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is Circulation theory "wrong" ???? (fwd)



Date: Sun, 17 Jan 1999 09:44:39 -0600 (CST)
From: David_Anderson <dfa@fnal.gov>

<PHYS-L@LISTS.NAU.EDU>,
tap-l <tap-l@listserv.appstate.edu>, scott@aa.washington.edu,
Jan-Olov Newborg <newborg@algonet.se>, jefraskin@aol, jsd@monmouth.com
Subject: Re: PHYS-L: is Circulation theory "wrong" ????

... an aerofoil obtains lift essentially by imparting
downward momentum to the oncoming airstream. In the case of a single
aerofoil, in an infinite expanse of fluid this elementary truth is
disguised, perhaps, by the way that the deflection of the airstream tends
to zero at infinity.

There should be no question that the net result of lift is that air is
given vertical momentum and energy. The confusion comes from the 2D
simulations of the aeronautical engineers. I think you will have an
easier time visualizing the 2D situation if you keep in mind that a 2D
airfoil is really an airfoil of infinite length. This fact is often lost
and is the cause of much of the confusion. As Scott and I discussed in
our paper, the efficiency for lift of a wing is proportional to its
length. A wing of infinite length diverts an infinite amount of air down
at zero velocity. Since the work done is proportional to the vertical
velocity of the air (squared), the infinite wing develops lift without
doing work.....
___________________________________________________________________________
____
David F. Anderson

While sympathizing completely with Anderson's approach, I had better
tighten up an assertion of his which is not quite sturdy enough to
serve as a stick for beating aero engineers, namely:
"A wing of infinite length diverts an infinite amount of air down
at zero velocity."

This assertion would doubtless be true for any wing of finite weight.
If, however, the wing loading were given in force ( or weight) per
unit length of span, then the weight to be reacted, also being infinite
would need finite net downwards velocity.

I hope and believe that Anderson and his associates would heartily concur
with my position. I have copied him on this note as a courtesy.


brian whatcott <inet@intellisys.net>
Altus OK