It does that
because there are fewer air molecules transfering momentum
to it from above, that there are from below. But it does not
do that via a coherent stream of air.

I guess I should have appended this to the previous response...

This is correct. However, a coherent stream of air is not necessary for
this:

The movement of the plane towards the earth is transferred to
movement of the air towards the earth, which it does until it eventually
transfers its momentum back *to* the earth, leaving the system with the
same relative momentum with which it began.

to also be correct. A coherent stream of air is not required, nor is it
what I am proposing.

A fluid can transmit force without flow in the conventional sense.
That is the basis for hydraulics.

We are not really talking about "flow in the conventional sense", we are
talking about microscopic collisions. Flow may be involved (as in the
flow that causes upwash upflight) but it needn't be (as in the case of
the microscopic dribbler).

The downflow observed from the wing initiates above the wing
and flows down behind the wing after the wing has passed.
It is not the air that suppors the wing.

Well, the only air that supports the wing is are the molecules that
impact it from below. They not only support the wing, they also fight
against the molecules impacting from above. They win, because there are
more of them. There are more of them because of downflow and the
collisions it causes.

Well then if the downflow is NOT balanced by upflow why doesn't
the upper atmosphere run out of air?

Because the wing is not of infinite weight. 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).

If a sufficient (huge!) number of aircraft took to the air, the upper
atmosphere would become measurably thinner. Maybe we should get a grant
to do this experiment using general aviation aircraft - for the good of
Science and the benefit of GA pilots.

Jose
--
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