I inferred coherent flow from 'downwash'.

That coherent flow is not necessary does not mean that coherent flow
does not exist. My point is that the downwash does not have to be
directly from the wing to the earth. It can be very indirect - in a
multiple collision scenario, the existance of new momentum somewhere
imples the existance of opposte new momentum elsewhere, mediated by
collisions which may or may not be "coherent", however you wish to
define it. Momentum is conserved. Always.

When we are discussing the microscopic transmission of momenta
between air molecules whic is the basis for presure, yes. Is that what
you mean by 'downwash' or downflow, as opposed to something that
involves a flow of mass?

There is downwash, involving a "coherent" acceleration of mass
downwards. Due to an increase in microscopic collisions below (and a
scarcity of them above), there is an incoherent transfer of momentum
(called pressure) to the surrounding air (and ultimately to the earth).

This leads to a condition described as "low pressure above, high
pressure beneath", or equivalently described as "less momentum
transferred via collisions above, more momentum transferred via
collisions below", which supports the wing, propping it up again and
again as it tries to succumb to gravity. We call this lift.

There are some neat bulk equations which help quantify this, which come
embodied in a concept which is useful for understanding this in some
contexts. However, an equivalent (newtonian) concept is more useful for
understanding in other contexts, and explains a few things that are not
addressed by the B word.

Then it doesn't matter which way the air above the wing flows. If
the air flows sideways, you still have lift.

No, at least not directly. If there is less momentum transferred from
above than from below, you have lift. This comes from lower pressure
above and higher pressure below. How you get that is ultmately
Newtonian, not magical. Once Newton has his say, Bernoulli can
reformulate it in a useful bulk form.

Consider a flying saucer, composed solely of two disks with no
appreciable space between them. The one below does not spin, the one
above spins rapidly. Should there be lift? Why? Does it matter if the
top disk is rough or smooth?

The upper atmosphere in
fact =is= deprived of air while the airplane is in flight... that air is
squeezed down below the wing, increasing the pressure on the surface of
the earth, in an amount exactly equal to the weight of the airplane
(divided by the area of the earth).

I think that the downflow dispaces other air which flows up to
replace it--conserving momentum and mass.

What happens to the downward momentum of the downflowing air when this
happens? The displaced air, flowing upwards, has acquired upward
momentum - where did that come from? (and so far, conservation of mass
has not been an issue)

Jose
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