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Re: Misconceptions: Physics of Flight



I appreciate the time John Denker has taken to reply to all of our
e-mails about lift. In response to my contribution he responded
line-by-line, and that helps me see where we agree and where we differ.

I am still having problems with the "whys" of this all. I am also
having trouble with laminar flow, separated flow, etc.

In the summary of his third chapter he says: "A wing is very effective
at changing the speed of the air. The air above is speeded up; the air
below is slowed down. Each air parcel gets a temporary change in speed
and a permanent offset in position."

The problem I am having is... why? Why is a wing very effective at
changing the speed of the air. I need to see this as mechanical
process involving molecules of air. And I think my problems with
turbulent/laminar flow and separated/non-separated flow might be at the
heart of this.

As an object moves through a fluid, surely the atoms/molecules ahead of
the object are being displaced to some other location; likewise the
atoms/molecules must "fill-in" behind the object or else we're left
with a vacuum behind the object.

For the moment, let's ignore what happens in front of the object
because it is behind the object that gives me the most problems. I
would say that the air going over the top of the wing is speeded up
because it is being sucked into the void created behind the moving and
tilted wing. Then the question becomes: in what manner does it fill
this void? If it turbulently swirls into this region it doesn't have
any obvious net momentum and it is difficult for me to imagine lift.
In addition, it seems that drag is high if "air friction" changes from
bv^1 to bv^(more than one) when turbulence sets in.

On the other hand, if it is laminar flow and the air follows the upper
surface of the wing, then it will have momentum directed along the
angle of attack (i.e. momentum typically directed downward). To me
that implies lift and also low air friction.

I am not sure how to understand turbulent flow versus separated flow
(words Denker distinguishes). John's computer generated wind tunnel
drawings, and real wind tunnel & smoke photos I have seen, certainly
look like laminar flow to me, and they look like turbulent flow once
the wing has stalled. Is this all wrong? I can imagine some degree of
turbulence that somewhat follows the wing shape. And I can imagine
some degree of separation without gross turbulence. But, in general, I
sure have the impression that a major stall has both gross turbulence
and separation behind/above the wing.

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