? Everyone knows that gravity provides the thrust needed by a glider to overcome drag.

It's very easy to demonstrate that lift is proportional to speed. Just turn your engines off, slow down and try and stay up there!

Nice video of someone converting excess height to excess speed to excess lift and back to excess height again....

I'm an ex glider pilot. What you fly?
 Recognitions: Science Advisor I've flown C-172 and a T-6 Texan, though, the later only briefly. So yeah, I know how to trade altitude for speed and vice versa. While a glider does use longitudinal component of weight to counter drag in steady flight, it doesn't fall under definition of thrust. Under standard aeronautical definitions, thrust and weight are two distinct forces, even when they are not orthogonal. I know what you mean, but it just doesn't seem to be related to what Lunar-Scooter is talking about. Lift is proportional to square of the air speed, yes. But your air speed isn't proportional to the thrust. Sure, as soon as you kill the engine, if you don't start trading altitude for it, you'll start losing speed. But you won't lose lift the instant you lost thrust. You'll lose it gradually, as you lose the air speed, which, of course, will force a descent, unless you increase angle of attack. The sum of longitudinal forces, including drag, thrust, and conditionally weight, determines longitudinal acceleration of the plane. While the instantaneous air speed is what determines the lift. Another good demonstration of this is when you are flying under constant thrust and hit a gust of wind. Your thrust doesn't change, but your lift certainly does. Point is, lift is independent of thrust. Yes, thrust is kind of important for flying, but it doesn't contribute to lift in any way. These are two completely separate forces generated in two completely separate ways.

 Quote by K^2 Bernoulli Equation doesn't explain lift at all. It can be used as a good estimate, under certain conditions, but in order to get exact lift, you need pressure at the actual boundary. And at the actual boundary the flow velocity is precisely zero and Bernoulli Effect is also zero. The only reason Bernoulli Equation works to estimate lift is because viscosity of the air is low, and if you are working in regime where air can be treated as incompressible, the pressure gradient near the boundary is low. So you can take air flow near the boundary, and use Bernouli Equation there to say that pressure at the boundary will be the same. At mach number << 1, you will be correct. There is one more problem with Bernoulli in that it is difficult to implement near the critical angle of attack. As the separation layer creeps up the wing, relevance of Bernoulli Effect to pressure at the boundary decreases further. So you really want a different method of estimating lift, and that's where Kutta Condition and Kutta-Joukowski Theorem come up, as you point out. But ultimately, it's the pressure differential on the wing surface that's the direct cause of the lift, and you cannot apply Bernoulli Principle at the boundary. You can use it as a motivation to look for an effect, but if you want to actually explain it, you can't do much better than, "Boundary conditions of the problem." Because that's the real reason for the pressure differential.
Have you read anything I've said? I agree (mostly) with you. I do need to point out that, as borne out by both theory and experiments, the wall-normal pressure gradient across a boundary layer is effectively zero, so if you use the displacement thickness to add to the shape of the airfoil and then run an inviscid simulation, you will get effectively the exact pressures at the wall. There have been many years of theory and experiments confirming this in both air and water. This is why Bernoulli's equation works for incompressible, unseparated flows. There are also compressible forms of the Bernoulli equation that solve this, but even they suffer from the same limitations.

Like I said, Bernoulli's principle is a tool that can be used I calculate lift, not the law that explains its origin. On that we agree.

 Quote by rcgldr The direction of the "wake" is mostly downwards (lift), and somewhat forwards (drag). For a propeller, the "wake" is mostly aftwards (thrust), and somewhat circular (drag).
This isn't true. The wake trails behind a foil passing through a fluid. On a plane. It is a long disturbance in the fluid behind and slightly below the plane. True it does have a vertical component, which is that downwash that corresponds to the opposite of the lift, however the wake does not travel downward for long. It is mostly directly rearward.

 Quote by rcgldr The force exerted by gravity onto the aircraft is in turn exerted by the aircraft onto the air, and eventually, that force is exerted by the air onto the earths suface (forces do not vanish), via a continous impulse (downward shifting of air) that originated at the aircraft.
Yes we know, forces balance. That's a fundamental law. The downward momentum flux of the wake of an airfoil is directly proportional to lift, and that lift is often calculable using Bernoulli (but no always, of course).

Some additional lift is lost due to the effect of wingtip vortices on a real wing, but even that, through the laws of physics, would still be shown in the balance of momentum in the downwash with the lift. That also over complicated the answer to the simple (on its surface) "how does an airplane really fly?"

