Fluid flow over aerofoil

Question

Why does the velocity of air flowing over the top surface of aerofoil increase, when there is no contraction in area over the top?

Answer 1

The aerofoil's angle of attack (angle of the wing to the airplane's path through the air) creates a low pressure area above the wing rearward from the leading edge that sucks in and accelerates air from the leading edge and circulates it upward. The lower surface of the wing tends to compress air, slows it down, and circulates it downward. The combination of low pressure on top and higher pressure on bottom lifts the airplane.

At some angles of attack, it's advantageous for an aerofoil's cross section to be convex on the top, and concave or flat on the bottom. The convex top path is longer than the concave or flat bottom path, which has given rise to the Longer Path or Equal Transit explanation of why air flow across the top is faster than air flow past the bottom.

The reasoning is that as the aerofoil moves at one speed through the air, if all the air meets at the trailing edge at the same time, but the top path is longer than the bottom path, then top air flow must move faster than bottom air flow, as the outside of a curve is longer than the inside of a curve. This assumption is incorrect and provides a misleading explanation. If both top and bottom air flows met at the same time, there would be little or no lift. Notice in the linked video that the angle of attack determines whether air flow is faster on top or on bottom.

Speed of air flow along the top of the wing is greater because the angle of attack creates an area of low pressure that sucks in air and circulates it upward. Speed of air flow along the bottom of the wing is less because the angle of attack creates pressure that slows the air and circulates it downward. This can be reversed if the angle of attack is reversed.

Answer 2

@Ernie "et al", There's nothing wrong with the equal transit time theory if it is expressed properly (In fact it is very good at explaining the thin, zero AoA aerofoil scenario).

Often it is not well expressed. Firstly I'd call it an approximation rather than a "theory".

Crucially the assertion is this:
That after the aerofoil has passed the air returns to its original state. This is modelled and approximated by considering that for that to happen the transit time of the air over and below the wing would need to be equal. It's an approximation. Then we consider what would happen if the air over the thin aerofoil maintains it's speed but not its velocity....
Since the over-wing path is longer it will have a longer transit time, or you could say there will be a path deficit. Essentially that's a kind of estimated vortex, from which can be calculated an approximate pressure deficit. As a result of this approximation we can estimate both
1) that the actual air speed over the aerofoil will increase as it is pushed into the low pressure region, and
2) the pressure differential above and below the wing gives an estimate of lift.

What's nice about this explanation is the horizontal (zero AoA) aerofoil can and does generate lift. It's an important solution because this mechanism is very drag efficient, so it's ideally the one we prefer wings to use in cruise mode. (Not during take-off an landing when the overriding concerns are power and control.)

What would be very, very, wrong is if any-one felt that 50 years ago people were significantly less capable of systematic thought than the current generation. Which is partly what compels my input.