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The wings of an airplane in level flight direct air downward with a force equal to the airplane's weight. If one were to build a large scale on the ground, as a

The wings of an airplane in level flight direct air downward with a force equal to the airplane's weight. If one were to build a large scale on the ground, as an airplane flies over it, the scale would register the weight of the airplane. The wings act like a scoop forcing air downward behind the wing. At least that's the way I think about it when I'm out flying around in my Cessna.

WanderPanda 3y ago
That’s my mental model as well. The incompressible fluid-based explanations never made much sense to me
ivanjermakov 3y ago
Although it is a nice mental model, that's not quite true. > The wings act like a scoop forcing air downward behind the wing Only bottom side of the wing acts as a scoop, creating positive pressure. Upper side, in opposite, creates negative pressure which "sucks" the plane into it, creating additional lift. It surprised me how much lift is coming from the negative pressure - about a half: https://aviation.stackexchange.com/a/16202 https://aviation.stackexchange.com/a/16202
- danmaz74 3y ago
  Actually, it is quite true. Gravity is exercising on the airplane a force F equal to the weight of the plane, towards the ground. For the airplane to stay at the same height, air needs to exercise a force that is equal and opposite to that of gravity. For an airplane buoyancy is negligible, so the force comes from accelerating enough air towards the ground so that F = M*A when M is the mass of air being accelerated, and A the (average) acceleration. Notice that this isn't a separate effect from the effect of pressure - it's just a different way of seeing the same effect. The wing is accelerating the air both upwards and downwards, but because the pressure is higher below the wing than it is above it, more air is accelerated down than it is accelerated up - which lifts the airplane, but makes the air go down.
  - topaz0 3y ago
    GP was not disputing the redirection of flow or the magnitude of force/air momentum change. They were just saying that not all of this is because of the "scoop" effect from the bottom of the wing: a significant part of the redirection also comes from the low pressure above the wing (at least in practical cases).
- jameshart 3y ago
  Except that negative pressure is not a thing. Air molecules are not grabbing the wings and pulling them up - they are just not pushing down on the top as much as the ones underneath are pushing upwards.
  - Tyrannosaur 3y ago
    Negative pressure is not a thing, except you just described it. If you take the difference between the pressures above the wing and below the wing, you get a negative number. A thing not existing absolutely can still exist relatively.
    - jameshart 3y ago
      That’s just a pressure differential, and not what the OP meant by ‘negative pressure’. 100% of the lift force on a wing is attributable to the pressure differential across it, after all. They (or their stackexchange source at least) are - like the referenced article and as is commonly done in aero engineering - subtracting out ambient pressure as a reference pressure, and then viewing pressure above the wing as ‘negative’ and pressure below as ‘positive’. It’s a convenient choice to make, for various reasons, but it is essentially an arbitrary one. The problem comes when you then go on, like OP did, to come across statements like “how much lift is coming from the negative pressure - about a half” Now, since in analyzing the pressure we have subtracted the reference pressure and made a zero point in between the low pressure value above the wing and the high pressure value below it, it actually shouldn’t surprise us at all that ‘about half’ of the lift seems to be attributed to the positive pressure below the wing, and half to the negative pressure above the wing. This is just saying that half the lift on the wing is attributable to the first half of the pressure