But your original text suggested that the fluid *had* to be
compressible.

Yes, it did suggest that. It shouldn't have. My clumsiness.

Anyway, remember that there is no
overall downward movement of the air unless there exist wingtip
vortices.

An infinitely wide wing has no wingtips. You suggest it could not
provide lift. I've read the "wingtip vortices provide lift" papers, I'm
not convinced that the correlation implies a causation in that
direction. I see it as: the wing causes downwash which provides lift
(action-reaction) and =that= creates vortices. The higher pressure air
underneath the wing has to go somewhere - around (the wingtip) and up
makes sense to me, and that is a vortex.

What happens in the two-dimensional case?

I'm skeptical that if you have an airfoil generating
positive lift, just tilting the trailing edge upwards slightly is
going to kill that lift.

I agree with you. The downward movement of air is being generated over
the entire wing chord, and has some depth to it too. I suspect (without
solving any equations) that tilting the trailing edge upwards slightly
(that's what ailerons do, sort of) does reduce lift, but as the airflow
right underneath the airfoil gets deflected upwards, the air further
away (below) does not, leaving a small lower pressure region just below
the upward pointing trailing edge. The rest of the mass of air below
this small low pressure region continues downwards through its momentum.

Jose
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