On Oct 7, 11:54 am, flightoffancy wrote:
In article . com,
says...

Now, in Chapter 3, section about airfoils, it actually says:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."

IMHO, the latter part of this paragraph is correct, but the former
part is wrong.

JC, you're confusing yourself.

[Note, I just re-read your post and realized that you too are implying
that a the top wing surface can accelerate air molecules downward.,
hence my double response.]

Instead of focusing on "fixed" wing, think for a moment about helicopter
blades and propellers. These are airfoils not fundamentally different
than one attached to the side of an aircraft.

Agreed.

Anyone who has ever seen video of a helicopter hovering or has been near
a helicopter hovering knows that air is being pushed down by the blades
with massive force and that is the equal and opposite force exerted by
the mass of air on the bottom of the blades that keeps the helicopter
from falling out of the sky.

Well that is certainly true.

A fixed wing aircraft is only different in that it pushes air under it
by moving forward, rather than in a circle.

True.

The bottom line is simple: an airplane can only stay aloft by pushing
air down.

This true and not true. A wing does not necessarily have to push air
downward to cause lift.

An airplane can stay aloft if rarefication is somehow created above
the wing. This is what's happening with the blow-over-paper trick.
The air below the wing remains more or less steady at ambient
atmosphere. The air above the wing is rarefied and therefore causes
less force above the wing. The net result of the difference between
the full force below the wing and the reduced force above the wing,
minus the weight of the paper due to gravity, results in an upward net
force on the paper, causing it to rise. As soon as you stop blowing,
the ambient atmosphere works to replenish the rarefied air above the
paper to its natural state, which causes a net force on the paper due
to pressures above the paper and below the paper to equalize [taking
normal vectors into account, yada..], and gravity becomes the
determining force, causing the paper to flop back down.

Note that, in the paper trick, the airspeed of the paper is 0, and,
for all practical purpurposes, the air beneath the paper has no idea
that you're blowing on top of the wing.

Yes, the angle of attack gives the greater impulse to knock the air
downward. But a curved upper surface gives even more downard force to
the air.

How? How can a the top surface of a wing cause a downward force on air
molecules that are on top of the wing?

The uppper surface of the wing can only exert a force on the air
molecules above the wing either in the upward direction, or in the
lateral direction due to friction. Ionized particles and charged
surfaces notwithstanding, it is not possible for a (theoretically
smooth) upper surface to exert a downward force on air molecules that
are sitting on top of it.

-Le Chaud Lapin-