I wish to apologize for hastily posting yesterday without adequately checking my work. In computing the volume of rubber I used the 32" motor length instead of total rubber length of 65.5". For thickness and width, I used a micrometer, holding the rubber up to the jaws and being and juuuust not letting any light through (reproduceable). I figure there's no point in measuring new rubber, since I want to know how many turns I can put into rubber that is fully broken in. Therefore, I used a spent motor for the dimensions as well as for the breakage test.
So, the formula I used is:
/w . √d . (x-1)√x/√2π
And the numbers I have are:
L is motor length=32"
w is weight=2.75g
d is density=weight of rubber /volume of rubber=2.75g / 65.5"x0.060"x0.040"= 17.49g/in3
x is breaking stretch ratio = 9.5
Which gives me N=4800, 90% of which is 4300. That's amazingly reflective of my empiric data and reassures me that we have decent rubber and technique, pending opinion(s) of the forum. The 4300 is initial and would need to be wound back to 4100 to get 0.3 in.oz launch torque. That doesn't seem like much unwinding and makes me cautiously optimistic that our rubber, plane, and prop are pretty well matched.
Since we've been launching at maximum 3750 turns, it looks like we can add another 350 turns. At 2400 rpm we can hope for another 8 seconds, from 1'27" to 1'35", which frankly doesn't seem like much of an improvement