A motor’s total energy and number of strands.

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    There is an age long debate on whether the total energy of a motor depends on the number of strands: 24, 26, 28 and even 30 strands for a F1B rubber motor.

    Recently, I made up a batch of June 2009 motors with mostly 28-strands. Since the theoretical number of turns seemed rather low, I restranded 8 motors down to 26-strands and measured their energy again. Before getting to the answer, I will describe the measuring method, influenced by Bror Eimar and Jim Bradley. Results are presented at (*).

    Each motor is made from a large loop tied in advance. The motor is stranded between two vertical poles (1/8 brass rods), plugged into a wooden board, with no overlaps. The distance between the rods is the raw distance. …. Each motor is stretched to 60 Lb for at least five minutes. No tension adjustments are made after the initial pull. Motors then “rest” overnight.

    Each motor is stretched to a length corresponding to 5200 PSI (Eimar). This corresponds to about 86 Lb for shortest motors, down to about 80 Lb for the longest motors. The stretching is done in three phases – to 60 Lb for about a minute and roughly 70-73 Lb for 20-30 seconds (to allow the motor to cool down) and then to max pull (80-86 Lb) – kept as short as possible to avoid damaging the motor. Tension measurements are recorded for 5200 PSA, 60 Lb, 45 Lb, 30 Lb, and 5 Lb (Bradley). The 5 Lb is measured by stretching the motor after it’s fully retracted.

    I label the energy between the max pull to 60 Lb as ”burst”, 60 to 30 Lb as ”knee” and 30 to 5 Lb as “cruise”. All tests factor in the ambient temperature. (#) However, these labels are inaccurate, since the length corresponding to the upper boundary (5200 PSI) varies across motors, while the other loadings (60, 45, 30 and 5 Lb) are fixed. (The latter should be defined in terms of PSI, but this is yet to be implemented.)

    The 26-stands motors were tested a day later. At that stage the motors were not completely “relaxed” (on average 1.5% longer) as measured by the length at max pull divided by the raw length. So the energy rations of each motor (26-strands to the 28-strands) was multiplied by (max stretch at 28-strands/max stretch at 28-strands) to net out this effect.

    (*) Got the following average energy changes for the eight motors: Burst: – 22.5%, Knee: -1.2%, Cruise: + 11.8%, Total: +1.9%. The increase in total energy is insignificant given the small sample. In other words, total energy remains unchanged in restranding. The drop in the burst energy is due to measuring the energy of the shorter motors (28-strands) over 86 to 60 Lb, while the longer motors (26-strands) are measured over 80 to 60 Lb. The opposite applies to the cruise results. So the “shift” in energy is an outcome of a flawed measurement methodology, as noted above (#). Constant total energy results for different strandings replicate previous tests. The result is general and applies to other motor weights.

    Which motors (28 or 26-strands) are optimal, for a given vintage, is another question. But at least they have the same total energy.

    Bill Shailor

    This is good information. Thanks!



    It may interest you that Russell Peers (famous GB F1B flyer) now strips his rubber to ~1/16″ square and claims better energy storage.

    Anyone else tried it ?



    Many west coast F1B fliers swear that 1/16” motors are superior (3-5%). Chinese only order 1/16” and John Clapp subscribes to this school. Well known European F1B fliers, as you point out, use these motors.

    The variation on a 1/8” motor between the shortest and longest raw motors is just under an inch. So, 1/16” motors would have half the variation – allowing a narrower range of turns.

    One issue is that these motors doubled the number of edges from four (1/8”) to eight (1/16”). Edges are the weak points of a strand, making it more likely to fail. Labor is another issue: I’ve tried to make up 1/16” motors using the method described above, but the amount of additional labor is significant, outwighing the possible benefits. Indeed, the father of two very well known Ukrainian fliers, who makes their motors, told them not to order any more 1/16” rubber.

    So far, the 1/16” motors have not dominated, so their excess performance claims might be overstated. But when the 1/16” guys start winning, I’ll join their crowd.


