[ivanjermakov]

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

[danmaz74]

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]

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]

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]

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]

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.

[topaz0]

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.