I once found an explanation that finally made it clear to me why the shape of the airfoil can create lift. Yes, the air above the wing needs to travel a longer distance with the typical section used in wings, which means that it goes faster than the air below the wing. It also leaves the wing moving downwards - and when this downward-moving, faster flux of air meets the slower one from below, the result is that a mass of air is pushed downwards - exactly as needed to lift the plane, as you correctly said.
As the article says, you can have lift by just changing the inclination of a symmetrical airfoil, but an asymmetrical one can generate lift even without inclination (and with lower drag). The article also explains that acrobatic airplanes have symmetrical wing sections exactly because they need to be able to fly just as easily inverted.
> Yes, the air above the wing needs to travel a longer distance with the typical section used in wings, which means that it goes faster than the air below the wing.
Both of these sub-clauses are true, but the "which means" connecting them aren't. There's no law of physics saying a fluid that has a longer path ahead of it speeds up in anticipation.
Isn't there an even more basic explanation: If incoming air hits a flat surface at an angle, and is deflected downwards, then by the law of action and reaction, the surface itself moves upward.
As a child, I quickly outgrew the airfoil explanation when I realized this.
That's exactly what is happening. But it is also not enough for the airplane to fly.
In a normally flying airplane, the wing compresses and pushes an amount of air under its wing. But there is actually even greater amount of air sucked down by the region of underpressure created above the wing and by the laminar flow directing it downward. Here, the drawing at the top of the page makes it clear: http://www.amasci.com/wing/airfoil.html
When you have a stall condition, what happens is that the air below the wing is still being compressed and directed downwards, but the air above the wing becomes turbulent and "unsticks" from the surface of the wing. Rather than being nicely directed downward, it just dissipates a lot of energy in turbulent motion that is not directed in any particular direction.
This turbulent air not only ceases to provide lift, it also prevents the air from below the wing to be directed downwards efficiently.
The main job of an airfoil isn't to create a pressure difference, it is to create conditions for the air to be laminar at as wide range of speeds and angles of attack as possible to make the plane nicely behaving and possible to takeoff and land. It is super critical for landing as you need to have higher the angle of attack the slower you fly and all planes essentially are driven as close to stall as possible during landing. Similar happens at high altitudes and high speeds, but for a bit different reason (read up on "coffin corner" if you are interested in that sort of thing).
Great explanation. In addition, flaps make the wing able to provide lift at slower airspeed at the cost of efficiency. Perfect for takeoff and landing.
Yes and no. The thing you describe happens, but it's not enough to explain the amount of lift generated by a wing, because a surprising amount of air hits also the top of the wing! The difference in pressure between top and bottom wing surface is just a few percent.
The reason wings produce significant lift anyway is that they deflect air far beyond their surface. Air several metres away from the wing is also deflected downward, even though it doesn't actually hit the wing itself.
So yes, Newton's third law is involved, but in a "spooky action at a distance" form, where the wing somehow manages to deflect a bunch of air it doesn't even touch!
My dad who worked on wind tunnels just flat said you can either integrate the pressure over the surface of the wing or the momentum change as the air passes over to derive the amount of lift.
Both give exactly the same results and are convertible mathematically.
For wind tunnel work it was easier to measure pressures.
I'm with you I don't think the standard hand wavy explanation gives you the ability to attack the problem mathematically. So it's basically wrong.
My understanding is that "which means" only makes sense with the assumption that what is being studied is the laminar flow of an incompressible fluid (which was described as a fair assumption for air and a wing at subsonic speed). But thinking more about it, it's probably right that this isn't about the fact that the air above needs to travel a longer distance, which would also be true for a concave wing section, but the fact that the layers immediately above the wing need to travel the same X distance through a thinner Y section - as in a tube which becomes thinner. Which forces the fluid to go at a higher speed, and have a lower pressure.
As the article says, you can have lift by just changing the inclination of a symmetrical airfoil, but an asymmetrical one can generate lift even without inclination (and with lower drag). The article also explains that acrobatic airplanes have symmetrical wing sections exactly because they need to be able to fly just as easily inverted.