You're right that pressure itself is a momentum change, but that
doesn't appear to be what the momentum change advocates are, ah,
advocating.

They (and I too) are talking about local effects. It's fine to say no
=net= downard motion, of course there is no net downward motion - the
earth gets in the way, and the increasing pressure pushes the air back up.

Suppose the earth wasn't really there... we have an infinite field of
air in all directions, and no gravity. A airplane comes through. It
will (at the proper AOA) experience lift, and will accelerate upwards
(upwards being defined wrt the wings on a cessna, downwards being
defined wrt the wings on a piper). Newton requires that air be
accelerated downwards.

If we introduce plane gravity (that is, gravity that magically only
attracts airplanes and leaves air molecules alone), then this gravity
will pull the airplane down, and will prevent it from accelerating
upwards. However, air will still have to be accelerated downwards to
keep the airplane from succumbing to gravity. There will be localized
high pressure below the wing, and localized low pressure above the wing,
and there will be a vortex as the air rushes around the wingtips, but
the air that is accelerated downwards will not have anything to stop it
(except other air, which molecule by molecule accepts the transfer of
momentum).

If we let gravity work on the air molecules too, then there will also be
a gravitationally induced acceleration of air downwards, since there is
nothing to stop it. The air will be in free fall (and pretty soon the
airplane will not be able to keep up).

It's only when you put the earth itself in the picture that it all comes
together. With a hard surface below the air, (momentum from) molecules
that have been "thrown down" by the wing will get transferred to the
earth, and the air molecules will bounce back upwards again. THIS
causes the pressure that feeds the upwash (and helps keep the earth from
accelerating upwards towards the airplane). If we let the system
stabilize, the air molecules will pile up near the earth, and be sparser
further up.

However, the pressure change below the wing isn't downwash.

If there is a technical meaning to that word, I am not using it that way.

Jose
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