Reason for lower air pressure above an airplane wing

Question

I am posing this question from the perspective of a novice. I read an article, from Scientific American, titled "No One Can Explain Why Planes Stay in the Air". The article explains how, while we understand how to create flight (with airplanes, for example), we still don't understand why there is lower pressure above the wing, allowing for the air above the wing to flow more quickly, thus generating lift. If you care about this topic, read the article I am referencing, it lays out the two competing theories for explaining why air pressure is lower above an airplane wing in flight. It also shows how both of the theories are incomplete.

I found it fascinating that we still don't understand this, so I decided to think about it a little. I am not a physicist, but I would say I have a basic understand of physics (having at least studied it during high school). The first aspect of what is going on when an airplane is flying that I decided to think about is (to me) the most obvious force involved in flight: gravity. I then conjured up the question, "how does gravity act on air?" If you type that into a google search, it will bring up this text box at the top:

As gravity hugs the blanket of air to the Earth's surface, what physicists call a density gradient is set up in the air. The air near the ground is pulled on by gravity and compressed by the air higher in the sky. This causes the air near the ground to be denser and at a greater pressure than air at higher elevations.

Google cites this as coming from https://www.uu.edu/dept/physics/scienceguys/2001Oct.cfm - I am assuming it is reputable if google is choosing to present it as a "quick answer" (if you will) for the search "does gravity act on air".

In this explanation, I think what it is saying is, the air that is closer to the ground is more closely packed together. It mentions the "density gradient," and I interpret that concept to mean that, as you get closer to the edge of our atmosphere (starting at ground level), the air will become less and less dense as you go higher and higher. So if that is how air behaves under gravity, and an airplane is flying through the air, this means that, as it is being propelled sideways (as it's wings "cut through the air"), by nature, it is going to split the density gradient of the air (which I think can also be directly linked to air pressure - more dense air, air closer to the ground, is higher pressure, and less dense air, air further from the ground, is lower pressure). In saying this, then I think I can say that, as the wing cuts through the air, there is no reason why it wouldn't cut through the air in a way where it creates a situation around the wing, where there is lower pressure above the wing, and higher pressure below the wing (even if it is only a small difference - much force is being applied sideways in this situation by the airplane), thus allowing for lift. There is no reason for the air that is higher up (the inherently less dense air) to pass below the wing, because it is already above the more dense air (the air that is already closer to the ground) as described by the density gradient. It seems to me, that this notion supports the idea that there would be lower pressure above an airplane wing (allowing for the air to move more quickly above the wing), and higher pressure below it. Everything I have said kind of seems pretty obvious, and seems to support the idea that lift will be inherent in any situation where something is slicing horizontally through the air at a rate of speed where it is able to negate any other forces that may act in an effort to make it move non-horizontally. I'm guessing I am missing something, and it isn't this (seemingly) simple - either way, with what I think I have discovered through my brief research, I basically want to know why what I said is wrong, if it is (I guess it would be pretty cool if I just figured out this problem haha).

Answer 1

"I'm guessing I am missing something"

Yes you are. Two things in particular:

Firstly, hot air rises, so there is a density gradient in opposition to gravity. This will tend to provide regions where there is effectively no density gradient, and therefore no lift, under your idea outlined above.

Secondly, a modern airliner has a wing depth, from it's top surface to it's undersurface, of say 100 cm on average. Earth's atmospheric density gradient is far, far less than this, so these aircraft manage to fly in air that has no density gradient, in real terms.

Answer 2

I will first summarise the question as I understand it, so that if I'm wrong you can quickly correct me without having to read through a lot. Here goes:

(1) There are competing theories about lift on a plane, and an SA article claims they are both incomplete.

(2) You have a third, simpler interpretation, and you want to know if that is complete.

Now, your interpretation is solely based on natural density differences in the atmosphere, and the upward force generated on these for the same reason. Consider two counters to this:

(a) A plane can also land, or lower it's altitude in the same atmosphere just as it can raise it's altitude. Going down does not require more fuel/effort than going up, which you would expect if density gradients were keeping it upfloat. Note that I'm assuming no change in speed here.

(b) It is possible for a plane to fly very close to the ground, where the density difference is pretty low (atmospheric density doesn't actually vary linearly with height).

There is a reason for airfoil design, and for flexible ailerons, namely that they can change the direction of airflow above and below the wing. These airflow directions actually generate the pressure differences and generate lift, in a manner basically similar to what you propose. Just that it's coming from airfoil design and not natural density differences.

Answer 3

As a line of air strikes the leading edge of the wing it gets deflected upwards.Then it bounces off air above but loses some speed in the vertical plane because the collision with the air above is not elastic. So as this bounced air moves down and along the wing it doesn't exert as much force on the wing. The bounced air also thins out because when it was deflected upwards it pushed the air above up and made more room for itself.

