Jose wrote:
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.

I inferred coherent flow from 'downwash'. Some persons, perhpas not
yourself, pointed to disturbances on the surface by low flying aricraft
as evidence of downwash. That sounds like coherent flow.


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).


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?

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.

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


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).

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

--

FF