On Oct 3, 5:56 pm, wrote:
So, if it has nothing to do with Bernoulli, what has it to do
with lift?

You can have lift of an object with no Bernoulli. It's simple vector
addition.

With tables and straws and the like we're talking static,
not dynamic pressures. The airplane uses dynamic AND static
pressures.

Right.

In cruise flight (low AOA), I would expect a reduced pressure
on the bottom of the wing, though not as low as on the top. The
velocity of air across the bottom will drop its pressure, there, too.

Right. The AOA matters here. Angle the wing up any significant amount,
and over-pressure will build under the wing.

Air has mass. Anytime you try to push it out of the way,
there will be some reaction. Newton says so. We know this as drag.

Hmm...ok, sure. I wouldn't call that drag necessarily. That's like
saying that a hydraulic piston assembly has drag. I more prefer to
think of drag as laminar fricitional forces of the fluid. Pushing out
of way implies that plane doing the pushing is perpendicular to the
direction in which pushing is being done (which is true at leading
edge of wing). But I guess this is acceptable.

But we also know it as lift reaction. A flat plate flying
through the air at some tiny angle of attack doesn't have much faster
air over the top than the bottom, if any difference at all, yet it
will generate plenty of lift. Try this on, if you want to think
outside the box:

Thinking outside the box is what lead me to refuse to accept hand-
waving explanations of aerodynamics from CFI's.

The airfoil we know is just that: a foil (device to
deceive) to trick the air into flowing over it without breaking up at
much higher angles of attack than a flat plate would let us. So the
leading edge has to have some radius so the air can get around the
corner from the natural stagnation point under the LE at high AOA, and
that curve must gradually taper off toward the rear or the now-
disturbed air would want to separate and turbulate, and if it did that
it would then slow down dramatically, pressure would rise and lift
would decrease. But, happily, Newton is still at work underneath so
the airplane falls, but not as if the wings fell off. We're still
moving forward and the wing is still shoving air out of the way
downward, so lift is still generated.

Yes this is true, but the explanation in the Jeppensen book is wrong.
it defines downwash:

downwash: - "the downward deflection of the airstream as it passes
over the wing and past the trailing edge"

It goes on to say:

"According to Bernoulli's principle, the increase in speed of air on
top of an airfoil profdues a drop in pressure and this lowered
pressure is a component of lift."

Ok, we really know that the lift results from what's under the wing no
longer being balanced out, but I won't nit-pick this explanation.

Next paragraph it says:

"In addtion to the lowered pressure, a downwar-backward flow of air is
also generated from the top surface of the wing. The reaction to this
downwash results in an upward force on the wing which demnstrates
Newton's third law of motion."

This is plane false. That is *NOT* what Newton said. Newton did not
say you could take any action and willy-nilly find what you think is
the reaction, and say, "Hey, this looks good, let's use this."

Newtons law, in fact, is better stated as reciprocity of force, IMO.
This says that, if you take two objects, one apply force to the other,
the other, by reciprocity, must appy an opposite force against the
first.

Newton's law, conbine with F=ma, also yields the notion of
conservation of momentum.

But getting back to Jeppensen, the downwash, if they mean what's
happening on the top surface of the wing, is *not* contributing to
lift. Note that they say "results in", but don't explain how. This
seems to be typical of books of flight dynamics.

I'd like to point something else out regarding Bernouilli's principle.

I haven't tried, but I suspect that I could build a contraption that
consists of surface where the velocity of air above the surface is
much higher than that below, but the pressure above the surface is
higher.

-Le Chaud Lapin-