Re: Judgement on opposing airfoil views, pt. 1

[William Beaty <billb@ESKIMO.COM>]

On Thu, 26 Aug 1999, JACK L. URETSKY (C)1998; HEP DIVISION,              ARGONNE NATIONAL LAB ARGONNE, IL 60439 wrote:

> To which I add my puzzlement about all the heated and unheated verbiage
> generated by this topic.  I have looked at John Denker's web site and
> everything I see there comports with my own (very conventional)
> understanding of aerodynamics.

Hi Jack!

I'll have to go review John's website again.  I've been mulling over the
messages and his critique of Anderson/Eberhardt paper for so long
that I haven't been back to re-visit his "airfoils" chapter in a very long
time.  Might you be assuming that we are fighting about John's "airfoils"
chapter?  We are not, as far as I know.  It is quite possible that his
"airfoils" chapter is perfectly correct, and only his critique-page of the
Anderson/Eberhardt paper is flawed.  It's is his critique-page that is
causing all the fuss.  I assume that you've read it?  :

   http://www.monmouth.com/~jsd/fly/lift.htm


>         Anderson came and gave a colloquium at Argonne, and I was assured that
> he and Eberhardt would calculate lift on an airfoil in the time honored way.
>         All parties believe in Bernoulli, Newton's laws, and the need for
> cirulation to cancel an infinity in the inviscid approximation.
>         Where they have heatedly differed is in the verbal characterization
> of their results.
>         My conclusion - it's hard to translate mathematics into colloquial speech.


I see my disagreement with John as involving a very simple consequence of
Newton's laws.  Suppose a three-dimensional wing is in level flight at an
altitude much higher than the ground-effect regime.  It seems clear to me
that, over time, if the wing does not impose a net downwards
momentum-change upon the parcels of air which it passes, then the wing has
no mechanism for generating a lifting force.  When an aircraft is high
above the earth, it derives lift by accelerating the air downwards.


Correct me if I'm wrong, but John Denker disagrees with me.  Instead he
says that, over time, a 3D wing in high-altitude flight DOES NOT need to
impose a net downwards momentum-change upon the air-parcels it passes.


We can discuss the details, but I think that the above description reveals
the core of the problem.  It also forms the core of John's critique of the
Anderson/Eberhard paper.  Does the wing need to "launch" matter downwards
as it flys along?  I say yes.  John Denker says no.

I recall hearing some people in the distant past saying "well, it's not
that simple".  I disagree.  It *is* that simple: either a wing creates a
force-pair between itself and the earth or it does not.  Either it employs
conservation of momentum or it does not.  A wing doesn't stay up there
because of bouyancy, and it seems clear to me that it's only other option
is to employ action/reaction and be lifted as it creates a "net downwash."


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