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Re: Physics of Flight; air deflection

At 05:59 PM 8/11/99 -0400, Michael Edmiston wrote:

It seems to me that Bernoulli analysis is fine once you accept that air
goes faster over the top of the wing.

Right.

Bernoulli doesn't explain why this is so.

Right. You need some other principle to give you the velocities.

It also is clear to me that the idea of "circulation" is
valid and also helps the analysis at some level.

Right. IMHO it is indispensible at every level beyond the second-grade level.

However, I have never
felt that Bernoulli is a good way to explain lift to young or novice
scientists. Here I make a distinction between an "analysis tool" and
an "explanatory tool." For some people Bernoulli is worthwhile
"analysis," but it does not strike me as a good "explanation."

I like the basic approach that air must be deflected downward to create
an upward force on the wing. I think this is an explanation physics
novices can buy. This also makes it clear why the angle of attack is
of extreme importance.

OK, but this begs the question of HOW MUCH air is deflected downward, and why.

However, if we use the air-deflection approach, the question becomes:
why must we pay any attention to the shape of the wing; why not use a
rectangular board with some appropriate angle of attack?

Why not indeed? A flat plate actually makes a passable (but not great)
airfoil.

The answer to this question is that we want to preserve laminar flow
over the top of the wing. Why? Because, if the air follows the top of
a slanted wing (angle of attack), we can deflect air downward (get
lift) from the top of the wing as well as the bottom of the wing; and,
of equal importance, it greatly reduces drag.

Cough, cough.... it's a lot tricker and more complicated than that.

If we have a flat board thrust through the air at some angle of attack
and sufficiently high velocity, we might deflect air downward from the
bottom of the board,

Choke, choke.... The notion of air bouncing off the bottom of the board is
a not a good model of how a wing (or even a flat plate) produces lift. See
http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_fluid
for an explanation of why not.


Now if we round-over the leading edge and taper-off the trailing edge
(i.e. make an airfoil) we might be able to eliminate the turbulence.

Technically, turbulence isn't the problem. Perhaps you mean "separation".
Some wings even incorporate "vortex generators" or "turbulators" to
*increase* turbulence in order to fend off separation. For more on this, see
http://www.monmouth.com/~jsd/how/htm/spins.html#sec_boundary_layer


It has already been mentioned in this discussion that the wing "stalls"
if the air separates from the top of the wing (i.e. if turbulence sets
in).

Again, separation is not the same as, or even necessarily caused by turbulence.

This happens at some combination of speed and angle of attack for
a given airfoil shape.

The stall depends on angle of attack, not speed.

When that happens, not only does the wing lose
the bulk of its lift,

Actually, the coefficient of lift decreases less than you might think, and
decreases only slowly as the angle of attack increases beyond the critical
angle of attack. At the stall, the airplane loses certain stability
properties, and this is much more noticeable than the loss of lift _per se_.

So I view the airfoil shape primarily as a method to keep laminar flow
over the top of the wing at the desired speed and angle of attack.

Again, I assume you mean "attached flow" rather than "laminar flow".

Engineers can choose various shapes and designs (such as with or
without camber) depending upon the desired performance (and other
things such as whether this plane will frequently be flown upside down,
i.e. a stunt plane).

Right.

In summary, (1) this approach makes sense to beginning students,

OK, but it doesn't really explain anything, since (without appeal to some
principles you haven't mentioned) it doesn't explain how much air gets
deflected.

(2) its puts a lot of importance on angle of attack (which is correct),

Right.

(3) but it also explains why the airfoil shape is important,

Actually it doesn't explain or even mention the most important important
thing about the shape of an airfoil -- the trailing edge. *THAT* is important.

(4) it easily
allows one to understand what a stall is and why it is so devastating.

Not really. If you would like some information as to what a stall is,
please check out
http://www.monmouth.com/~jsd/how/htm/spins.html