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Author Topic: Calculating max turns/torque for indoor rubber sizes  (Read 1842 times)
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BG
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« on: August 26, 2008, 04:47:25 PM »

Ok indoor gurus...how do you do it? How do I get at the max turns for my 500 mmx 1mm x 4 strand pnut motor...I know it is over 2000 but how do I get the absolute max without blowing ten motors in testing....what do you guys do?? Is there a good rubber formula that I can use??

thanks in advance

Bernard
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thymekiller
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« Reply #1 on: August 27, 2008, 08:16:48 AM »

Theres a chart on this forum for outdoor rubber , if you want that as a guideline im sure I could find it. Dont know about indoor. Ill be watching this thread because I too would like to know....

thymekiller
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BG
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« Reply #2 on: August 27, 2008, 08:26:17 AM »

Yes I know about the out doorchart...it seems to work ok but it is for Tan II and most of us are using super sport these days. Also I am not sure how the numbers translate to smaller rubber sizes. I wish we had an equation where I could just test a few pieces of a given motor and inset the test results to get average breaking turns and torque.
b


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Wout Moerman
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« Reply #3 on: August 27, 2008, 09:59:01 AM »

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I know it is over 2000 but how do I get the absolute max without blowing ten motors in testing

You know the advice Duco Guru used to give (and probably still does)? Something like: keep on winding till it blows, then back of one turn. It seems like you already know when it blows. I think you just have to use that knowledge and don't bother about the formulas. Or even better, improve the formulas for the type of rubber you are using!

A few options in getting more turns before it blows:
- lubricate well
- inspect the rubber to see if there are visible imperfections
- Stretch well and wind very slow
- use the weight per meter and the total weight of the motor for the calculation instead of the metrical thickness.
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Hepcat
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« Reply #4 on: August 27, 2008, 01:10:35 PM »

Bernard
I developed a turns formula from first principles some years ago and it seems to work very well. I have attached a .jpeg which gives the equation. This was an appendix to a propeller article so some of the words will not be applicable but I think the essentials of the rubber equation are there.

You will see that the equation is formed from three terms; the first concerns the weight and length of the motor, the second the density of the rubber and the third the maximum stretch that can be attained. One approach would be to assess the density and stretch characteristics of your present rubber and apply them in the full equation. However I think the simpler approach is to take the third form of the equation where I have already combined the density and stretch into one constant which only leaves the length and weight of the motor to be dealt with. Make up a 2 strand motor, check the length and the weight very carefully and then wind it until it breaks. Then just use the test figures and solve the equation for the constant and then that constant will enable the maximum turns to be calculated for other motors made from that batch of rubber. Obviously good practice would be to break several test motors and get an average value for the constant.

[I am concerned as to how large the .jpeg will appear. If it is not clear I can send it to you as an email attachment, unless anyone has a better idea.]

John
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Re: Calculating max turns/torque for indoor rubber sizes
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Dave Andreski
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« Reply #5 on: August 27, 2008, 01:19:01 PM »

Hello John,
Your attachment is just fine.

Dave Andreski
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Hepcat
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« Reply #6 on: August 27, 2008, 01:23:17 PM »

I forgot to mention that the number of turns that Super Sport will take is very similar to the number that Tan2 would take. However there is a general feeling that Tan2 gave slightly better cruise torque figures. Difference between batches often seems greater than differences between Tan2 and SS.

John
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BG
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« Reply #7 on: August 29, 2008, 07:01:38 AM »

John,
thanks for the equation....it is just what i was looking for. Is there a similar one for calculating max torque?

Bernard
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