Comment by civil_engineer
2 years ago
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.
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
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.
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).
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.
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.
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That’s my mental model as well. The incompressible fluid-based explanations never made much sense to me
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
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.
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.
... 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).
> 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.
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
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.
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The simple newtonian deflection model is correct however, As you engineer your deflector to have the least possible drag the airfoil shape naturally falls out.
Actually that is a bit of a lie, the airfoil shape only falls out due to a third implied force that needs to be accounted for. the wing needs to be strong enough to hold itself up. if you had infinitely strong materials the deflector shape that would fall out would be like a slightly bent piece of paper.
A clarification note on fluids: you are deflecting fluids, and everything this implies. just because I say newtonian deflection don't think I mean billiards balls, or if it has to be billiard balls think trillions of them simultaneously
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
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