differential across the wing, and about half the lift attributable to the other half. One of the problems of using a relative pressure and thinking about negative air pressure is that it gives the impression that negative air pressure, like positive air pressure, can grow arbitrarily large. It can’t. You can’t have a negative air pressure lower than negative ambient air pressure, because the absolute air pressure cannot go below zero. But what you’re talking about is a relative pressure differential. We can have an arbitrarily large negative pressure differential because we can have an arbitrarily high pressure on one side of it.
bloppe 3y ago
Ya, I was hoping for more nuance related to this. I'm sure the air foils generate lift, but atmospheric pressure at cruising altitude is ~4psi, and the pressure differential across the foil must be only a tiny fraction of that. According to my understanding of Bernoulli's principle, you'd have to quadruple the speed to cut the pressure in half, and I can't imagine the top air traveling that much faster than the bottom air. Yet a 747 can produce 850000 pounds of lift with only 729000 square inches of wing? Feels like a very incomplete description at best
- SAI_Peregrinus 3y ago
  The pressure differential is what causes the direction change of the flow, pushing the air down. The shape of the wing and the angle of attack cause the pressure differential.
- p_l 3y ago
  The airfoil shape causes formation of vortex around the wing, which ridiculously changes the relative speeds and pressures involved. At low pressure you compensate with speed, which is squared in lift equation.
p_l 3y ago
... I'm honestly surprised it's possible to get PPL(A) without learning about wing vortices responsible for lift generation. In order to use "scoop" approach for lift, you need to have either very low wing loading (think paper airplanes) or very high speeds (above transsonic range).
Rapzid 3y ago
> If one were to build a large scale on the ground, as an airplane flies over it, the scale would register the weight of the airplane No, it wouldn't. I think the article does a pretty good job building a more complete understanding than the simplistic "deflection" mental model.
- turtledragonfly 3y ago
  I think what they were saying is that from a pure "Newton's 3rd law" standpoint, if the plane has an upwards force, then the air has a corresponding downward force, which must go somewhere. Yes, it is spread out and complicated and turbulent, etc, but ultimately must balance out. If we could somehow "draw a box around" the entire plane+air system, then the plane's upward lift will create a corresponding downward force on the box, one way or another. So, in the broad sense that you push the earth away from you when you jump, the plane also pushes the earth away from it when it flies (mediated by a bunch of fluid dynamics). Or, classic example: if a (sealed) truck full of birds is jostled so that they start flying, does the truck weigh less? [1] [1] https://www.youtube.com/watch?v=lVeP6oqH-Qo&t=35s https://www.youtube.com/watch?v=lVeP6oqH-Qo&t=35s
  - Rapzid 3y ago
    It's wrong though. A large, hypothetical scale under the plane would not register the weight of the plane as it flies over. And not just because diffusion but that being one of many reasons.
    - turtledragonfly 3y ago
      I'm curious to know your reasoning more. Certainly if we flew the plane very low over the ground, the air pretty directly pushes down on it, and the hypothetical scale would register something. Just look at the grass when a helicopter hovers over it. As the aircraft flies further up, we'd need a bigger scale to capture the full area affected, and if it's moving there would be increasing lag between the location of the plane and the (large) area where the downward force hits the ground. Or do you disagree with that? At what point does the scale stop working? Obviously there would be practical limitations — that force is so spread out that it would be hard to measure. But let's not have practice get in the way of theory (:

