Turns and torque

[bjt4888]

Attached is the rubber turns and torque formula that I recommend to my students. Also, a sample calculation is written out. The sample is for a 12" x .087" motor. The 2 in the denominator under the square root is the number of strands of rubber in the motor and the .04 under the other square root is the "thickness" of typical tan ss rubber. This formula is from a book that I believe was written by John Barker.

Of course, the sample may suit a particular airplane with a particular propeller and propeller pitch and you will have to do lots of testing and alteration of variables to discover the optimal number of turns and corresponding torque and backoff turns for your particular airplane and for your flying site.

I am posting this in response to a private message sent to me through this wiki. I thought this might be helpful to others.

Brian T.

[DoctaDave]

I have found that with certain batches of TSS the motor can be wound to a much higher torque than spit out by the formula. This should be used as a starting point and you should always do some tests of your own to make sure you aren't missing out on a couple hundred turns.

[bjt4888]

Doctadave,

You are correct that breaking torque on some batches (or some sections of batches) of TSS is much higher than this formula predicts as this formula was developed for Tan II. We use the formula as a starting point and also wind some practice loops to destruction. We have found that it is not necessary to wind close to max before backing off to get maximum duration and this result has been corroborated by other national champs level competitors. Although breaking torque for TSS in this thickness range may be as high as 1.4 in oz or even 1.7 in oz (and occasionally 1.9 in oz by our testing), my students were getting the best duration by winding to 1.0 or 1.1 in oz (approximately 80% to 83% of max predicted by the formula) before backing off and launching. Our data indicated that it seemed to hurt the mid-range of the torque curve to wind to 90% or 95% before backing off.

Good points,

Brian T.

[DoctaDave]

That's something that I have never thought of as I always just tried to maximize the amount of turns I put into the motor, but what you're saying does make some sense. I think once I have some time, I will make a bunch of motors and see what the unwind torque curve is starting from 95% and 80% and see if that can help shed some light into the topic. I'm also interested in seeing what the difference in torque curves are for motors that have been used multiple times. F1D motors on their 4-5th wind could be consistently wound to over 110% of the predicted breaking turns, which would increase times by 10%. I would like to see if the same could be done on the thicker TSS motors used in Wright Stuff as well.

Of course if you have found more success only winding to 80% then it would be best to stick to what works. The next time I go flying I'll definitely try to see if I can get better times by winding to only 80%

[bjt4888]

Doctadave,

Our tests were done on a small number of samples and were not exhaustive. I'm interested in learning what results you get. My students best duration results were on winds of 11/2013 tan ss of .0545 g/in (approximately .083" thick, 17.25" motor at 1.91 grams of rubber and .08 grams for black rubber o-rings) were about 1,900 turns and 1.0 in oz. The formula would predict breaking turns of 2,400 for this motor. All of my student's flying was done at sites with ceilings between 21' and 30', and the usual backoff was to about .45 in oz launch torque. The flaring prop that I pictured in last year's wiki was used on a slightly modified Freedom Flight kit.

Thanks,
Brian T.