On Oct 5, 1:11 am, Jim Logajan wrote:
Le Chaud Lapin wrote:
I think for the purposes of piloting that one does not need to know the
Navier-Stokes equations.

That's true.

Because quite honestly the real explanation of
lift is to be found in the differential equations (or integral form)
governing mass continuity, momentum, and energy.

Yes, I agree.

All those simple
explanations are just that - simple and obviously incomplete. Rest assured
you're correct in your observations on the inadequate explanations - but
don't make the mistake of assuming a simple _and_ accurate one must exist,
if only you think hard enough on it.

I never thought that my explanation was complete or accurate. I
offered it because, IMO, it has a sificant influence on airfoil
dynamics. But after reading several books that ignore the partial
vacuum, and one book that was wrongly employing Newton's thereom...
[which is intolerable on any grounds, let alone aerodynamics], I had
to read more.

In his book, "The Proficient Pilot", on page 8, Barry Schiff writes:

"There is, for example, this amusing fable: 'Air flowing above the
wing has a greater distance to travel (because of camber) than air
flowing beneath the wing. Therefore, air above the wing must travel
faster so as to arrive at the wing's trailing edge at the same time as
air flowing underneath. This is pure nonsense."

In his book Learning to Fly With Rod Machado, on page 70, Rod Machado
writes:

"Because air flowing over the wing bends, it is forced to travel a
greater distance than the air flowing underneath. Because it travels a
greater distance, the bent air must move faster on its journey over
the wing. It's this relative increase in wind speed above the wing
that lowers pressure and productes lift."

Since I was new to piloting, I asked a few pilots if they understood
aerodynamics about an airfoil, and they all said yes. The reason they
gave is the one that Rod Machado gave. I asked the older pilots if
they knew who Barry Schiff was, and they said yes. I asked them if
they realized that Barry Schiff did not think that was the reason, and
they said I was mistaken, that Barry Schiff would never be as confused
as I seemed to be about flying. I asked them if they knew who Rod
Machado was. They said yes. I asked them if they thought that Rod
Machado and Barry Schiff would agree. They said yes. I asked them if
they were 100% certain, and they said yes.

Thus began my earnest interest in aerodynamics. I had always been
interested in flying, but this accelerated the interest quite a bit.

I took a sheet of paper and blew over it, the trick that we've all
done, and it went up of course, but I suspected that the reason it
went up was not the reason that was being given by so many people.

I visited a few aero/astro departments online, and while no one is
going to argue with the mathematics of field theory...I did get the
feeling that there were experts in the field saying the opposite
things. So before digging into the math [afterall, there is nothing
wrong with math], I decided to refresh my understanding of Bernoulli's
theorem from my old physics book, and while reading that book, it
occured to me that Barry Schiff's view is more likely right than
not...that is, until I read his explanation of relationship between
Newton and downwash on the next 2 pages of his book. I found a couple
of other books that gave the explanation of downwash that was
similar. Most importantly, I also notice that there was a ***HUGE***
amount of hand-waving going on, far more than one would expect in a
field that has been researched for over a century. I still need to
find a book that I can trust.

I went to the WWW and started reading more aero-astro excerpts, and
concluded that not only is theory still in flux, the experts do not
even agree on the basics. The very basics. Huge amounts of money had
been spent on wind-tunnels. But after all that, I could not get two
experts to agree on the basics. And this was a not simply a matter of
different styles, using integral instead of differential form of
equations, for example, or, deciding where to put a constant, as we
electrical engineers do in our expressions of the Fourier
integrals...there was *fundamental* disgreement about what causes lift
on an airplane.

I asked one of the pilot's again..."How sure are you that the aviation
world understands the basics?" He said he was very sure. He started
rattling off things about NASA.

I begin to imagine airflow over a wing and concluded that pinching at
front of wing must be very important, more important than one would
think, reading the explanations. I also concluded that a glider
should have wings that are very long but with a short cord, which aslo
turned out to be true. I then revisited my physics book...and it
_appeared_ that the application of Bernoulli to flying is wrong in
many contexts, but I decided to not discuss that just yet because it
would be too controversial.

I ask one of the pilots if he thought rarefication had anything to do
with it. He said, "No, it's all Bernoulli." I asked him what would
happen if I did the paper experiment, the one mentioned in my OP, and
he did not know, but said it does not matter because a piece of paper
is not a wing and it does not fly through the air. I asked him if he
understood why the paper would move, and he said, again, it does not
matter. I asked him if he saw any relation between my paper experiment
and the movement of a airfoil, and he said, finally, no, there is
none, because it is all Bernoulli.

I asked a CFI one last time, and he too said it was all Bernoulli,
precisely the argument that Barry Schiff refutes in his book.

So I started imagining, with no mathematics, what goes on with fluids
around surfaces, which lead me to these various experiments.

If you are interested in some books on the subject I'd be happy to make
some suggestions.

Sure.

-Le Chaud Lapin-