Derek Rowell wrote in message
news:E4o9c.108024$1p.1536914@attbi_s54...

Repeat the experiment with the "rudder" on the inlet side of
the fan
(transmission in reverse). Is there a turning effect (torque)
or not? Is
there a sideways thrust on the "rudder"?
You tell me - I just did it. The answers to all four questions
is yes.

Yes, in reverse there is a torque on the rudder. But (if I read
your hinges correctly) it may be caused by a solely fore and aft
force on the rudder. Either would certainly explain the rudder
kick I have experienced.

What we're actually looking for is a net force at right angles to
the centreline of the boat, so the proper hinge for this
experiment would be parallel to the boat centreline, above the
flow. We'd then look to see if the paper was still deflected.

Just nipping off to play with some bits of wire and card . . .

JimB