On Thu, 1 Apr 2004 11:45:50 +0100, "JimB"
wrote:


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

So I hung the spatula just behind the fan. Lo and behold, the
same
thing happens but just a little less. When I rotate the
spatula to the
left, there is a noticable *left* motion to the blade... i.e.,
it's not
only drawn forward into the blade but it also moved to the left
from
where it was when the spatula blade was perpendicular to the
fan. When
I turn it to the right, the spatula swings to the right.

Steve, that was the experiment I first did. Then I realised that,
to yaw the boat, I had to look solely at lateral force. To do
this I had to constrain the card so that it could only hinge
laterally (no fore and aft motion permitted). This is where the
bits of wire came in. The card had a bit of wire attached rigidy
to the top, sticking at 45 deg horizontal angle to the card. The
card end of the wire bent down to stop the card swinging around
the wrong end of the wire. I hung the card (your spatula I
guess!) through two loops (hinges) first mounted parallel to the
centre line of the fan, then at right angles.

This gave a different result, very little lateral swing, lots of
fore and aft swing. Of course (a weakness in the experiment) it

Your experiment seems to be flawed if you're trying to look solely at
lateral force with no fore and aft motion permitted and yet you get a
lot of for and aft swing.

To prove to myself again that there is a lateral force even with no fore
and aft movement, I put a string around the bottom end of the spatula
which would allow it to swing laterally but hold it from being moved
toward the fan. So, we have a plastic spatula hung by the little
hanging hole at the top from a hook which allows it to swing in all
directions like a pendulum but I can firmly control the angle of the
blade by turning the hook. And there is a string looped around the
handle just above the blade which I can hold to prevent the blade from
moving towards the fan so there's no fore and aft motion.

Result: same thing. When it's behind the fan and you turn the blade so
that it's not perpendicular to the fan, the spatula swings *only*
laterally since there's a string keeping it from moving toward the fan.

My initial conclusion has only been reinforced.

Steve