I often hear about planes stalling when they lose lift due to low airspeed/too high angle of attack. Why don't birds stall? Does it have to do with the structure of their wings and their flexibility, or their higher power/weight ratios relative to aircraft?
[Physics] Why Don’t Birds Stall
aerodynamics
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I've never seen actual figures but, in general, articles I've seen about flight state that "most" lift is generated from the angle of attack and relatively little from the Bernoulli effect. I suspect the exact figures are rather variable and probably depend on whether the plane is climbing, descending, banking, etc and will also vary from plane to plane. Maybe this is why exact figures seem not to be quoted.
The pressure difference between the top and bottom of the wing is quite real, though note that on the top of the wing it's not a vacuum as the pressure doesn't decrease that much. The lowered pressure above the wing will indeed tend to pull the skin off the wing, or more precisely the air within the wing that is at normal atmospheric pressure will try to push the skin off. Once again I can't give you exact figures - I must admit I thought ballpark figures would be easy to calculate, but Google has failed me.
Incidentally, there's a good NASA article on this subject at http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html and it even includes a Java applet for you to play with the details of the wing. A longer slightly more staid article is at http://www.free-online-private-pilot-ground-school.com/aerodynamics.html
Later:
If an approximate answer would be OK then you could could use Bernoulli's equation as described in http://en.wikipedia.org/wiki/Bernoulli%27s_equation#Incompressible_flow_equation. Although this really only applies to incompressible fluids, and air is obviously compressible, the article suggests it would be a reasonable approximation for low speeds.
Rewriting the equation to make it more useful for our purposes gives:
$$P = \rho A - \rho \frac {v^2}{2} - gh$$
where $A$ is some constant and $h$ is the height. We don't know the constant, but let $P_{bot}$ be the pressure below the wing and $P_{top}$ be the pressure above the wing then we can take the difference between them i.e. the pressure drop between the bottom and top of the wing. If we assume the height is constant i.e. we can ignore the thickness of the wing we get:
$$\Delta P = P_{bot} - P_{top} = 0.5 \rho (v_{top}^2 - v_{bot}^2)$$
I don't know what speed you plane flies at, but let's guess at 30 m/s and let's guess that there's a 10 m/s difference between the air speed at the top and bottom of the wing, so that's $v_{bot} = 30$ and $v_{top}$ = 40. Google gives the density of air at ground level as 1.225 kg/m3.
$$\Delta P = 0.5 \times 1.225 \times (40^2 - 30^2) = 429 Pa$$
429 Pa is 4.29 grams per square cm or 0.06 pounds per square inch, so it's completely insignificant.
My apologies, I won't be reading your entire question.
But still I will provide an answer. Why is that? Because flight does not require any of the things you talk about.
You could build an airplane that would fly with no "airfoil" shapes. You could build an airplane that would fly with completely flat rectangular wings made out of plywood. The important thing would be the angle of attack of the wings to the air. Consider a flat piece of wood, like plywood. Push it through the air in a direction exactly parallel to its flat dimensions and it develops no lift. Tilt the wood so the leading edge is "up" compared to the direction it is moving and you can feel the lift.
The lift can be thought of a few ways. Think of the air molecules hitting the surface of the wood. They bounce off, in a downward direction. Well if we are pushing air downward, we must have an equal and opposite force, which is the lift. Or another way: we are gathering air under the board, it gets a little pressurized. The pressure is pushing up on the wood. This is really the same picture as the first if you think about it.
All the rest with airfoils and so on, all this has to do with developing lift efficiently, developing lift while minimizing drag. An airplane with flat plywood wings would fly, but it would have a lot of drag and would therefore be very inefficient.
Best Answer
A bird's wings have muscles, unlike the wings of a plane. It's true that we can control them, but they're metal(\m/). Human mechanisms are not as flexible as those of nature.
Compare driving a car with running in your imagination: If you are walking and someone is going to crash into you and you see him, you could dodge it easily. But if you are driving a car and uou want to move left as dodging when you see some of the obstacles, you cannot do that.
It's exactly the same as birds, they can control their wings 100%. We can control planes only half of the bird wings, so it may make a difference.