Number of Turns and Geography + Rubber Storage

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    A F1B motor’s turns can be calculated using the Bror Eimar formula:
    Turns = 1.22 * (0.96/Weight[gr])^.5 * (ML/RL) * (RL[cm])^1.5

    My RL is the motor’s raw length, as measured between two vertical posts with no overlaps. (Convensionally stranded motors with overlaps are probably 3/8″ shorter.) ML is the motor’s length at 4800 PSI, after breaking in the motor. This approximates the standard 80 Lb pull test. ML/RL is the motor’s stretch ratio, dividing the length at about 80 Lb by its raw length. (The carrot ^ means raising to the power.) For example, a motor with raw length 13.5″ (which is 34.29 cm), with a stretch ratio of 9 weighing 29.5 grams will have
    turns = 1.22 * (.96/29.5)^.5 * 9 * 34.29^1.5 = 398 turns.

    The constants are: 0.96 is rubbers specific mass (water is 1.00), which is divided by the motor’s weight in grams. The number 1.22 is empirical, based on actual flying data. For example, my Aril ’07 BE average turns number was 427, and the average actual number was 438 turns, about 2.56% higher. Note that the motor’s stranding is not factored in.

    The BE turn formula does not incorporate a temperature correction. At outlying temperatures (below 50 F and above 80 F) one should back off a bit. This depends, of course, on the rubber’s vintage. The turns formula has been publish many times with minor variations (#). Most these formulas based on a motor’s raw length and its stretch ratio after break-in will give reasonable predictions.

    The table has BE turns for different raw motor lengths (with no overlaps) as columns and rows of stretch ratios based on rubber between April ’05 and June ’09:

    BE F1B motor turns for motors without overlaps
    XX 13.0 13.3 13.7 14.0 14.3 14.7 [inch]
    8.6 360 374 388 402 416 431
    8.7 364 378 392 407 421 436
    8.8 368 382 397 411 426 441
    8.9 372 387 401 416 431 446
    9.0 376 391 406 421 436 451
    9.1 381 395 410 425 441 456
    9.2 385 400 415 430 445 461

    Very short motors (13”) with low stretch rubber (8.6) can only be wound to about 360 turns, while long motors (14.7”) made from stretchy rubber (9.2) can be wound to about 461 turns. (Note that of one uses motors with overlaps that are 3/8″ shorter, it would also increase the corresponding stretch ratios.)

    So what is the optimal number of turns? Evidently, there are two schools of thought. Many fliers in the east seem to prefer about 400+-20 turns, to help the model blast through the abundant ground turbulence. (narrower rubber will have a smaller range.) In the west, most fliers seem to prefer around 450 +-20 turns. (Motor runs will depend on the prop’s pitch but generally increase with the number of turns.) A saying attributed to Alex Andriukov is that “A long cruise climb will outperform a spectacular launch”. In other words, a 450 turn motor run is preferred.

    This applies to those who regularly fly in western desert fields that lack vegetation and particularly trees and have no small scale turbulence. But when Western F1B fliers fly long motor runs in the east, say at Muncie, their models have more trouble working through the ground turbulence layer, as note by Bill Shalor.

    So the moral of this story might be: when you are flying in the east, app. 400+-20 turns are preferred; and when you are flying in the desert west, app. 450+-20 turns are preferred. (Including all the associated climb trim adjustments.) (Will anyone argue this point???)

    Well, this is might be a first approximation….

    (# A more compact example was posted by Carrol Allen on August 26 in “2009 Rubber Issues”.)


    Aram, one correction. I measured the density of the latest rubber by doing a float/sink test with an alcohol water mixture. I found that the rubber was neutral in 40 proof alcohol. Looking the density of 40 proof alcohol on google yielded an density of 0.93. The same as the “engineering handbook” gives for natural rubber.

