Mixed motors

[billdennis747]

I am aware that some people will use two different rubber widths to make up an 'odd' motor, eg one loop of 1/8 and one of 3/16. I've never done it because it strikes me that if they were separated, the thicker one would take many fewer turns. Does it matter?
Bill

[cvasec]

I use one loop 1/8" and one loop 3/16" in my P30. The number of winds is the same as five strands of 1/8".
Ron

[MKelly]

Bill, 

I use mixed motors in many of my models, haven't found it to be a problem. Most of my dime scale models fly on a loop of 3/32" plus a loop of 1/16", some of my larger models fly on mixed loops of 3/16" and 1/4". I regularly wind them to 80-85% of breaking torque without problems.  When they do break there is no consistent pattern of which size fails first.

Cheers,

Mike

[billdennis747]

Thanks Mike  and Ron - that'll do for me. The perfect answer is 1/16 sq but it's such a pain to organise!

[calgoddard]

In theory, the overall cross-section of the rubber motor should determine both the breaking turns and breaking torque, assuming that the loops of different width are all made from the same batch of rubber.

I have not found that mixing different widths of rubber in he same mult-strand rubber motor to be deleterious in terms of either reducing the breaking turns or reducing the breaking torque.

I have built and flown over a dozen P-30 models from high-tech models to my low tech Three Nite P-30 kitted by Volare Products. Attached is a motor sizing chart for P-30 rubber motors that others may find useful. Since the maximum weight of the lubed rubber motor is 10 grams per the P-30 rules, the flier need only determine the optimum cross-section for a given model. This of course depends on the prop, the weight of the model, the design of the airframe, etc. A 6 x 1/8-inch rubber motor will produce a rocket climb but a short motor run. A 4 x 1/8-inch rubber motor will produce a very slow climb and a long motor run, but can only be used with a P-30 at or near the 40 gram minimum and only under very calm air conditions.

I understand that the people that fly those $3,000 (US) F1B models at the highest level of FAI competition all fly with 24 x 1/16-inch rubber motors. The theory is that if one strand breaks, the other strands will hold the broken strand together enough so that the launch and cruise are not seriously compromized.

Indoors, with duration stick models like Limited Penny Plane (LPP), single loop motors are used exclusively. The rubber is stripped to very specific density in terms of grams per inch in order to maximize flight times. One of the greatest US indoor fliers of all time is Bill Gowen. I think he experimented with using more than two strands on indoor duration stick models and found no advantage. He still holds many of the indoor records. Sometimes Bill would use three strands because he was using up extremely valuable remnants of the very best batches of TAN II rubber. The last price I heard for May 99 TAN II rubber was $30 (US) per gram! But keep in mind that the maximum weight of a lubed rubber motor for an F1D model is only 0.4 grams.

I hope that people who read this post find it to be helpful.

[lincoln]

I had some rubber from a really good batch and I gave it away to a kid who was taking F1D seriously. It's probably a good idea that I didn't know what it was worth.

[Jmk89]

Another way to make an "odd" rubber motor is to use an odd number of strands of the same cross-section rubber.  This is very handy if you have made up your motor to the correct weight already.

I do this by putting a loop at each end of the single-strand strip (I just turn the end back on itself for about an inch, put a turn or two of darning wool around the two strand part and then cyano the loose end to the main strand).  I next put a front-end fitting (typically a Grey hook) through one loop and pin it to my building board.  Then I pin the rear-end fitting (some kind of bobbin, typically) to my building board at the correct length for the motor and lead the rubber back and forth between the fittings trying not to introduce any stretch or slack - with luck the second end loop will just reach the rear-end fitting and I finish the motor by putting that loop-p onto the rear-end fitting.  So if the same motor organised as 10 strands is too weak and 12 strands is too strong, I can make an 11 strand motor without having to use different rubber by just undoing the joining knot, putting a loop at each end of the single strand and re-stranding as described.