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Old Dec 13, 2012, 01:12 PM
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The 'Wack, BC, Canada
Joined Oct 2002
12,808 Posts
There used to be charts for the turns per inch for a few different cross sections. But so much depends on the qualities of each batch of rubber that these days those charts mean little. Also to some extent the number of turns you can get into a motor depends on how slack it is between the hooks.

So there's really nothing more to do than to make up a sacrificial test motor and wind it correctly and go until you feel it tighten up with the strong rise it creates just before it snaps or to wind through that point until it breaks and then figure out your own 80% max turns for a safe winding for the models.

At 80% of max you can wind and fly all day long with very little risk. But you do want to inspect for edge tears every couple of flights and especially at the end of the day. Small nicks and "hairs" of rubber at not a big deal unless they extend across the entire edge surface to connect the two faces. When you see that sort of nick in the edge then toss the motor.

As for stripping the rubber down you can try the pull through a single blade styles. But in the end you'll likely find, like I did, that the Harlan style strip cutter is the way to go. I always got variable results with the fixed blade pull through styles to where I had only questionable control over the final width. 5 to 10 thou of variation over a few inches was not at all uncommon with the pull through styles I tried to make. And that simply wasn't good enough for the EZB's and microfilm models I was flying back then. For penny planes and indoor scale it's likely fine but I soon gave up trying to strip my own and simply used to buy the envelopes of pre-stripped rubber from the indoor specialty places back then.
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