    My first posting in this thread discussed testing a motor to a target PSI – corresponding to a 80-86 Lb depending on the raw motor length and then measuring the distance at fixed tension: 60, 45 ,30 and 5 Lb. However, as pointed out, comparing across motors with such fixed boundaries is flawed.

    If all motors are stretched to a target PSI, say 5600 in the case of Carrol Allen, or 5200 in the examples above, and additional observations are taken as the motor retracts, the overall energy of motors can be compared across motors. But this does not apply their distribution of energy.

    A solution is to consider a series of intermediate PSI targets. PSI is calculated as = Tension (Lb)/cross section. The cross section is product of the strand’s cross section (width * thickness) multiplied by the number of strands in a motor. This has to be divided by the expansion or stretch ratio which is the overall length at the given tension divided by the raw motor length. So a target PSI = (X lb * length at X lb)/(width * thickness* number of strands * raw motor’s length), were all lengths are in inches. This equation has one unknown – the PSI. We can solve for any other variable such as the overall length – given a target PSI, a tension level (Lb) and the motor characteristics.

    For example, given a raw 14.00″ motor, 28 strands, with 1/8″ .042″ thick. To match 5400 PSI one would need (75 Lb at 137.6″ total length) or (77 Lb at 132.3″) or (80 Lb at 129.0″) or (83 Lb at 124.3″) …… etc. This differs from the commonly used tension targets were the distance is recorded, or distance targets were the tension is recorded.

    Suppose that the max tension of all motors is 5400 PSI. One then can then pick lower PSI targets corresponding roughly to 60, 45 and 30 Lb tension (Bradley) set at 71%, 48% and 30% of the max value. (These magical percentages are subjective – a first pass at trying to match the tension values for a motor stretched to 80 Lb.)

    Each PSI level is measured to the appropriate (tension at length) pair. The pair is recorded on a worksheet with the appropriate energy calculations. One can now meaningfully compare the distribution of energy across motors in terms of their burst (100% to 71% PSI), knee (71% to 48% to 30% PSI) and cruise (30% PSI down to 5 Lb).


    I read with interest the above posts.From basic physics principles,I would expect the total energy to be independent of the number of strands,unless more or less is dissipated as heat or sound.
    However ,what counts is the total kinetic enery coming from the prop.Supposedly,the force produced at any one time should turn the prop at its ideal revs,given its weight,dia,pitch,section,drag.The variable pitch is therefore a crude eqivalent of a gearbox.
    So,what needs to be researched is the optimum properties of the propellor given the type of motor to be used,and to do this necessitates the length and number of strands of rubber to be set first. In practice this mostly boils down to experience of individuals,and mimicry of the best performing models.
    Perhaps an experiment to measure force (thrust) against time as the motor unwinds would be the way to proceed.The area under the curve is the total energy created.The experiment could then be repeated after altering any one of the important variables-prop pitch diameter,section,profile etc.
    What most of us do is try this rather randomly with test flights.Maybe the winners have sat down and proceeded scientifically.A big problem is the variability of rubber and the quatity of it required in a lot of experiments.

    Roger Morrell


    for whatever it’s worth I know that Igor Vivchar uses some of those instrumented testing that Doc James talks about. By using tested motors from the same batch he and his brother fly essentially the same airplanes back to back , one with a new item such as prop blades and the other with the production part. Seeing Igor’s father job in the family business is to make and test motors he does have a source of calibrated power. He also uses altimeters during testing to measure altitude and sink speed.

    In Igor’s case building F1Bs is his business so he tests things that he believes will improve the performance of what he sells. I know that people have gone to him and said why don’t you test such and such a new feature and been disappointed when he declined to do it. Clearly it’s his business, he has already many ideas and generally has a good idea what will work or not. He also believes that his airplanes have to give a good performance to a wide spectrum of the flying population, so would not want to sell something would require a high level of expertise to get a good performance.

    I know that with the lack of availability of Tan2 he reduced the length of his super sport motors to gain comperable performance. I will be interested in seeing what he does with the latest super sport. Personally I’ve not found it necessay to make trim changes. It is clear that you can get more turns and this may change the way one decides how many turns one can get for a given length of motor.