    As an aside, The engineering handbook say that rubber is preserved in water. What do you people think of keeping our rubber in a glass or plastic jar immersed in water? Distilled or deionized water would probably be best.
    I was also considering ordering some of the oxygen getters that one sees in a lot of packaging. Keeping oxygen away from the rubber seems to be the best for long term storage, also low temps.

    (Everyone getting ready for the MM?)

    George Reinhart

    You make a good case for what what I think could be the ultimate storage system.
    Imagine if you will the following procedure for long term storage.

    Measure out your prferred weight of rubber.
    Wash using your preferred method and carefully dry.
    Make up the motor or not (your preference)
    Place individual motors in a latex (natural rubber) condom.
    Fill the condom witth enough distilled or deionized water to exclude oxygen bearing air. An alternate metod would be to displace ambient oxgen bearing air with dry nitrogen.
    Seal the condom using any preferred method.
    Store individually preserved motors in a dedicated deep freeze until required time of use.
    Carefully thaw the preserved motors (motorcicles) in advance of the required time of use.
    It would be a lot of trouble but rubber is expensive, and good rubber is worth taking proper care of.

    I hope this adds a small measure of knowledge to the art and science of that arcane branch of model aeronautics known today as “rubber flying”




    Congratulations to Aram for mastering the “rubber” topic and scoring first in f1B at the 2009 NATS!

    As far as storage, I am already getting flack for storing so much rubber in our second refrigerator. If I increased the volume for storage by requiring the freezer, it would be out with the rubber. I already have some old stuff in a garage closet that stays especially cool in our Minnesota winters, and somewhat cooler than outside temperatures in the summer.

    Maybe we can send all of our prize rubber to Pete for cyrogenic storage, kind of like Stan the Mans head?

    Can you imagine if the grandkids or some other family member were to open the frig door, and have a bunch of frozen condoms drop out.

    All of this kind of reminds me why I don’t fly rubber events seriously.


    I mixed up proof with percent in my note on measuring density. It should have said 40% alcohol. (I guess I shouldn’t have been sipping some of it at the time.)


    Way back in the 70’s when we were using Perilli I found a steel case with a rubber seal in a second hand store. I think it was used to ship some sort of communication gear. Anyway I stored rubber in it dry with a mason jar stuffed with wet steel wool. The thought was that the steel wool would oxidize and take up all the oxygen. I still have some of the perilli in it and the rubber is not broken down. It doesn’t break easely it has just lost it’s elasticity, only about 600% elongation compared to about 800% when it was new. It is useful for catapult but that’s about all.

    George Reinhart

    Carrol, Dave,
    In a more serious vein, I wonder if anyone really knows what are the critical factors in rubber ageing.
    We know that UV is sure death.
    Cooler temperatures seem to slow the ageing process just as heat above some optimum temperature seems seems to cause deteriorating performance.
    I was unconcerned and unaware that ambient oxygen could be a factor.
    Some types of rubber lube are said to cause long term damage.
    John Clapp told me that the 06/06 rubber initially tested as mediocre until it aged a couple of years in storage, and then was pretty good stuff.
    I am not a chemist but would like to know if the oxygen molecule has any effect on the chemical make up of the finished rubber strip.
    You can probably think of a few other factors as well.


    Called up Fred Pearce, the rubber guru, to see what he thinks about storing rubber. As Fred does not have access to the net, the following summarizes our conversation.

    According to Fred, the major rubber text book sates that without any free sulfur, rubber will last 20 years , provided it is kept in a steady temperature in the dark. Fred himself tested July and October ’97 Tan II rubber in 2006 0r 2007 for energy and elongation and found almost no degradation. The rubber was stored in his home without refrigeration.

    As long as the rubber has no free uncombined sulfur or other chemical elements, it will remain stable. Combined sulfur is benign and can last for a long time. But storing the rubber in water or other substances might trigger unwanted chemical reactions. An agent depriving the rubber from Oxygen might inadvertently also trigger a chemical reaction.

    Refrigeration might help by keeping the rubber at a constant low temperature.

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