Cue the conversation on how wing works.
NASA has a bunch of aerodynamics educational material on their old Glenn Research Center website. Here's a page on these two different ways to look at lift [1].
Very briefly, the gas flow has to simultaneously conserve mass, momentum, and energy.
One approach is to consider conservation of energy. When you work out the implications of that you get different flow velocity over different parts of the wing and different pressure due to those differences. When you integrate the pressure over the whole wing you find that you get lift.
The other approach is to consider conservation of momentum. Working out the implications of that, you get velocity differences in the flow. If you integrate those around the whole wing, you find that there is a net turning of the flow downward. Conservation of momentum requires that the wing gets momentum opposite of that and we have lift.
or both?
Both of those approaches are equal. Or, to put it another way, it isn't that the fast moving air and the slow moving air "causing" a pressure differential which then "causes" lift... it's one integrated process. They aren't separable. You can't have different levels of pressure without some effect on air velocity, you can't have forces in the air without some effect on pressure, etc. The whole argument is all "But what causes what?" and the only correct answer is that it's an equality, not an implication. It can only be properly understood as a single whole that can be looked at in several ways.
https://www.scientificamerican.com/article/no-one-can-explai...
Answer: because a surface of a certain geometry produces a force perpendicular to fluid flow as per the Navier-Stokes equations. But that's not a good explanation to give a room full 2nd graders, therefore your pet-explanation will have to make a compromise somewhere. You are all equally right and wrong at the same time. Unless you say "equal transit theory". Then you are just wrong.
It is true that most of the popular simplified explanations are incorrect. Flat plate airfoils generate lift if they have positive angles of attack. Airplanes can fly upside down. At fractional mach numbers, pressure above and below the wings is essentially equal.
If you are not flying near the speed of light, Newton's laws apply. So, if you want simple explanation, the wing deflects air downwards and that pushes the airplane up. If you put your hand outside the car window at an angle, you will feel a force. Should be simple enough for 2nd graders.
The momentum theory of lift is simple, intuitive, but unfortunately incomplete (just as the differential pressure explanation). It's covered in the article.
> But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is cambered. It is only when an airplane lands and comes to a halt that the region of lower pressure atop the wing disappears, returns to ambient pressure, and becomes the same at both top and bottom. But as long as a plane is flying, that region of lower pressure is an inescapable element of aerodynamic lift, and it must be explained.
Also I want to address this:
> At fractional mach numbers, pressure above and below the wings is essentially equal.
Surely you mean density? Air pressure is certainly not the same above and below, as differential pressure integrated over the surface is equal to the lift force generated by the wing. So no, while the Newton's explanation is a great explanation for a second grade classroom, it is not complete.
What? The pressure is lower on top of the wing and higher below because the air is being pushed downwards by the wing. I will happily explain this to any second-grade classrooms you find yourself having trouble with.
> Nevertheless, there are at this point only a few outstanding matters that require explanation. Lift, as you will recall, is the result of the pressure differences between the top and bottom parts of an airfoil. We already have an acceptable explanation for what happens at the bottom part of an airfoil: the oncoming air pushes on the wing both vertically (producing lift) and horizontally (producing drag). The upward push exists in the form of higher pressure below the wing, and this higher pressure is a result of simple Newtonian action and reaction.
> Things are quite different at the top of the wing, however. A region of lower pressure exists there that is also part of the aerodynamic lifting force. But if neither Bernoulli’s principle nor Newton’s third law explains it, what does? We know from streamlines that the air above the wing adheres closely to the downward curvature of the airfoil. But why must the parcels of air moving across the wing’s top surface follow its downward curvature? Why can’t they separate from it and fly straight back?
I’m sure smarter people than I wrote the article, but the way I explain it to myself is that it’s a manifestation of the same basic force or effect where the lower pressure area needs to be filled somehow. Like how the wind blows, cyclones form, etc., except in this case giving the wing lift somehow ends up being part of the most efficient “fill the void” solution.
