---------- Forwarded message ----------
Date: Sun, 17 Jan 1999 09:44:39 -0600 (CST)
From: David_Anderson <dfa@fnal.gov>
To: William Beaty <billb@eskimo.com>
Cc: "phys-l@lists.nau.edu: Forum for Physics Educators"
<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" ????
Bill,
I found your last email somewhat stimulating. I believe that I can shed
some light on the situation. I will first start by giving a quote from
Elementary Fluid Dynamics, by D.J. Acheson, Clarendon Press, Oxford:
Notwithstanding the importance of circulation, the Kutta-Joukowski
condition, and the theorem of [the previous section which relates lift to
speed and circulation], 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.
It is the loss of the fact that 2D simulations are of infinite wings that
give rise to the (silly) misconceptions that lift does not require work
(the basis for the book Stop Abusing Bernoulli! How Airplanes Really Fly,
By Gail Craig), that if one were to look far enough back there would be no
net momentum transfer, that circulation drives lift (rather than the other
way around), that a wing with symmetric circulation has lift, and that the
induced drag is just due to the nasty wingtip vortices.
The point was brought up in the email that many textbooks and expert
cannot possibly be wrong. Through the years of developing the view that
is presented in our paper, many a time I almost quit because of something
that I read from an expert. I would be stopped by the thought that if
what was said were true, that I really didn't know how a wing developed
lift. Fortunately for me, Scott was there to explain the origin of the
misconceptions. This work has been more of an effort in learning where
misconception came from than figuring out how planes really fly.
I hope this is of some help.
David
_______________________________________________________________________________
David F. Anderson
Fermi National Accelerator Laboratory
PO Box 500, MS-220
Batavia, IL 60510
(630) 840-3471; FAX (630) 840-6039
_______________________________________________________________________________