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Old Aug 01, 2014, 02:52 PM
Pedro Salomón
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Deciding rubber motor size

Hey Guys i would like to learn how to calculate the proper size for a rubber motor. For example i have a pacific ace of 30" wingspan that weights 45 grams. What is criteria to define the rubber either lenght and width?.

Thanks for advise!
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Old Aug 01, 2014, 10:16 PM
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I use enough rubber to try to get to about 25% or higher of the total weight as rubber. On my 30 inch Pacific Ace (25 years old) I use 1/4 inch FAI (that size is no longer available) that is about 12-14 feet long or about 24 grams of rubber. When tied in a loop and braided so that I have 4 strands it works well with a 10 inch prop.

If think in terms of the cross section of the rubber, mine is one inch. You could achieve similar results with twice as many strands of 1/8 inch rubber.

For your plane at 45 grams (lighter than my old plane) you could use about 10-12 grams of rubber as a starting point. Then later gradually increase the rubber content to 15+ grams. Hope this helps.
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Old Aug 01, 2014, 11:02 PM
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Gene Wallock created a helpful rubber motor chart for a variety of planes. Find a plane similar to yours and use his recommendation (I'd say something like a Phantom Fury)

Here's the link http://www.antiquemodeler.org/sam_ne..._list_2010.pdf

Norm
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Old Aug 02, 2014, 12:36 AM
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Gold Coast Australia.
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As a guide, Senator at 30in span built (32in on plan) uses 30 grams of rubber and a 12 or 13 in. prop.
1/4 rubber = 6 strands, 3/16th = 8 strands and 1/8th = 12 strands.
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Old Aug 02, 2014, 03:04 AM
Pedro Salomón
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Joined Apr 2005
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Hi guys thanksfor comments, will try with this recomendations to get best performance
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Old Aug 05, 2014, 06:55 AM
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United States, MD, Lutherville-Timonium
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Good starting points described! Trial and error works ! Remember the prop has a lot to do with the rubber size, it is basically a governor dictating the speed of the rubber to unwind as well as the thrust provider. Pitch and diameter of prop are your friends, get familiar with them ! I've built a lot of planes using a combination of an 8" prop with 2 loops of 1/8" rubber, and if you can center the motor weights on the cg you can almost pick your rubber length to any duration !
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Old Aug 05, 2014, 07:59 AM
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Kevin

Good to see your name here. Been a long time since I have seen you.

Pat
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Old Aug 05, 2014, 11:16 AM
B for Bruce
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The 'Wack, BC, Canada
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There's really no rhyme or reason to choosing the motor.

In terms of weight we've got Coupe d'Hiver models with 10gms of rubber in a 70gms model. So that's a mere 14% of model weight for the rubber. But the old unlimited class models and the early morning "Dawn Patrol" unlimited models run a 100% motor in lots of cases. But happiness is somewhere in between for most of us.

For myself I tend to go with anywhere from 20% to 30% of the model weight for the motor weight. I then arrange the loops of rubber so that the motor is anywhere from 1.25 to 1.4 longer than the hook to hook distance of the model so there's some slack but not too much.

For all but the very smallest one or two loop motor models I always make up the prop block so it has a tension stop. That is the prop shaft is spring loaded so as the motor runs down and the tension is coming out of the rubber the prop shaft moves forward just a little and catches on a stop pin so the last few turns hold the motor slightly taught instead of laying in the belly and bunching up in the nose or tail.

Others have had good luck with braiding the rubber to avoid this bunching. I've never been able to make it work. Or I'm missing out on some factor since the braided motors become LONGER for me . I also clean off the rubber after a couple of days of flying and with plain loops I find it easier to wash than with a braided length.
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Old Aug 05, 2014, 11:18 AM
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Braided motors should be shorter in length, not longer. Maybe you are winding the wrong way.
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Old Aug 05, 2014, 04:12 PM
B for Bruce
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Like I said Pat, clearly I was missing SOMETHING....

Not a big deal. Like I say, I like to wash out the rubber often enough that I'd rather find another way to keep it tight between the peg and hook. So I'd be constantly needing to unwind and re-wind the braiding. It's just easier for me to leave it all straight.
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Old Aug 12, 2014, 08:37 PM
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I use rubber lube that is good for the rubber and doesn't really need to be washed off, though I suppose if one had water soluble lube it could be removed by a bit of soaking, even from a braided motor. I don't usually braid, as I'm too lazy, but it's worked for me before.

The way I braid, at least for motors that are only one loop, is to wind before making the loop. I use a Magic Marker or something similar to mark the center of the strip, then I put in maybe a couple of turns per inch. (Very much a TLAR procedure. ) Then I grab the middle with one hand, and the ends with the other two hands. ;-) I tie the two ends together. I have a knot that works only when lubed, so everything is lubed before I put the winds in. Once the knot is in, I hold the motor by the knot and whirl it around a bit, then massage it until it looks like a proper motor, and it's ready to use. Best if the front hook is right at that center mark, and of course the knot at the very back.

---------------

For an outdoor scale model, an upper limit on motor width is the amount of torque that the model can withstand in flight. This is variable as a heavier motor means more airspeed and therefore an ability to withstand more torque. Perhaps for a mass launch model you could use a slightly wider motor at first and back off just a tad in the early rounds, or perhaps just not wind quite as hard. In the later rounds, the extra width will keep the model climbing. I find a rubber stripper very handy for this, at least on the relatively small (24" or less) models I fly. But that's not cheap.
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