WanderPanda 3y ago

That’s my mental model as well. The incompressible fluid-based explanations never made much sense to me

ivanjermakov 3y ago

Although it is a nice mental model, that's not quite true. > The wings act like a scoop forcing air downward behind the wing Only bottom side of the wing acts as a scoop, creating positive pressure. Upper side, in opposite, creates negative pressure which "sucks" the plane into it, creating additional lift. It surprised me how much lift is coming from the negative pressure - about a half: https://aviation.stackexchange.com/a/16202 https://aviation.stackexchange.com/a/16202

danmaz74 3y ago
Actually, it is quite true. Gravity is exercising on the airplane a force F equal to the weight of the plane, towards the ground. For the airplane to stay at the same height, air needs to exercise a force that is equal and opposite to that of gravity. For an airplane buoyancy is negligible, so the force comes from accelerating enough air towards the ground so that F = M*A when M is the mass of air being accelerated, and A the (average) acceleration. Notice that this isn't a separate effect from the effect of pressure - it's just a different way of seeing the same effect. The wing is accelerating the air both upwards and downwards, but because the pressure is higher below the wing than it is above it, more air is accelerated down than it is accelerated up - which lifts the airplane, but makes the air go down.
- topaz0 3y ago
  GP was not disputing the redirection of flow or the magnitude of force/air momentum change. They were just saying that not all of this is because of the "scoop" effect from the bottom of the wing: a significant part of the redirection also comes from the low pressure above the wing (at least in practical cases).
jameshart 3y ago
Except that negative pressure is not a thing. Air molecules are not grabbing the wings and pulling them up - they are just not pushing down on the top as much as the ones underneath are pushing upwards.
- Tyrannosaur 3y ago
  Negative pressure is not a thing, except you just described it. If you take the difference between the pressures above the wing and below the wing, you get a negative number. A thing not existing absolutely can still exist relatively.
  - jameshart 3y ago
    That’s just a pressure differential, and not what the OP meant by ‘negative pressure’. 100% of the lift force on a wing is attributable to the pressure differential across it, after all. They (or their stackexchange source at least) are - like the referenced article and as is commonly done in aero engineering - subtracting out ambient pressure as a reference pressure, and then viewing pressure above the wing as ‘negative’ and pressure below as ‘positive’. It’s a convenient choice to make, for various reasons, but it is essentially an arbitrary one. The problem comes when you then go on, like OP did, to come across statements like “how much lift is coming from the negative pressure - about a half” Now, since in analyzing the pressure we have subtracted the reference pressure and made a zero point in between the low pressure value above the wing and the high pressure value below it, it actually shouldn’t surprise us at all that ‘about half’ of the lift seems to be attributed to the positive pressure below the wing, and half to the negative pressure above the wing. This is just saying that half the lift on the wing is attributable to the first half of the pressure differential across the wing, and about half the lift attributable to the other half. One of the problems of using a relative pressure and thinking about negative air pressure is that it gives the impression that negative air pressure, like positive air pressure, can grow arbitrarily large. It can’t. You can’t have a negative air pressure lower than negative ambient air pressure, because the absolute air pressure cannot go below zero. But what you’re talking about is a relative pressure differential. We can have an arbitrarily large negative pressure differential because we can have an arbitrarily high pressure on one side of it.

bloppe 3y ago

Ya, I was hoping for more nuance related to this. I'm sure the air foils generate lift, but atmospheric pressure at cruising altitude is ~4psi, and the pressure differential across the foil must be only a tiny fraction of that. According to my understanding of Bernoulli's principle, you'd have to quadruple the speed to cut the pressure in half, and I can't imagine the top air traveling that much faster than the bottom air. Yet a 747 can produce 850000 pounds of lift with only 729000 square inches of wing? Feels like a very incomplete description at best

SAI_Peregrinus 3y ago
The pressure differential is what causes the direction change of the flow, pushing the air down. The shape of the wing and the angle of attack cause the pressure differential.
p_l 3y ago
The airfoil shape causes formation of vortex around the wing, which ridiculously changes the relative speeds and pressures involved. At low pressure you compensate with speed, which is squared in lift equation.

p_l 3y ago

... I'm honestly surprised it's possible to get PPL(A) without learning about wing vortices responsible for lift generation. In order to use "scoop" approach for lift, you need to have either very low wing loading (think paper airplanes) or very high speeds (above transsonic range).

Rapzid 3y ago

> If one were to build a large scale on the ground, as an airplane flies over it, the scale would register the weight of the airplane No, it wouldn't. I think the article does a pretty good job building a more complete understanding than the simplistic "deflection" mental model.

turtledragonfly 3y ago
I think what they were saying is that from a pure "Newton's 3rd law" standpoint, if the plane has an upwards force, then the air has a corresponding downward force, which must go somewhere. Yes, it is spread out and complicated and turbulent, etc, but ultimately must balance out. If we could somehow "draw a box around" the entire plane+air system, then the plane's upward lift will create a corresponding downward force on the box, one way or another. So, in the broad sense that you push the earth away from you when you jump, the plane also pushes the earth away from it when it flies (mediated by a bunch of fluid dynamics). Or, classic example: if a (sealed) truck full of birds is jostled so that they start flying, does the truck weigh less? [1] [1] https://www.youtube.com/watch?v=lVeP6oqH-Qo&t=35s https://www.youtube.com/watch?v=lVeP6oqH-Qo&t=35s
- Rapzid 3y ago
  It's wrong though. A large, hypothetical scale under the plane would not register the weight of the plane as it flies over. And not just because diffusion but that being one of many reasons.
  - turtledragonfly 3y ago
    I'm curious to know your reasoning more. Certainly if we flew the plane very low over the ground, the air pretty directly pushes down on it, and the hypothetical scale would register something. Just look at the grass when a helicopter hovers over it. As the aircraft flies further up, we'd need a bigger scale to capture the full area affected, and if it's moving there would be increasing lag between the location of the plane and the (large) area where the downward force hits the ground. Or do you disagree with that? At what point does the scale stop working? Obviously there would be practical limitations — that force is so spread out that it would be hard to measure. But let's not have practice get in the way of theory (: