Re: Physics of Flight (and Anderson/Eberhardt)

[Rick Tarara <rbtarara@SPRYNET.COM>]

First:  Let me suggest that much of this debate between
Beaty/Denker/Anderson (BDA) needs to be taken off the list and back into
private.  The problem for list readers is that many of the references are to
previous private communications and to year old arguments which may well be
fresh in the memory of the debaters but are lost on the rest of us.  Also,
the tone of the arguments has turned quite nasty at times and therefore have
no place on this list.  I don't want to shut off the whole discussion, just
keep it relevant for the whole list.

OTOH:  While BDA discussions rage on, I think many of us who have at best 15
minutes in an introductory level physics course with which to discuss flight
and airplanes are still at a loss.  Below William characterizes the two
poles of the debate.

----- Original Message -----
From: William Beaty <billb@ESKIMO.COM>
> Bernoulli-ist position:
>      Airplanes fly because of pressure differences, and they
>      need not deflect any air downwards.
>
> Newton-ist position:
>      Airplanes fly entirely because they deflect air downwards, and the
>      pressure-differences are a consequence of the changing
>      momentum of the air.


 From the 15 minute/intro class point of view let me offer/ask:

1)  Does air really move faster over the top of the wing and is there (as
Bernoulli suggests) a pressure difference?
    a)  If yes, then WHY?
        i)  The (apparently discredited) path length difference AND
continuity argument (if the air
            doesn't move faster it would pile up on top of the wing--akin to
why there are river rapids)?
        ii)  The low pressure above the wing 'sucks' air into this
region--but how does the low pressure
             form initially--from air deflected away from the top surface by
the leading edge and/or the
             angle of attack??
        iii)  Other?
    b)  If yes then why isn't there any discussion of this portion of the
lift.  If there is a pressure
         difference (lower on top) then looking at the vertical components
of the RANDOM motion
         of the air molecules there would be more collisions with the bottom
of the wing (each
         molecular collision imparting some momentum to the wing) than the
top?
         i)  How big is this effect (this is what is generally calculated in
texts, but the velocity differences
             given may not be realistic)?
         ii)  What percentage of the lift can be attributed to this
effect--perhaps a lot for some craft
              and almost nil for others?
2)  Anyone who has flown commercially knows that in flight the nose of the
plane is up thus making
     it easy to buy into the SIMPLE 'angle of attack' view that the bottom
of the wing deflects air
     downwards and from N3 the wing must therefore be deflected upwards.
I'm not clear if the Newtonists
     are using this effect or go along with the Decker view that the
important downflow of air happens
     behind the trailing edge of the wing.  However, it is the direct
'bouncing' of the air off the bottom of the
     wing that (I think) students can most easily see.  How important?

While I have personally just dropped the topic from my syllabus
(unfortunately FLUIDS in general seem to be a casualty of the 'less is more'
movement), when I did include the topic I included both effects.   I haven't
had students do calculations for many years (and then they were Bernoulli
back-of-chapter calcs) so I have no sense of the proper mix of pressure
versus delta-p, and take it from the BDA discussions that this _simple_ view
is fundamentally flawed anyway.

So if there is an ultimate question in all this, it is:  What (if any) is
the 10-15 minute discussion that can adequately convey the essence of flight
to introductory physics students (even those in purely conceptual level
courses)?


Rick

*****************************************************
Richard W. Tarara
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

FREE PHYSICS INSTRUCTIONAL SOFTWARE AVAILABLE
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