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.
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.
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.
It's not arbitrary: negative gauge pressure above the wing means that (by definition) there is a pressure gradient increasing away from the wing (because the absolute pressure far from the wing is ambient pressure), so the net force on the air there is downward.
> made a zero point between ... shouldn't surprise us
Whether or not you are surprised is immaterial, but it is not guaranteed a priori -- you could get a net upward force with ambient pressure above the wing and positive pressure below or with ambient pressure below the wing and negative pressure above (meaning gauge pressure, relative to the ambient pressure distant from the wing, to be clear). The person who started this thread seemed to be implying that the former was a good mental model, and the person you replied to was just saying that in fact for practical wing designs it is somewhere in between.
FWIW it is very common to talk about positive and negative gauge pressure. Some people may say that without understanding what is going on, but it is a mistake to assume that they don't understand just because they use that language.
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).
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]
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.
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 (:
Planes fly through gas, not solid particulate. Gas has intrinsic kinetic energy when energized. Diffusion plays a huge role in all this of course.
The airflow is split at the leading edge. The area of positive pressure is not entirely below or focused under the wing. The top and bottom of the airfoil are both involved in turning the air flow.
The pressure under the airfoil increases a bit, but the pressure above decreases by as much to much more depending(2-3x or more). This hypothetical scale is under the aircraft but much of the lift occurs by decreasing forces on the top surface.
Scales measure weight/mass. Barometers measure changes in atmospheric pressure. So it's not even the tool for the job even if the stone skip theory of lift was accurate.
Perhaps my mention of Newton's third law gave you the impression that I was advocating for that "stone skip" theory — I assure you I wasn't! Especially as presented on that page, it is obviously junk (:
But surely you agree, broadly, that if birds are flying inside a sealed box, the box still weighs the same amount as if they were standing, right? (modulo some fluctuations)
All of the pressure differentials and whatnot have the net effect that an upward force on the wing results in a downward force elsewhere. The purpose of the scale is to measure that force — like measuring the weight of the box with birds in it.
In the hovering helicopter example, wouldn't you agree that a (large) scale directly under the helicopter will measure a weight corresponding to the helicopter's lift force? Like if I blow directly onto a kitchen scale — it will measure some grams.
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
I did not say reflector as implied by that link but deflector, a thing put in the fluidstream to move it somewhere else. airplanes lift because you are moving air down. People get hung up about the convex side of the airfoil but what else is the fluid going to do, stay a vacuum? it is going to move in the way the deflector shaped, adding to(actually providing most of) the downward flow. There is a lot of engineering that goes into it but at the end of the day an airfoil is the shape that moves enough enough air downward with the least drag. The only reason it is a thick teardrop shape is it has to be strong enough to support itself and the airplane. otherwise the ideal shape would be super thin shaped like the upper surface of the wing bending slightly from the cord(aspect directly into the stream) to the trailing edge(a few degrees of slope).
