Band doubling

[smithchrystal]

Is it ok to double-up a single rubber band on itself so that it's half as long and twice as thick?

[bernard]

The rule regards motor mass, so yes as long as you are under the maximum.

[retired1]

Is it ok to double-up a single rubber band on itself so that it's half as long and twice as thick?

Out of curiosity, why would you do that?
At least one national AMA competitor with light aircraft is experimenting with using 2 thin bands together with the thought that he can get a few more turns on the band than if it were one band of the same total size.
Time could probably be better spent with rubber vs torque vs pitch for virtually everyone.

[RJohnson]

My experience from experimenting with multiple bands this year is that it generates a higher, flatter torque curve... There are some limitations with 4 or 5 bands because it's difficult to get the necessary number of winds in, but I increased a helicopter flight time by 20% by doubling up the bands from one to two.

Happy flying!

[jander14indoor]

That much improvement is a little surprising. What kind of rubber are you using? What thickness for your strip? Did the increase occur using the same thickness and doubling the strands, or did you halve the thickness when you doubled the strands?

Jeff Anderson
Livonia, MI

[RJohnson]

Rubber width went down by a factor of 2.2 to approximately match sustained torque. Increased length to maintain rubber weight. It's tan super sport 2, the stuff you can get now. The batch was from back in June. I don't measure thickness even though I use a Harlan rubber stripper, I go by g/m and it was somewhere around 3-4 g/m if I remember correctly, 3/16 is usually around 9 or 10. The double stranded band was definitely a much better match for the copter. Not sure that it alone caused the improvement, but I can't match the performance with a single strand

[calgoddard]

All other variables being constant except for the number of strands, the torque produced by a rubber motor is solely a function of its cross-section. There is no empirical evidence that I am aware of that supports the proposition that the duration of the flight of an indoor rubber powered aircraft can be extended by increasing the number of strands in the rubber motor. If this were possible, world record holders such as Kang Lee and Bill Gowen would be using rubber motors with more than two strands of rubber. As far as I know, they both fly their models with single loops of rubber.

Here is some anecdotal evidence that tends to disprove the proposition. Under the rules for the international F1G class (Coupe d’Hiver) the minimum weight of the airframe (with prop but excluding rubber motor) is 70 grams and the maximum weight of the rubber motor (lubricated) is 10 grams. I regularly fly my F1G models with 24 x 1/16 rubber motors and 12 x 1/8 rubber motors depending on availability. The former rubber motors have not produced longer flights of my F1G models on any consistent basis compared to the latter rubber motors.

[retired1]

[quote="calgoddard"] If this were possible, world record holders such as Kang Lee and Bill Gowen would be using rubber motors with more than two strands of rubber. As far as I know, they both fly their models with single loops of rubber.

Last time I talked to Bill Gowan, He had started working on double loops with the thought that he could get a few more winds vs a single loop out of the same weight. He was also looking at 1 1/2 loops-IE: 3 strands.

The FF kit rubber, smallest of the 3 included in the kit is about 1/16 of an inch, which is small to me.

[calgoddard]

retired1 -

Thanks for the additional information.

A 3 strand rubber motor would require knots at each end of the rubber motor. It would seem to me that this configuration would result in less maximum turns than a 2 strand rubber motor (single loop) having the same weight and cross-section as the 3 strand rubber motor.

Let me know if Bill Gowen eclipses his own world records using 3 or 4 strand rubber motors. This result would be counter to 80+ years of indoor flying experience. However, Bill is one of the very best indoor fliers in the world, so if anyone can do it, he might be the one.

[bjt4888]

Calgoddard and Retired1,

Thanks as always for your excellent posts on the SO forums. Here is my recollection of Bill Gowen's discussion of multi-strand motors. In some cases, Bill was using three strands in order to make use of some particularly good 5/99 rubber that was already stripped so thin that the motors needed to be 3 strands for proper x-section for the particular model/prop. I believe that one of his high ceiling F1M world records was with a 3 strand motor.

Some of Bill's later posts on the subject indicated that he was exclusively using 4-strand motors for F1M and getting improved duration. One reason for improved duration that he conjectured was the smaller knot. My measurements on SO motors of about .055 g/in (which is close to F1M size) showed that the knot for a two strand motor of this thickness used 0.5" of rubber. So, a 4-strand motor of 0.0275 g/in knot would use only about .25" of rubber, resulting in a 4-strand motor of the same weight as the 2-strand motor would then have .0625" greater overall length. The maximum turns formula I use would predict that this .0625" of additional motor length would allow about 9 more max turns. As our SO airplanes were flying using a prop speed of about 6 rps last year, this additional number of turns, if fully utilized, would result in about 1.5 seconds more duration. Also, as you know, the F1M is a special model and Bill's good results with 4-strand might be more pronounced with F1M's using mechanical variable pitch and super-high initial pitch and rubber torque.

My students tied many 4-strand and two strand motors last year and found that what mattered most for motor quality was the particular piece of rubber. We were using 28" "blanks" for motor cutting in order to maintain consistency. One 28" blank would produce 1.75 in oz summed unwind torque readings over the cruise and let-down range and the very next 28" piece of the same batch of rubber would produce 1.6 in oz summed torque. We also discovered that it is important to test flying with "best" motors and not to just save them for the big competitions to be sure that prop pitch and softer vs. stiffer flaring settings were correct.

Here is an MSExcel chart comparing unwinding torque of a number of 2 and 4 strand motors that my students did last year. Note that one of the 2-strand motors and one of the 4-strand motors were the best (yellow highlighted sections of the unwind data that corresponds to the cruise and let-down).

Brian T.