Answer 4

The air UNDER the wing "ie" is a measured distance of 10 the air ABOVE the wing is a measured distance of 12, the 10 and 12 begin and end at the the same time intervals, thus the ABOVE the wing portion has tiny gaps of less air because they start and stop at the same time yet 12 had to move faster and leave tiny gaps in the air making less pressure ABOVE the surface of the top of the wing, We learned it from nature's Birds!..

Answer 5

Emphasis should be not on the fact that pressure is lower above wing, but that pressure is higher below wing because this additional pressure is caused physically by an air flow speed $u$ due to dynamic pressure $1/2 \rho u^2$ of flow. Flow arises, because plane moves in the air, so it's wings induces force upon air mass parcels, which due to Newton third law induces force back on wings.

This is the core working principle and it is understood fully from the $1738~\text{year}$ when Swiss mathematician and physicist Daniel Bernoulli published it's work in his book "Hydrodynamica". So article claim

No One Can Explain Why Planes Stay in the Air

Is complete lie,- just a bold sentence to attract journal readers very fast, since it's a marketing trick.

Answer 6

The wing comes along and scoops away some air. The scooped-away air goes below the wing. So there is less air left in the region above it. Less air = less pressure.

Obviously the full dynamics are more complicated, but this simple picture is enough to make it entirely unsurprising that the pressure is reduced above the wing.

Answer 7

As mentioned in another response here, if the gravity-caused pressure gradient were the cause of lift, then the wing would float up even without forward motion. That should've been quite obvious at the start. In addition, that pressure difference is many, many orders of magnitude below what it takes to get lift.

If you want to understand the true physics, you can't pick and choose only the fundamental principles you know how to apply. You must include all fundamental principles that apply.

You should learn what Newton's first law actually tells us that a force is the cause of acceleration of an object with mass:

AIR HAS MASS! A FORCE IS REQUIRED TO ACCELERATE IT!

Then inertia is a resistance to acceleration and that is what Newton's third law tells us. So here's what everyone else is forgetting to apply:

The reason the pressures change around the wing, both above and below is air's inertia.

When you say you understand how the air and the bottom surface of the wing are coming together and, therefore, that causes an increase in pressure; what you're saying is, without realizing it, is that air's inertia provides the resistance to that downward acceleration that is imposed by the surface, because the air and surface cannot pass through each other. The air's inertia resists that acceleration and, therefore, the pressure at the lower surface is increased.

Think about it. If a mass did not have inertia, when you came up to it and bumped into it, it would just move along and there could not even be any force developed. Then, when you stopped, it would also just stop. If there were no inertia, there could never be a force applied and it wouldn't keep moving after you stopped trying to push it.

So, what you also fail to understand is that when the air is deflected up over the leading edge of the wing, it wants to keep going in that upward direction and the inertia tries to keep it going that way. This has the effect of working against that much higher atmospheric pressure that holds the flow against the upper surface. Therefore, is the inertia tries to continue away from the surface, it pushes less on the surface, therefore lowering the pressure. By the way, this reduction is quite small, on the order of $0.08 \,\text{psi}$ which is only about a reduction of $0.6\%$ for a typical Cessna 172.

Now that you have a low pressure above the wing, you should be able to understand that the pressure gradient, that is, a higher pressure near the leading edge and the lower pressure above the wing, is the cause of the acceleration of this air towards the trailing edge. Speed absolutely does not lower the air pressure. Bernoulli does not say anything about the speed of air along a surface.

This video of mine should make it much clearer that inertia causes the pressure decrease above a wing. I also step through the full explanation of lift here as well as the correct understanding of Bernoulli's principle and I review why the demonstrations are misinterpreted and other explanations are incorrect.

Answer 8

While you blame pressure gradient for the lift, why do you need the plane to "slice" through the gradient? There is a pressure gradient across the wing as it is waiting to be boarded. Here's another thought in the same line of reasoning : use a wing that has an even thicker profile, thereby creating more lift due to greater pressure gradient exposed and once the plane is airborne, shrink the profile to reduce the enormous drag. The thing is that pressure gradient produces negligible gradient, which in fact would be changed merely by the temperature gradient effected by the engines, etc. Coming to the article in Scientific American, unfortunately, popular science book writers, in an attempt to attract interest from a wide audience, pretty often make bold statements. Just like "no one can explain why planes stay in the air", exclamation mark. Actually it is very well explained, as the article later admits. "But by themselves, equations are not explanations" it goes on to add, which is just a semantic game. Equations are just representations of explanations. If you can't explain it, you can't generate equations just by scribbling on a piece of paper. Then the article accepts this but does not give up and claims "but the theorem does not constitute a complete explanation of lift". Actually it does, but the author is just not aware of it. I personally would suggest reading more serious stuff than Scientific American.