Re: Explanation of airfoils

[William Beaty <billb@eskimo.com>]

On Wed, 11 Feb 1998, Chuck Britton wrote:

> > The flier text reads:
> >
> >   The Bernoulli explanation of flight that we were taught
> >   is at best incomplete and very often misleading.  A wing
>                ^^^^^^^^^^          ^^^^^ ^^^^^^^^^^
> But I notice that the blurb doesn't actually say WRONG!!!!!
> Supersonic wings are in fact symmetric, top and bottom. (or so my flyboy
> collegue tells me)

  Disclaimer: I'm no aerodynamics expert, just a "misconception collector"
  and interested bystander.

Whenever I get into this debate with people, they usually vehemently deny
that the "wing shape" explanation is wrong.  "Misleading and incomplete" 
is accepted, but "wrong" is not.  Our egos must be protected.  Too bad,
because textbooks and FAA pilot training materials would have been changed
long ago if enough people could bring themselves to state that the
material is flat out wrong. 

Where is the error?  These two assertions from the usual "wing shape"
airfoil explanation are wrong: 

  1. Upwash ahead of a wing is equal to downwash behind the wing, therefor
     wings do not impose an overall deflection on the air.  Superimpose 
     circulation onto constant flow, and there is no net deflection.
     Lifting forces come from pressure differences, not from reaction
     against accelerated mass.

  2. Parcels of air which divide at the leading edge of an airfoil must
     rejoin again at the trailing edge.

No. 1 violates conservation of momentum.  "Levitating" against gravity
requires either bouyancy forces or reaction forces against downward-
accelerated mass.  And if we use massless fluids in our Bernoulli-based
explanations, then we build in a violation of Newton's laws.

Smoke-pulse windtunnel photos easily disprove no. 2.  The air flowing
above a wing moves far faster than that below the wing, and its "parcels" 
overshoot the lower parcels by a large amount.  The high-speed air flowing
above a wing cannot be explained by appeals to differences in path length. 
But if statement no. 2 is wrong, the entire "wing-shape"  explanation
collapses, no?

(Recently someone told me that no. 2 also violates the "Kutta-Zhukovski
Theorem", and so even aerodynamic theory says no. 2 is wrong.  I'm not
familiar with the Kutta-Zhukovski Theorem.) 

> I can't beleive that for sub-sonic flight, the only reason for the 'air
> foil' shape to STILL be used is that of inertia from the Wright brother's
> original wind tunnel measurements.

But don't asymmetrical airfoil shapes prevent "detachment" of upper air
flow (stall)?  Even if the shape did not directly contribute to the
lifting force, suppression of the stall phenomenon is a sufficient reason
to search for particular airfoil shapes. 

A "Newtonist" reason for airfoil asymmetry: When a wing stalls, its upper
surface stops deflecting air downwards, and so lifting force decreases
dramatically.  The Coanda Effect demands careful shaping of the airfoil
upper surface to prevent triggering of stalls throughout a wide range of
airspeed and attack angles. 

If not for the stalling problem, airplane wings could look like tilted
flat plates.


Here's a separate issue for everyone to contemplate.  The following author
points out that because pilots misunderstand simple airplane physics,
accidents occur.  Now I'm waiting for it appear on the 6-o'clock news:
THE 'BERNOULLI' EXPLANATION KILLS PEOPLE!!! 

  Airfoils chapter...
  http://www.monmouth.com/~jsd/fly/how/htm/airfoils.html

  ...from SEE HOW IT FLYS, an online pilot training manual
  http://www.monmouth.com/~jsd



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William J. Beaty                                  SCIENCE HOBBYIST website
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