Re: refutation of the scoop theory

[John Denker <jsd@MONMOUTH.COM>]

At 07:52 AM 8/21/99 -0500, David_Anderson wrote:
> Mr. Denker has been attacking our work for over a year.

Dr. Anderson and I have been exchanging private email since 23 December
1997.  I pointed out to Dr. Anderson and to Prof. Eberhardt many
misconceptions, and on various occasions they thanked me for doing so.
This started over a year *before* they submitted their article to _Sport
Aviation_.

>  Other than his
> web page renouncing our article in Sport Aviation
> (www.aa.washington.edu/courses/aa101/lift.htm) (which was not pointed
> out to us by Mr. Denker) his attacks during the last year have been behind
> our backs.

I communicated the details of my critique to Prof. Eberhardt, in various
emails in January and February 1999.  The last traffic was four emails I
sent on 2 February 1999, to which there was no response.

They didn't need my permission to put up their web page, and I don't need
their permission to put up my critique,
  http://www.monmouth.com/~jsd/fly/lift.htm

> We will not argue fine points.  The bases of the disagreement can be
> pretty much boiled down to a single question: Does lift require work?  If
> we understand Mr. Denker's web site the wing rides its own upwash and thus
> does not require work.

It is an observable fact that long-winged aircraft achieve remarkably high
lift-to-drag ratios.

It would be an oversimplification to say lift requires *no* work.

> (He tries to explain this in his rebuttal to our
> article with a very confusing discussion of bouncing balls between a table
> an a floor.)  The simplest rebuttal to his work-free lift idea is that a
> wing traveling at Mach 1 or faster has no upwash.

If I were trying to achieve a high lift-to-drag ratio, I wouldn't be flying
at supersonic speeds.  I would be flying a high-performance glider at a
rather low speed.  I believe I have made careful and useful statements
about typical general-aviation wings in normal subsonic flight.  A
discussion of supersonic flight is not even tangentially relevant.

> We know where the concept comes from. We discussed it in our article.  The
> efficiency for producing lift with a wing is proportional to the length of
> the wing.  A longer wing diverts more air and thus requires a lower
> velocity of that air to produce the same lift.  (Less kinetic energy for
> the same momentum transfer) That is why high-performance gliders have such
> long wings. In aeronautics thy work with 2-D airfoils which are really
> wings of infinite length.  Thus, they are infinitely efficient and develop
> lift without downwash or expended energy.  But we are discussing finite
> wings on planes.

That's fine.

> If the wing accelerates air down to develop lift, and that air keeps
> going, then lift requires work.

If one accounts for all the momentum and all the energy, that's true.

> Before the wing came by the air was
> standing still and afterward it has some kinetic energy.

Depending on the niceties of one defines "before" and "standing still",
that statement may or may not cut to the heart of the central fallacy in
the Anderson / Eberhardt paper.

> Now we all know
> that a propeller develops thrust by blowing air back, and that a
> helicopter develops lift by blowing air down.

No, we don't know any such thing.  There's actually quite a bit more
involved, especially in the case of the helicopter.  As I have said in this
forum more times than I care to count, a simple "throw something down"
model is too simple to give a passable description of induced drag.

> If Mr. Denker were right and lift did not require work, either would
> propulsion, which is the same physics.

Again I say that is a bit of an oversimplification.  I doubt I was ever so
incautious as to say lift does not require any work.  I wish people would
not put words into my mouth.

> It is easy to see that we have a fundamental disagreement with Mr. Denker.
> Until we resolve the question of power and lift it would only be a
> distraction to get involved with any of his lesser complaints about our
> article.

The scoop model makes quantitatively wrong predictions about the effect of
span and chord on induced drag, as I wrote to Prof. Eberhardt at 06:09 PM
1/7/99 -0500.  He wrote back at 01:08 PM 1/8/99 -0800.  He disparaged my
analysis by pointing out a 15% correction term that I had omitted for
simplicity.  I reiterated on 08 Jan 1999 19:06:51 -0500 that the scoop
model was in error by something like 600%, with or without the 15%
correction terms.

I have several times posted various versions of the key argument to this
forum:  Consider two airplanes with reasonably long wings, initially flying
in formation, wingtip to wingtip.  Then they move apart, maintaining
constant airspeed at all times.  When they move apart, there is an increase
in the induced drag force.  This is fundamentally a nonlinear effect.  It
is not a small nonlinearity;  there is (to a good approximation) a 100%
increase in induced drag.

The Anderson / Eberhardt scoop theory does not appear capable of explaning
this.  Indeed any theory that involves only a simple "throw something down"
process is too simple to provide a passable description of induced drag.