Like how just behind a driving truck there’s an abrupt region of lower pressure; however, the air obviously doesn’t just keep on going straight forever but rushes in (incidentally, giving a boost to whomever happens to be tailgating). The gradual shape of the wing changes the scale of the effect, so that it happens constantly with tiny air ‘parcels’, each filling in the minuscule lower pressure region. (And, probably not unrelated to the fact that it’s intuitively unnatural for air to flow that way, lifting the wing a little apparently turns out to help even out that void most efficiently.)
It’s interesting to ponder for sure.
I doubt it. This daft idea that wings are hard to explain is kept alive by pop science writers because it's a reliable source of money for old rope, that's all.
What? Of course it isn't moving from above the wing to below - just that it's being compressed below the wing, and decompressed above it.
"Why can’t they separate from it and fly straight back?"
Because... they are parcels of gas, full of molecules flying in all directions at high speeds, bouncing off one another and things nearby, and if that parcel of air flies straight back it will find itself above a bit of space that contains nothing at all, and the molecules which are going in that direction will find they are able to do so unopposed (until they hit the wing) - so some of them will do so.
As a result, the mass of the gas will spread out into a larger volume, the number of molecules colliding with the surface of the wing per unit of time will drop (as they are more diffuse), and the pressure will drop.
This doesn't seem very mysterious.
Also just a minor point of pedantry: wings don't compress air. At least not in low Mach number flows. The static pressure changes as a result of the relationship between pressure and velocity. Compression is when the total pressure (static + dynamic pressures) changes. Total pressure in a low-mach number flow remains constant.
Because I'm pretty sure wings cause an increase in air pressure.
I guess the idea of 'dynamic pressure' is 'pressure that is caused by colliding with air just because you're moving relative to it'. But surely in the moving reference frame of the wing, that looks, locally, quite a lot like compression...
A book will say:
"In low-speed aerodynamic flow, pressure is constant along a streamline"
and then one chapter later say
"Pressure changes with a change in velocity along a streamline"
The first references to total pressure while the second refers to static, but at first glance they seem contradictory.
However I would fully avoid the word "compression" since implies that we are squeezing more air into a fixed volume (a.k.a. an increase in density) which is NOT what is happening in a low-speed air flow. The definition of compression strictly applies to total pressure. Although most people don't learn this, since compression is often used in relation to stationary flows to begin with (such as in pressure vessels) where the dynamic pressure is 0, thus static pressure equals total pressure.
The ideal gas law applies, at least nearly enough. So PV = nRT. By saying the density is equal between the top and bottom, you are also saying the pressure is equal. The air around the wing is having it's momentum changed, not it's pressure. At least, at sub mach speeds.
I should also mention that this pressure absolutely does change a lot over the flow field, and is commonly used to experimentally and mathematically quantify lift. The following is an image of the pressure distribution of a NACA 2412 airfoil at low speeds.
https://www.chegg.com/homework-help/questions-and-answers/n-...
Just to explain the chart a little bit, in aerodynamics, pressure is usually simplified to a Pressure Coefficient (CP) value. A CP of 0 is when static pressure equals atmosphere. A CP value of 1 occurs at the stagnation point (where velocity is 0, therefore static pressure equals total pressure). Note how this type of chart has an inverted y-axis (a common convention so that the wing upper surface is at the top). Notice how the static pressure on the lower surface is roughly atmospheric, while the upper surface pressure suction peak is high. In this case roughly equal in magnitude to the dynamic pressure. This is a typical pressure distribution for most airfoils, with the suction peak increasing in magnitude as angle of attack increases.
This plot can be obtained mathematically using some sort of potential flow scheme (see: XFOIL for 2D airfoils), or experimentally using pressure taps on a wind tunnel model. The area between the upper and lower surface curves is directly proportional to lift. The larger the difference between upper and lower surfaces, the more lift.
But yeah, there is just not an explanation that is both simple and complete and journalists have a pretty rough time dealing with that.
Pressure distribution: https://www.researchgate.net/publication/262976779_Aerodynam...
Pressure contour plot: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.ripubl...
As to explaining why air above the airfoil has to go downwards it seems pretty obvious that the air gets sucked into the gap left by the airfoil, but more importantly the article left out the Kutta condition that air doesn't wrap around a sharp trailing edge. This is both an intuitive phenomenon and crucial to the estimation of lift.