Comment by ivanjermakov
2 years 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
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.
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.
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