On Oct 16, 6:31 am, Thomas wrote:
On 9 Oct, 21:08, Le Chaud Lapin wrote:
You may want to check out my web pageshttp://www.physicsmyths.org.uk/bernoulli.htm
andhttp://www.physicsmyths.org.uk/drag.htm for a closer examination
of the physics behind the aerodynamic lift and drag.

The main point I am making there is that it is physically nonsense to
claim that changing merely the tangential velocity of the air stream
relative to the surface would in any way produce a resultant force (at
least for a non-viscous gas).

What one needs for a pressure change
(and thus a force) on the surface is a change in the numbers and/or
the velocity of the molecules hitting it, i.e. it is only the vertical
component of the velocity that is relevant here. Only this can produce
the lift for an airfoil, either because of the increased number of
collisions on the lower side or the decreased number of collisions on
the upper side (both situations lead to a lift).

I agree, but there are some that seem to think the contrary, as you
know, with the Coanda effect.
http://en.wikipedia.org/wiki/Coand%C4%83_effect

What is troubling about many of these theories is that, at the precise
moment where the reader is most alert in anticipation of the meat of
the explanation, the hand-waving begins. In the link above, the clause
entitled Causes, it is written:

"The effect of a spoon apparently attracting a flow of water is caused
by this effect as well, since the flow of water entrains gases to flow
down along the stream, and these gases are then pulled, along with the
flow of water, in towards the spoon, as a result of the pressure
differential. "

Hmmm...."and these gases are then pulled"...

pulled? By what?

And it should be
obvious that for this to be the case, one must either have the lower
side of the wing facing to a certain degree into the airstream, and/or
the upper side facing to a certain degree opposite to the airstream.
This is why one either needs a certain 'angle of attack' or a
correspondingly shaped airfoil. And it should be obvious that in order
to have an asymmetric force (i.e. a higher upward than downward force)
one needs the surfaces of the airfoil to be orientated in some way
asymmetrical relatively to the airstream. So a perfectly symmetrical
airfoil (front to back) at a zero angle of attack (like I indicated in
Fig.1 on my pagehttp://www.physicsmyths.org.uk/bernoulli.htm) should
not produce any lift as the upward force (from the rear part) is
exactly equal to the downward force (from the front part). All that
would happen is that the wing experiences an anti-clockwise torque.
This is the reason why the rear part of the wing (behind the apex)
must always have a larger surface than the front part. At least I have
yet to see an airfoil where this is not the case and where it can be
used at a zero angle of attack.
(the Bernoulli principle is in direct contradiction to this as it
would also predict a lift for a perfectly symmetric airfoil in this
sense).

I just read both your web pages.

BTW, your explanation of d'Alembert's Paradox and the blow-over-paper-
attached-to-table experiment could both use diagrams. I am trying the
blow over the paper experiment now and I am not sure if I am doing it
as you described. Could you provide a more vivid description so I can
make sure?

-Le Chaud Lapin-