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nxtscholar
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Re: Winding

Post by nxtscholar »

I'll rely on the more experienced indoor freeflight plane builders to correct/verify:

That really depends on the ceiling height and condition. What I've found is that the lower the ceiling height, the thinner the motor you want to use. The thicker a motor, the quicker it will climb. For regions, I ended up using 0.081 inch in thickness because the effective ceiling height was only 16 feet, and there were girders and wires that would kill times if the plane hit them. If the heights are around 20-30, I use 0.087 inches. anything 30+, I use 0.094.

I use about 80-100 turns. But to be honest, my data is still pretty limited based off of only 27 flights. :P
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Re: Winding

Post by bjt4888 »

Chris_L,
nxtscholar's information is good and he or she should be congratulated for compiling a good set of data and analyzing. Other factors that will affect the rubber thickness and corresponding rubber motor loop length that gives best duration include: propeller pitch, propeller blade area, airplane trim (a forward center of gravity may result in increased decalage angle and more drag requiring a different optimum propeller and rubber), airplane design (some designs generate more drag than others).
The students I am coaching have found that a more propeller area requires a slightly thicker rubber motor (on the design they are using this means about .094" rubber) and a lower blade area propeller is working best with a thinner rubber motor (.087" or .090"). My students are not varying motor thickness to adjust for different ceiling heights, they are varying the number of backoff turns and corresponding launch torque.
Also, for low ceilings, a good variable to experiment with is minimizing the left wing washin to reduce the climb rate (remembering that reducing washin also alters decalage angle and might require an increase in wing incidence) . The first circle of a well trimmed low ceiling airplane may exhibit a banking characteristic (maybe 5 or 10 degrees) and may only climb a few feet, or less in this first circle. This banking characteristic will usually reduce in the second circle, or soon thereafter, but the climb rate should remain just a few feet each circle. If you are looking for pretty significant improvements in performance, a flaring propeller (Google search "Chris Goins Double Trouble" for the concept) would also be a good thing to try.

Good luck, keep testing and keep logging data,

Brian T.
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Re: Winding

Post by Toms_42 »

Quick question, why do people unwind their rubber to adjust for ceiling height? Why not just wind it less in the first place?

I've also had a lot of inconsistency with winding rubber, they all seem to snap at totally different points. My 0.87 with armor-all has been snapping anywhere between 1700 and 2200 turns, am I doing something wrong?
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Re: Winding

Post by nxtscholar »

Soinc.org has a great guide on Wright Stuff that I suggest you look at; helped me out a lot. But here is the answer:

"If you have a well-trimmed airplane and fly it in a
low-ceiling site, such as a school gym, with maximum
turns in the motor, it will rapidly climb into
the beams and wires and baskets and all sorts of
other trouble. To avoid that happening, the flyer
takes advantage of a characteristic of rubber by
unwinding some of those hard-won turns before
launching. The available torque from a rubber
motor decreases rapidly as the first few turns are
taken off. With reduced torque, the propeller rpm
is also reduced, the airplane flies a little slower
and doesn’t climb as fast. It is much better to
wind the motor to its maximum (1300 turns) and
then unwind to 1200 turns than it is to wind to
1200 turns and launch. The initial torque of the
motor wound to 1200 is much greater than that
of the motor reduced to 1200 turns after being at
1300."

With regards to snapping your motor, more details please? There are a lot of factors involved beyond just the number of turns you have.

1.For example, what iteration number were you on? A rubber motor that's been winded hard the first few flights will be more liable to break later on. I bring this up only because you may have mixed old motors with new ones? Idk. It's possible based on only the given information.
2. But something about your question tells me that's not the case. It could also be along the lines of how far you stretch the motor. Some teams stretch the motor up to 3-4 times its length unstretched while winding to fit in more turns. Eventually, you'd need to walk toward the winder (or vice versa) to slowly lessen how much you stretch the motor. Assuming you do this process, you could be stretching and unstretching at different rates.
3. The amount of lubricant you add is inconsistent?
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Re: Winding

Post by retired1 »

A couple of years ago, we got rubber that had some very tiny nicks on the cut edge. You could not see it unless you looked closely while stretching and relaxing a section of the band. The nicked band would fail upwards of 20% sooner.
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Re: Winding

Post by bjt4888 »

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.
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Re: Winding

Post by Toms_42 »

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.
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Re: Winding

Post by bjt4888 »

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
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Re: Winding

Post by bernard »

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.
Attachments
Motor Winding Torque Curves 2.png
Motor Winding Torque Curves 1.png
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Re: Winding

Post by Less_Incidence »

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?
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