Calculating Torque

[Anonymous]

Hey guys I’ve got a question about rubber motors I bet someone in here can answer. I am wondering if one has, through testing, several data points about a particular batch of rubber, if other data points can be extrapolated. Specifically, if one is testing a particular batch of ¼” rubber and finds that a 2 stranded motor breaks at X torque, a 6 stranded motor at Y torque and a 12 strand at Z torque can 8 and 16 strands be extrapolated from this data? I’m thinking it can, but I don’t know how.

In William McCombs booklet, “Making Model Airplanes Fly,” section 8.3 and 8.4 he suggest superimposing known data on a predetermined curve, which he supplies, if I understand his presentation correctly. BTW as a newcomer to the sport I have found Mr. McCombs book to be very helpful. My concern is that different batches of rubber may have different characteristics and the “one curve fits all approach” may be inaccurate. On the other hand I may be all wet.

Someone in here can answer this question and I would be most grateful.

Stardust

[Bill Shailor]

Stardust,
My experience is that it is batch dependant. Moreover, as the torque per winds ratio changes with variables of strand and length, the only way to test the maximums of rubber is the old fashioned method of winding to destruction. Other variables of temperature, humidity, storage and age of rubber also change the characteristics.

[Anonymous]

Thanks for the response Bill. The question I’m asking is if motors of (say) 2, 6 and 12 strands have been wound to destruction and the torque noted at failure, could motors of other numbers of strands be extrapolated from these three data points? I am assuming that if the amount of torque needed to break 2, 4, 6, 8, 10, 12, 14 and 16 stranded motors were plotted on a graph one would get a curve ascending to the right. If one knows just three of these points, could the others be determined?

[JIM MOSELEY]

I would feel that any figures extrapolated would only refer to the torque readings on one specific meter rather than to be a general application.

For example it may be okay to talk of, say, 36 inch/ounces at max turns that a model is trimmed to but that trim stage may well be at a different reading if an alternative meter is then used – we found that when I loaned my winder to a friend who had forgotten to pack his.

[Bill Shailor]

I am sure that you can, with the same batches, extrapolate between known and unknown strands. The problem is that with increased strands, the lengths shortens and the amount of friction caused by a greater number of strands increases. I think the curve would generally look the same, though, between 2, 4, 6 etc. strands.

[Author]

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