---------- Forwarded message ----------
Date: Mon, 23 Aug 1999 13:32:48 -0700 (PDT)
From: Scott Eberhardt <scott@aa.washington.edu>
To: William Beaty <billb@ESKIMO.COM>
Cc: David Anderson <dfa@FNAL.GOV>
I arrived at work and spent 3 of the last 4 hours sifting through the lift
thread that arrived since SATURDAY!!!!! My conclusion is that I cannot
afford that sort of time. The arguments aren't going anywhere in my
opinion. I continually see Denker retreat into positions that are
defendable and using that as reasons why our paper is wrong. There are
many points that Denker makes that are correct. But, he is wrong in
assuming that because one piece is correct, the entire picture is correct.
He is correct that the lowest pressure is near the leading edge. And,
therefore most of the lift is near the leading edge. But, the upwash,
which is in front of the leading edge is a different thing. The upwash is
being bent upwards so imparts a downward force on the wing. This is our
"control volume" view. The surface view says that the air pressure
goes from a high pressure at the stagnation point to a very low pressure
just around the leading edge. If you integrate the pressures, you'll be
able to calculate the lift. Denker is correct and I have no argument
about that. But, that surface air is not the upwash. Also, Denker's
point of upwash balancing downwash is entirely correct for airfoils. It
is not for wings which are three-dimensional.
I know that David sees a central difference between the two approaches
(Denker's and ours) is whether power is required for lift. In the 3D view
it most certainly is. That's what induced drag is all about. In the 2D
view no power is required (so energy is conserved as Denker states). Both
explanations are correct, provided you know you are working in two or
three-dimensions. I have discovered teaching my students that most have a
difficult time conceptualizing an airfoil but they all understand wings.
It might seem trivial, but it's amazing how confused students get when you
try to have them work in a 2D world. In the 3D world Newton says that for
a wing to have lift it must impart a force on the air. Since the lift is
upward the net force on the air is downward. Since the air has been
changed, work was done. The work done in a given time is power. If
people have trouble agreeing with that it's time to break out the
introductory physics texts. All the other arguments that have been passed
back and forth deal with peripheral examples and mis-understanding of
messages. In my opinion it's not productive.
I think you hit the nail on the head when you describe Denker and
Anderson/Eberhardt as rivals. I don't think we will ever convice John
Denker that our Newtonian model has the right physics. David and I are
satisfied that it does, as are my collegues, and John's attempt to blow
holes in it just seems to be peripheral to our central theme: momentum is
exchanged and a force results.
The bottom line is that although I would like to add my two cents to the
PHYS-L group, I do not see it as a productive use of my time. I
appologize to all the other members of PHYS-L that will not have the
opportunity to hear my arguments. It is just too overwhelming for me
spend the time to defend David and my paper against someone who we will
never convince and who appears to have a vested interest in discrediting
us.
Scott
_____________________________________________________________________________
Scott Eberhardt
Associate Professor, Dept. of Aeronautics and Astronautics
BOX 352400, University of Washington, Seattle, WA 91895-2400
(206) 543-6508; FAX (206) 543-0217
scott@aa.washington.edu http://www.aa.washington.edu/faculty/eberhardt/eberhardt.html
_____________________________________________________________________________