Date:	Sun, 14 Aug 1994 20:26:14 -1000
From:	tanaka_be@swam1.enet.dec.com (Bert Tanaka)
Message-Id: <9408150623.AA19316@mts-gw.pa.dec.com>
Organization: Division of Applied Sciences, Harvard University
Subject: Sail stretch on a spinoff

*From:   SWAM1::US3RMC::"lord@eskimo.com" "David Lord" 13-AUG-1994 03:08:18.17
*Subj:   Re: Sail stretch on spin-off

*                    (stuff deleted)
*                 ... If the sail billow changes due to stretch then it
*can be redeemed by adding a spacer to each lower spreader and the upper
*spreader. You should strive for the same close up angle as the original.
*If you get the billow in the ball park and readjust the bridle because
*of the aspect ratio change the kit should fly pretty close to the
*original, maybe better. I have always preffered higher aspect ratioi
*designs. In commercial designs extra cutting of spars or spar waste runs
*the cost of the kite up substantially at the retail level.
*                                               Dave Lord

In an earlier note, I suggested, a method of sewing to diminish the
size of the stretch in the lower part of the sail, or, to buy a sail
made out of Icarex, because this fabric stretches less than regular
ripstop.

While I agree with Dave that adding a spacer to each lower spreader will
decrease billow, my own experience with this change is that it will make
the kite's performance quite unlike the original's.  I experimented
with a Hawaiian Team Kite, and a North Shore Radical.  I used a temporary
tee that I could vary the additional length added to the lower spreaders.
In both cases, the kites experienced degraded performance, mostly in loss
of tracking, unless you consider oversteer an improvement, which in the
case of these two particular models, I don't.  And, more zippy acceleration,
which can be good, but probably not for these kites, either.

One thing to consider is that the sail stretch is not uniform.  It appeared 
that the portion of the sail that stretched the most, was the trailing edge.  
The second largest area was in the lower center of the sail on each side - 
the pockets.  The sail does not stretch along the leading edge.  The stretch 
in the upper part of the sail, above the upper spreader, is relatively small. 
I pulled the skin off a new NSR and an older one (mine) and compared them by 
laying one on top of the other.  Crude, but at least I got a rough idea of
where the sail was distorted.

I ran into some problems trying to increase the size of the lower spreaders.  
Increasing their length makes them  not line up straight across.  In further 
experimentation, I cut my sail to allow the tee, and the spreader/edge 
connection to relocate to try and preserve alignment of the lower
spreaders, and to try and compensate for changing the angle that the 
whiskers will make with the sail with all these other alterations.  Using 
a longer top spreader makes the sail tighter where its not stretched.  Worst 
of all, the kites flew poorly with these changes.  I think that the large 
billow is sort of like a big buffer that in the case of the Hawaiian, makes 
the kite very even speeded.  The Hawaiian can actually fly pretty fast in 
strong wind, but it changes speed slowly, and never flies as fast as it would 
if it had a flatter sail. Increasing the size of the spreaders made it more 
twitchy, but not in a way that was desirable.  I was flying team with this 
kite at the time and so the speed and tracking changes were unpleasantly 
noticeable.  With the NSR, I attempted to fly pairs with an old NSR and 
a new NSR when the same situation occurred.  I was especially fond of
the NSR - I learned my basic groundwork with this kite, and was mostly
looking for a way of extending the life of what was then, a very expensive
sail.  Perhaps some other kites would be affected less with these changes.

I would be extremely interested in a method of restoring stretched sails,
because I've got a couple of older kites that it would be great to put new 
life into.

	      -Bert


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