Scioly.org

Toms_42,
nxtscholar and retired1 have both given you good advice to inspect rubber, check experimental procedures and wind rubber properly. Reading the info already available on soinc.org is a very good idea. 0.087" thickness Tan Super Sport rubber should make a 17" to 18" two gram motor. This motor should take approximately 2,300 to 2,400 or more turns to break if tied, lubricated and wound correctly. The students I am coaching are winding to about 85% of maximum turns and backing off turns to the desired launch torque. Their torque meter is usually reading about 1.0 inch ounces when this motor is wound to 85% turns (about 2,000 turns) and their usual backoff is 90 to 120 turns to launch torque readings of anywhere between 0.35 inch ounces and 0.5 inch ounces depending upon the specific propeller and airplane. For our particular airplane and propellers, these specs are about right for a typical high school gym ceiling of 25' to 27'.
nxtscholar is correct that you will have significantly more winds in the rubber motor if you wind to maximum torque (or as close as you determine is needed) and then back off to launch torque. In the example above, winding to 2,000 turns (about 1.0 inch ounce) and then backing off 120 turns to 0.35 inch ounces provides you with 1,880 turns. You will find that it might only take 1,200 turns to wind directly to 0.35 inch ounces. 1,880 turns is a lot better than 1,200 turns! There is a significant duration benefit to winding to maximum (or near maximum) and then backing off. Experiment and keep data on your tests to determine what works best for your particular airplane, propeller and rubber. Increment launch torque up gradually by winding each full power flight to maximum and backing off less and less on each successive flight and observe how the airplane flies. This will allow you to gradually increase maximum altitude and make full use of your ceiling height, hopefully without getting your airplane stuck in the girders, lights, etc. at the top of the gym. A reasonable initial torque increment amount would be 0.03 to 0.05 inch ounces. For example, if your first flight with minimal turns (about 500 - 750 with .087 rubber and a good airplane) results in level flight and no climb, then you could proceed to a launch torque of 0.32 inch ounces and then to 0.35 etc. till you determine the optimal torque to get as close as you want to get to the ceiling.

Good luck,
Brian T.

Chris_L wrote:What are some of the best rubber thicknesses you guys have found? And with how many winds?

That really depends on the prop/plane. We have been using 87 rubber with some success, with upwards of 1800 winds on it. With our other props, we use less winds on 94 rubber, but that is less successful. Currently I'm working on making even smaller props to try and use 81 rubber with >2k winds.

Note that to wind this much you should be lubricating your rubber with something like armor-all. Also, anyone know an alternative for rubber o-rings to hook on the rubber? They twist and are heavy. I tried cutting out rings from a straw, but that too twisted and snapped.

Toms_42,
Congratulations on your successful testing so far. You will see in the soinc.org write-ups and past Wright Stuff threads that there are recommendations for use of o-rings sliced off off 1/8" nylon tubing. An easy source for this tubing is hobby stores that sell better quality radio control cars and trucks. The 1/8" nylon tubing that is used to house the car's radio antenna is perfect for o-rings. Try to get a colored tubing (not clear) as this is easier to see while attaching the motor onto the prop hook and rear hook. Nylon o-rings of this type weigh about .008 grams per pair vs. the black rubber o-rings at about .083g per pair. This weight savings will translate into another 1.5" of rubber (or a motor that is .75" longer) resulting in a motor that will store more turns at each torque value. The nylon o-rings will only have a hole of about .0625" and you will need to one end of your rubber while making a motor to a thin point so that you can poke it into the o-ring and pull it through. It's no big deal that the o-ring fits sort of tight on the rubber. One disadvantage of this nylon o-ring is that it doesn't stay centered well on the Ikara propeller hook; it will have to be played with to unbunch from the hook. This is not a problem if you are making custom propeller hooks with a better shape and mounting your propeller with a Ray Harlan "pigtail" thrust bearing. The best propeller hook to shape for the nylon o-ring is the "reverse-s" (there is a youtube video showing how to make one). The rubber o-rings are heavier, but they are easier to attach to the prop hook and rear hook and they stay centered on a simple "diamond" shaped prop hook pretty well.

Good luck and good flying,

Brian Turnbull
AMA and indoor flying since 1972

jander14indoor wrote: PS, I encourage you to challenge all this by collecting some data, it will make MUCH more sense if you do.
Make up 3-5 motors as identical as you can.
Wind one taking torque data vs winds until it breaks. Plot it. This establishes the max torque for those motors
Take a second motor and wind to 90% of breaking torque and unwind taking data both ways. The area under the unwind curve is the energy available to fly.
Wind that motor a second time, same max torque. Plot again. Torque should be lower on unwind, but turns much higher. Figure the area again, see if it is more than first wind.
Repeat that sequence until the motor breaks. Compare results
Do that with a third and fourth and fifth motor (until you patience wins out, but hey, repeatability is a critical issue in science and engineering).

I've attached some graphs that I generated earlier this season with motors made from the Tan Super Sport in the Freedom Flight kits. Motor 1 was a motor that had been used before, and Motor 2 and Motor 3 were new. I wound Motor 1 and Motor 2 to breaking, and for Motor 3, I wound it to one under breaking and then started unwinding. With my winding technique, breaking was at around 147-148 turns on a 10:1 winder.

Discussion of error: I made my competition motors on a scale with 0.001g accuracy; I don't remember if I made the motors used in these graphs on that scale, but I think I did. Even with that precision when making motors, I've noticed that some motors of the same mass can have loops that are up to 2 inches longer. I don't remember if I checked my motors for similar lengths when I wound them.

Very nice graphs, and data! Does the utility you used have some kind of a Riemann sum function so that you could estimate the stored energy in each motor?