Jose wrote:
Wrong. Momentum change is momentum. The time rate of momentum
change is Force. Neither is pressure.

What I wrote (or intended to write) is that pressure is manifested by
many momentum changes. Each molecule that collides with a wall
transfers momentum to that wall, and the net force caused by all those
momentum changes (over time) manifests itself as pressure, (which is
force divided by area). The essential point is that pressure arises
from momentum transfer on a molecular level.

IOW, the causual basis for macroscopic gas laws is governed
by statistical mechanics. Agreed.


Why does Newton require that air be accelerated downwards?

To counterbalance the wing being accelerated upwards due to lift. The
wing is at an AOA which generates lift.


I think you are refering to Newton's third law, often stated as: "For
every
action there is an equal and opposite reaction."

For an aircraft in level flight, the upwards acceleration due to lift
is
counterbalanced by the downward acceleration due to gravity.
This satisfies Newton's third law.

For a wing in level flight, the vertical component of momentum is
zero. Therefor, conservation of momentum requires that the vertical
componenet of momentum of the air also be zero. The wing imparts
as much upward momentum to the air as it does downward momentum.

The pressure differential through the wing, from bottom to top,
integrated
over the wing area, provides an upward force for a wing in level
flight.

The downwash behind the aircraft, which is counterbalanced by a more
diffuse upwash around it, is real but not relevent to the issue of
lift.

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FF