Re: REPLY TO ALL--Misconceptions:Airfoil

[William Beaty <billb@eskimo.com>]

On Thu, 12 Feb 1998, Richard W. Tarara wrote:

> I've been convinced enough by all our previous discussions on this to pull
> the airfoil from my instructional set (although it's still in my Animated
> Chalkboard software--too lazy  to replace that one), but I still have
> questions.  Doesn't the air flow faster over the top of the wing than the
> bottom for certain airfoils (for whatever reason) and doesn't that produce a
> pressure difference--the essence of the Bernoulli approach.

Rick, I think my communication isn't clear enough.  "Bernoulli" still
applies, and when an airfoil provides lift, the flow above the airfoil is
always faster than the flow below it.  100% of lifting force can be
explained by pressure difference across the wing. 

The misconception comes in when we try to explain why the air above the
airfoil flows faster.  The usual explanation (path length differences) is
wrong.

Since 100% of lift can also be explained by deflection of air (by *BOTH*
surfaces) of an airfoil, then perhaps Bernoulli should take center stage
when explaining flight. 



> Yes other
> things are going on, but with such a shape is NONE of the lift attributable
> to a pressure difference?

Pressure difference explains 100% of lift.  Deflection of moving mass also
explains 100% of lift.  Two viewpoints, one complex physical phenomenon.

>  What about low speed airfoils--I'm especially
> thinking about para-foils and human powered craft such as the Gossamer
> Albatross--are those explainable totally in Newtonian versus Bernoulli(an)
> terms?

I think low-speed airfoils demonstrate the deflection of air more clearly
than high-speed airfoils.  Low speed airfoils have large camber, and their
trailing edges are at an obvious angle with respect to direction of
flight.  At high speed, sufficient lift arises even from tiny camber or
tiny attack angle. 

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