I don't understand Jack Uretsky's response to George Spagna. Here's
how I would answer George.
(1) Yes, Bernoulli is a conservation of energy statement, and yes it
applies to a particular streamline of air. Bernoulli should be used to
compare the pressures within a given streamline when some regions of
that stream are flowing faster than others.
(2) Yes, the streamlines over and under a wing are separate
streamlines. But if we compare the streamline over the wing to the
same streamline some distance in front of the wing, we see it is moving
faster on top of the wing and hence has lower pressure than it had
before. Likewise, if we do the same thing for a streamline under the
wing, we see it has higher pressure than it did before.
(3) Assuming we chose points sufficiently ahead of the wing that the
pressures of the two streamlines are the same at that horizontal
location, then we have the situation that: top is lower than ahead,
bottom is higher than ahead, therefore top is lower than bottom.
Although I personally do not like Bernoulli analysis as a means to
explain lift, I believe it is an analysis that is reasonably valid.
However, to use it you have to know the velocities. These velocities
are best measured in a wind tunnel. But once you have a particular
airfoil in a wind tunnel at some angle of attack and some overall air
velocity, why measure velocity differences and calculate lift when you
can directly measure lift (and drag) from your airfoil support
mechanism?
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