    I get back home next weekend and have a week to prepare for a comp with my Vivchar F1B.I have no Tan 2 as I came back to free flight when there was only supersport.I have been sulking because of this!!I am really looking forward to using the June 09 SS. I will be happy to just get to the flyoff as my model doesnt have a VPP; and I really concentrate on F1G more seriously where I do have 3 really good Bukins.At the moment I feel F1B is too expensive to get into seriously.My building skills with carbon are not up to standard yet,and as I am retired I find the cost of a Ukranian supermodel prohibitive.With my Bukin F1Gs,I find I can just concentrate on trimming and competing,which is what I enjoy.I fly vintage and P30 also but I seem to spend a fair bit too much time repairing and improving!!Also I am up against flyers who have been competing for decades.
    So for the moment I am stuck with talking about flying!,but next week I will be able to get on with it and make excuses after!


    Another flier reporting no trim changes with 06/09 is Omri Sirkis, an Israeli F1B flier (in Hebrew via e-mail). Omri uses 420-35 turns and notes, however, that the motors are tougher and could potentially be wound more.

    As some fliers repost no effect and other, like me, discover the need to make adjustments (AR, wing twist). The question is why? My hunch is that fliers in the second group tend to uses shorter motors, with about 400 turns. These shorter motors have, on average, higher torque and switching to 2009 rubber augments their torque leading to trim changes. However, this is only a guess.

    (Note that the thread’s topic has drifted to 2009 rubber issues.)


    Sorry about the drift in the thread.However it is all interesting.Keep posting .


    I think of the stored energy in the rubber as being due to its elongation only,but we achieve this by both elongation and twisting.So there is some sideways streching of the rubber.Also some energy must be lost by friction between the strands.Why do we not use 1 strand of say 8 or 10mm square.Is there research or experience of this?Is it theoretically best to use a large number of very small strands?My experience makes me think so.Could it be that with more strands there is less sideways stretch and more elongation which is more useful energy?


    Rubber manufacturing is an interesting topic, but outside our (rubber fliers) control. Theoretically, a round rubber chord is the best shape as it lacks edges. Round rubber cords are made in small diameters and substitute rubber bands used for fastening stabs and wings. Rubber chords are extruded from a large molten rubber ball using pressure. In contrast, the standard rectangular rubber is stripped from large flat rubber sheets, made from two layers approximately .021” thick. The two processes are completely different, in terms of adding ingredients and curing, requiring a degree in Chemical engineering.

    Personally, I happen to like the 3/32” rubber (06/06). However, the other customers who buy the rest of FAI Model Supply’s rubber do not use this width.

    (A rubber motor’s torque, not tension, powers the propeller.)


    My objection to one sixteenth rubber other than the handling of so many strands is the increase in surface area by 25% which requires more lube. It may not be significant but say you use 0.5 grams of lube on a 1/8″ stranded motor. A 1/16″ motor would require 0.628 grams for the same lubed surface ratio. That’s about 1.5 inches of 1/8″ rubber you would be missing.

    The other discussion of stored energy whether torsion or elongation is complicated by the knotting of twisted rubber. Try googleing twisted rubber fibers, the math is quite complicated.


    With my F1G models,the weight of the lube is quite significant.I find the silicone lube the heaviest and equivalent to the loss of more rubber than I would like.I suppose lubrication is a whole new topic,but I would be interested in comments on this. In a comp I use a new motor for each flight and often find a strand broken afterwards.At present I am using soft soap and glycerine.


    Here is a picture of the rubber tester I built back in the 70’s. It plots acurve on graph paper of force Vs extension. The area under the curve is the stored energy. The curve as the motor is being returned is the usable energy to the prop. I assume Arams fixed extension points can be picked off the plots. The area under the curve does vary from motor to motor but not in any particular way. I have plots from Pirelli rubber from the 70’s and 80’s that have almost identical curves. I could post those but someone pinched my camera at the NATS!

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