Note that the following stuff is sort of subtle, and would only be taught after the simpler concepts were solidly learned. It’s about the reasons why wings fling air downwards, or more specifically, the reasons why ‘circulation’ appears.
I mean, use a blunted, non-sharp trailing edge. If the trailing edge was just as blunt as the leading edge, then even if the wing was cambered and even if it had a positive attack angle, the wing would still not deflect air downwards, and lift would vanish.
Certain kinds of wings will fail to deflect air downwards, since the air flowing beneath the wing would collide with any downwards-flowing stream and push it back upwards again.
In terms of those diagrams, below is what happens if either the trailing edge is too blunt, or if the viscosity of the flowing fluid is high enough to damp out all the inertia effects:
WING WITH NO ‘KUTTA CONDITION’
Notice that the flow pattern in the above diagram is symmetrical, where the flow approaching the leading edge has the same shape as the flow departing the trailing edge. The wing is unable to fling the air downwards and have it keep flowing downwards. The air divides and lets the wing pass by, but the wing doesn’t deflect the flow in a permanent way.
If instead the trailing edge is made sharp, then intertia effects become strong, and the pattern of flow changes to this one:
WING IN NORMAL FLIGHT
Are you aware that “sucking” is different than “blowing?” For example, if you have a pump with its inlet and outlet having identical shapes, then the flow pattern near the inlet is radial like a sunburst, while the flow pattern near the outlet looks like a narrow stream. Inertia is the cause. Air that was flowing as a narrow jet inside the pump will continue flowing as a narrow jet when it leaves the outlet. But air which approaches the inlet has no reason to form itself into a long narrow jet. (And you can blow out a candle from far away, but just try sucking out a candle from the same distance!)
This type of inertia stuff is what happens in normal flight as shown in the second diagram; the leading edge cannot form distant air into a narrow stream, but the trailing edge can do this easily because of inertia. The leading edge is surrounded by a pattern resembling “suction”, while the trailing edge is surrounded by a “blowing” pattern: a long narrow jet of air. The wing essentially “grabs” the air and flings it downwards, the air continues moving after the wing has gone past, and the “claw” that grabs the air is located at the sharp trailing edge.
(Those diagrams are from 3 Airfoils and Airflow)
PS
In three dimensions the air continues flowing downwards forever (or until it hits something, or until it slows down from friction.) But this topic is never mentioned except in the most advanced textbooks. But it’s simple! (Well, it’s simple if you have an animated diagram.) I thought these concepts should be part of introductory teaching, so I put the math-free version on my website here:
http://amasci.com/wing/rotbal.html
Note the animated GIF that’s a ways down on the page. When I first figured this stuff out (while fighting with J. Denker!), I was amazed to discover that wingtip vortices aren’t just an unimportant detail which should be ignored. Instead they’re right at the center of any full-blown explanation of flight. They don’t just trail behind the wing. Instead, the wingtip vortices are part of the air which remains moving downwards after the wing has passed by. Planes fly BECAUSE they create wingtip vortices. I think this whole part of wing explanations is ignored in introductory texts because they focus on 2D diagrams (which simplify things a bit TOO much.)
