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RE: Raskin's Coanda effect article



On Tue, 28 Jul 1998, Marlyn Jakub wrote:

Bill Beaty wrote:
The Airfoil-Lifting-Force article from QUANTUM magazine is now online on
Jeff Raskin's website:

http://freebie.cfcl.com/jef/


Bill,
On 13DEC95, I wrote a criticism of this article for Phys-L readers. Again, I
would strongly urge readers of Raskin's logic to add lots of salt before
believing any of Raskin's claims.

Unfortunately I missed the Quantum article at the time, and so didn't
grasp the points of your critique. Does anyone here know if the PHYS-L
archives from that era are still online?


In short, Raskin uses air jets which are smaller than the wing object. This
can be totally misleading. E.g. blow through in inverted small (5mm) funnel
and you can hold a ping-pong ball inside the funnel...but with a downward
blast of ping pong ball diameter, there's no lift for the ball to stay in
the larger funnel.

I agree that this can cause serious problems. But if his little models
were converted into proper airfoils and placed in a large-diameter air
flow, would they suddenly act very differently? Are the experiments in
his article simply crude, or are they actually misleading? Crudeness need
not mean "wrong", crudeness in pursuit of simplicity is understandable, as
long as the crudeness doesn't completely change the answers given by the
demonstration.


Raskin never mentions circulation because he cannot have much with flow
only over the top of the wing.

I'm confused. If the flow above the wing is large, and the flow below the
wing is small (zero), does this not indicate that circulation is large? I
thought that zero circulation only occured when above-flow equals
below-flow.

With circulation, flows over the wing are much larger than those under
the wing, so the 2% figure only shows that he needed circulation in his
model to get the better velocity estimates.

It appears to me that his point about 2% is NOT intended to find fault
with the "Bernoulli" viewpoint. Instead he is exposing the flaw in the
common airfoil misconception. The common misconception is this:

Lift is created only by the differing lengths of upper and lower wing
surface. Parcels of air which are divided by the leading edge of an
airfoil must rejoin at the trailing edge.

Clearly this is not the "Bernoulli" viewpoint, since it focusses on the
lengths of the wing surfaces rather than the lengths of streamlines.
Other authors have pointed out the same flaw: if the difference between
upper/lower wing surface is measured in actual aircraft, we find that it
is far too small to be useful in explaining the origin of the lifting
force.

The "common misconception" should be eliminated from our explanations of
wings, regardless of whether we take a Bernoulli or a Newton viewpoint.
The upper/lower air parcels do not recombine at the trailing edge of the
wing, and the length of the upper/lower wing surfaces are too similar in
size to explain the lifting force.


Raskin claims that the back of the airfoil is most important, but the
primary pressure lift profile is at the front third of the typical wing,
where Raskin's arguments would seem to lead to the opposite conclusions.

They seem to lead to opposite conclusions, but I'm convinced that they
actually do not. Suppose we have a cylinder moving broadside through the
air. If I add some odd feature to the moving cylinder, is it necessary
that all changes in pressure must appear only at the surface of that added
feature, and nowhere else? I would say no. And where a wing is
concerned, I would expect that large changes in the geometry of the
trailing 2/3 of a wing might have drastic effects on the pressure
difference across the front 1/3 of the wing.

An analogy: suppose I release an inflated rubber balloon so that it flys
around the room. Isn't the reaction force distributed all across the
surface of the balloon, even though the narrow jet of air is the cause of
that reaction force? In the same way, the linear air flow along the rear
2/3 of the wing would act as the "rocket exhaust", while the front 1/3 of
the wing would act as the "rocket engine bell" where the important forces
arise.

Plug up the exhaust port, and pressures elsewhere will vary radically.
Change the aileron setting of a wing, and the pressure difference at the
front 1/3 will be significantly altered.


I have no conformation on the claim about Einstein's wing design or on
Raskin's role in developing the Mac computer...perhaps Jobs or Wosniak can
help on the latter.

Is there a missing ":)" here? It sounds to me as if you suspect Raskin
of dishonesty. A keyword search of +macintosh +"jeff raskin" should do
wonders.


If Raskin's info on Einstein is wrong, the error must lie with his quoted
reference (which does sound like a hobbyist magazine):

Grosz, Peter M. "Herr Dr Prof Albert Who? Einstein the Aerodynamicist,
That's Who!" WWI Aero No. 118, Feb. 1988 pg. 42 ff


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