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This is all a bag of worms, depending on how accurate you want your explanation to be.
The current ATPL (JAA) explanation, aka the 'air deflected downwards / equal and opposite reaction makes lift' explanation is (a) a step in the right direction (versus the 'equal transit time' fallacy) and (b) at least doesn't violate Newton 3. However, it does violate momentum conservation.
Just say to the bloke something along the lines of: "In your explanation, consider level flight. The aircraft hasn't changed its vertical velocity component, but if the air has been deflected downwards, so momentum has not been conserved"
I've spent a lot of time thinking about this over the last couple of years. Over my career I've worked as a ground school instructor teaching PofF, a school teacher of Physics in addition to my flying. What I do not have though is a degree level of understanding regarding fluid dynamics.
In general, for explaining anything mechanical, my yardstick for something to be a good model is: Newton 1,2,3 not violated. Momentum conserved. Energy conserved.
Also, everything else being equal, scalar based explanations are simpler than vector based explantions, e.g. Energy rather than forces (doesn't help much with explaining lift, but helps a lot with explaining induced drag).
As far as I can see , it is impossible (by my yardstick) to give a good model for lift production if you are only looking at a 2d aerofoil. However, once you go to 3d the world is your oyster.
Anyway, I'm short of time to post further at this point.
pb
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