Brian Whatcott wrote in message
...
On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"
wrote:
....

Asking for explanations from experimental rigs is the royal
road to
progress. Congratulations!

Limitations of the experiment:
It didn't check for associated force changes at the fan
The scale of 'rudder' against fan size is way out
The wire had a little flexibility
Fag ends produced smoke which rose too fast
Reynolds numbers were wrong.

And, just in case you mis-understood, my hinges were pendulum
hinges which did not allow the 'rudder' to rotate around its
vertical axis (except in the 'rudder kick' experiment). They only
allowed pendulum movement laterally, or when re-oriented, fore
and aft (subject to wire flexibility).

If a hinge surface is hinged more than about 1/4 aft of its
present
leading edge it is unstable in the fluid flow. ('rudder kick')

Agreed, and not necessarily a proof that there's a net force at
right angles to the centreline of the boat (my earlier
assumption)

If a surface *is* hinged about 1/4 from the leading edge, it
can
still break into oscillations which are quickly destructive,
unless
the mass is balanced closer to the hinge line.

Good old flutter.

If a FLAT surface is inclined slightly ( 20 degrees) to the
fluid
flow, the flow over the 'upper' surface is faster and provides
lower
pressure than the flow over the lower surface. The streamlines
do not
follow the (flat) surface of the test article (of course!),
they kick
up in a smooth curve over the top. This applies to an airfoil
flown
upside down too. The streamlines look similar to the
streamlines
over a right way up foil, but less efficient and with lower
pressure
difference from top/bottom.

If the foil is asymmetric.

Agreed, though Jax seems to challenge the association of local
water speed and pressure. I'll suck him in a bit further on that
one.

It is not necessary for a lump of fluid dividing past the foil
to
join up again after it has passed..
When providing lift, the lump of fluid does not join up again,
in
fact.

We seem to agree on basic aerodynamics. I'm looking forward to
hearing more about modern advanced fluid dynamics from Jax in the
'lift over foils' thread. Perhaps you can act as moderator?

JimB