I've been hanging back on this topic because I'm no expert, and the
viewpoint I have is one I don't often see. But what the heck, let's
throw it out and see... if it flies.
It seems to me that Bernoulli analysis is fine once you accept that air
goes faster over the top of the wing. Bernoulli doesn't explain why
this is so. It also is clear to me that the idea of "circulation" is
valid and also helps the analysis at some 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.
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?
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.
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, but we will likely have turbulence on top of the
board (because of the sharp top leading edge of the board). We might
be able to fly this if we have sufficient power to overcome the drag
and maintain the speed, and if we don't break the board. But that
turbulence is going to create a lot of drag force, and we won't be
getting much if any lift from the top of the board.
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.
This not only reduces drag, but also allows the air going over the top
of the wing to be deflected downward and provide additional lift. If
students accept the idea of viscosity, and the idea of laminar flow
around the shape of an object, they can readily accept that a wing with
a positive angle of attack will deflect air downward both from the
bottom and top of the wing. You can decide whether to tell them more
specific details about upwash and that the top of the wing actually
provides more downward air motion (lift) than the bottom of the wing.
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). This happens at some combination of speed and angle of attack for
a given airfoil shape. When that happens, not only does the wing lose
the bulk of its lift, it also gets slammed with a great amount of drag,
and that's why a stalled wing just about drops out of the sky.
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.
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).
In summary, (1) this approach makes sense to beginning students, (2)
its puts a lot of importance on angle of attack (which is correct), (3)
but it also explains why the airfoil shape is important, (4) it easily
allows one to understand what a stall is and why it is so devastating.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817