Ron, I couldn't agree more about the importance of the relationship between
centre of pressure and gravity of a kite. About 2 years ago when I built my
first stunter, I spent hours in the field adjusting the bridle to no avail.
I was under the belief that it didn't matter what the kite shape was, the
bridle would compensate for it. Well it just doesn't work that way and alot
of the kites I hear on the 'net' that have problems seem to exhibit bad shape.

Not knowing where to start, I looked for things that I could measure and see
if there were relationships between them. The only two things I found easy to
measure were the centre of pressure and the centre of gravity.

The way I find the centre of gravity is similar to yours Ron, it's just that
I leave the sail on and balance the spine on my finger. I mark two points, one
being the position where the frame is horizontal and the other where the
chamber is horizontal.

Now that leaves the centre of pressure. They talk about this force alot in
books, but never give a clue on how to find it. The best method I have found
is to take the stunt kite to a park when there is a nice breeze blowing and
stand behind the assembled kite as if to launch it. Now try and balance the
kite with one finger on the spine. Once this point is found it doesn't matter
what the wind speed is, it will always remain balanced.

I then built a kite where the centre of pressure was between the two centres
of gravity. This turned out to be my first successful stunter. The bridling
took a matter of minutes to get right. With this good balance the kite behaves
very solidly in the sky. When the wind drops off or you give it a hard tug the
kite still keeps its same attitude in the sky.

Of course this is just the tip of the ice berg. Other things like the chamber
depth, aspect ratio, positioning of standoffs all play vital roles.

Grant McCauley
Australian Centre for Unisys Software (ACUS)
Internet: grantm@syacus.acus.oz.au
UUCP: uunet!munnari!syacus.acus.oz.au!grantm

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