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 Quote by CWatters K^2 wrote: . . . . . Nice video of someone converting excess height to excess speed to excess lift and back to excess height again.... http://www.youtube.com/watch?v=9mGTNffSEc4 I'm an ex glider pilot. What you fly?
I couldn't get much from that movie because the camera was (almost certainly) mounted on the wing so how could you tell its speed or height from the picture. Whilst I believe the theory, how does showing a change of angle wrt the horizon prove anything?

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 Quote by boneh3ad This isn't true. The wake trails behind a foil passing through a fluid. On a plane. It is a long disturbance in the fluid behind and slightly below the plane. True it does have a vertical component, which is that downwash that corresponds to the opposite of the lift, however the wake does not travel downward for long. It is mostly directly rearward. Yes we know, forces balance. That's a fundamental law. The downward momentum flux of the wake of an airfoil is directly proportional to lift, and that lift is often calculable using Bernoulli (but no always, of course). Some additional lift is lost due to the effect of wingtip vortices on a real wing, but even that, through the laws of physics, would still be shown in the balance of momentum in the downwash with the lift. That also over complicated the answer to the simple (on its surface) "how does an airplane really fly?"
Presumably the faster the plane flies, the slower the downwash would be. This most be along the same lines as when boat is planing; the faster it goes, the less of the hull is below the surface. Impulse is force times time. The force must be equal to the weight of the plane / boat but at high speed, that force acts on a section of air for a shorter time so the air moves downward slower (all other things being equal).

The effect of downwash can be pretty catastrophic for light aircraft flying through the flight path of large aircraft as they land (at 'low' speed).
 Recognitions: Science Advisor It's mostly the vortex state that causes problems for other aircraft, not the downwash.

 Quote by K^2 It's mostly the vortex state that causes problems for other aircraft, not the downwash.
Same thing. Downwash is inner half of each vortex.

 Recognitions: Gold Member Science Advisor Cracking picture. I can see the rotational motion is much greater than the bulk downward motion. Wouldn't like to fly into that in a Cessna!

 Quote by sophiecentaur Cracking picture. I can see the rotational motion is much greater than the bulk downward motion. Wouldn't like to fly into that in a Cessna!
Well, the rotational motion is simply the downward moving air combined with the corresponding upward moving air to fill the void. So downward and rotational should be essentially the same.
 Recognitions: Gold Member Science Advisor That figures. I now realise there can be no net motion. It's just a matter of how big this vortex region is. ?

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There IS a net motion. If there was no net motion, there would be no net lift. You see where the could would be if it wasn't for the plane on that picture? All that cleared space is the net downward air displacement.
 Quote by DaveC426913 Same thing. Downwash is inner half of each vortex.
Well, first of all, it's not the same thing, because you can reduce vortices, with winglets, for example, but you can't reduce downwash.

But more to the point, yes, you aren't going to come across one without coming across the other, but the downwash is not what's dangerous. If it was just downwash, you'd correct for it by increasing angle of attack, throttling up, and then you continue as normal. You can get hit by a downwards jet of air at high altitude. Happens all the time, and quite a few of these are significantly worse than any downwash.

Vortex, on the other hand, will at best start flipping you over without any chance of correction until you are clear. At worst, it will overstress the aircraft. Vortex state is what gets you. Not the downwash.

 Quote by K^2 There IS a net motion. If there was no net motion, there would be no net lift. You see where the could would be if it wasn't for the plane on that picture? All that cleared space is the net downward air displacement.
Well, true. You can look at it as downwash slightly greater than upwash, or you can look at it as one big vortex with net downward motion.

Either way, true, the net motion is downward. Though I suspect the speeds in the vortex greatly exceed the speed of the downwash.

 Quote by K^2 Well, first of all, it's not the same thing, because you can reduce vortices, with winglets, for example, but you can't reduce downwash.But more to the point, yes, you aren't going to come across one without coming across the other,...
Yeah, they're both components of the same thing. But yeah, you can look a them independently. Which is what you were saying originally, though it was not apparent at first.

 Quote by K^2 but the downwash is not what's dangerous. If it was just downwash, you'd correct for it by increasing angle of attack, throttling up, and then you continue as normal. You can get hit by a downwards jet of air at high altitude. Happens all the time, and quite a few of these are significantly worse than any downwash. Vortex, on the other hand, will at best start flipping you over without any chance of correction until you are clear. At worst, it will overstress the aircraft. Vortex state is what gets you. Not the downwash.
Agreed. Downwash leaves a stable craft. Vortex demolishes the craft's stability.
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 Quote by DaveC426913 Though I suspect the speeds in the vortex greatly exceed the speed of the downwash.
There would have to be regions like that to pull the condensate upwards as evident on the photo, yes.