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lechassin wrote: ↑October 15th, 2019, 2:05 pm Alright, I found this formula here: https://www.hippocketaeronautics.com/hp ... opic=983.0

N=√L/w . √d . (x-1)√x/√2π

L is length=32"
w is weight=2.75g
d is density=2.75g/32x0.0625x0.0625=22g/in³
x is breaking stretch ratio: 23 feet is 276", divided by 32"=8.625

Solving gives N=4572 turns, which is amazingly close! We got to 4500 turns just as the torque was starting to climb very fast, which IMO represents imminent breakage. The motor broke on the next run at low torque, which confirms that 4500 turns mortally wounded the rubber.

90% is 4115, so we'll shoot for 4100 and back off to launch torque for now. That's 137 cranks on our 30:1 winder.

Wait, how is your whole airplane 2.75 grams?

DatSciolyBoi wrote: ↑October 15th, 2019, 3:36 pm

lechassin wrote: ↑October 15th, 2019, 2:05 pm Alright, I found this formula here: https://www.hippocketaeronautics.com/hp ... opic=983.0

N=√L/w . √d . (x-1)√x/√2π

L is length=32"
w is weight=2.75g
d is density=2.75g/32x0.0625x0.0625=22g/in³
x is breaking stretch ratio: 23 feet is 276", divided by 32"=8.625

Solving gives N=4572 turns, which is amazingly close! We got to 4500 turns just as the torque was starting to climb very fast, which IMO represents imminent breakage. The motor broke on the next run at low torque, which confirms that 4500 turns mortally wounded the rubber.

90% is 4115, so we'll shoot for 4100 and back off to launch torque for now. That's 137 cranks on our 30:1 winder.

Wait, how is your whole airplane 2.75 grams?

He's talking about his rubber, not his plane. The John Barker formula is a formula used to calculate max potential out of the motor.

CrayolaCrayon wrote: ↑October 15th, 2019, 3:39 pm

DatSciolyBoi wrote: ↑October 15th, 2019, 3:36 pm

lechassin wrote: ↑October 15th, 2019, 2:05 pm Alright, I found this formula here: https://www.hippocketaeronautics.com/hp ... opic=983.0

N=√L/w . √d . (x-1)√x/√2π

L is length=32"
w is weight=2.75g
d is density=2.75g/32x0.0625x0.0625=22g/in³
x is breaking stretch ratio: 23 feet is 276", divided by 32"=8.625

Solving gives N=4572 turns, which is amazingly close! We got to 4500 turns just as the torque was starting to climb very fast, which IMO represents imminent breakage. The motor broke on the next run at low torque, which confirms that 4500 turns mortally wounded the rubber.

90% is 4115, so we'll shoot for 4100 and back off to launch torque for now. That's 137 cranks on our 30:1 winder.

Wait, how is your whole airplane 2.75 grams?

He's talking about his rubber, not his plane. The John Barker formula is a formula used to calculate max potential out of the motor.

Ohhhh, scared me for a second lol

Well, I ran the formula again using better numbers and I'm not so keen on it right now:

Stretch is 9.5 on another sample, which is more in line with what we expect, width 0.060", thickness 0.040"

N comes out to almost 6800, 50% more turns than the breaking point on our test sample. Is there that much to be gained by fine tuning our winding technique??

It just seems like the formula is way off without some constant applied to this size rubber.

You’re missing something in application of the formula. It’s pretty accurate. The easiest way to get a good understanding of the formula is to have the textbook page that describes its use. Maybe the resource you found didn’t have a full explanation of the formula’s use. I have a .pdf of this textbook page and will look around for it and post. If you’re in the mood for some research, I have posted the textbook page in this forum previously. Probably in the 2014 or 2015 WS thread.

I use the version of the formula that substitutes rubber width and thickness for density. I think it’s easier to calculate, but you need to back into the width using actual measured density using the method I previously mentioned.

Using a formula to estimate maximum turns and torque allows us to more efficiently test many different rubber motor lengths and densities to make the best use of limited gym time. And it’s fun for the students to see these mathematical relationships.

Brian T

Eric,

Sorry, I see that the textbook page is in the link you posted. I’ll have to look at your calculation for 32” of .060” later today. Prepping for first group of four students building four airplanes on Saturday.

My calc using the formula for breaking turns for this rubber length and .060” (.0398 g/in) is 5,240. As you showed with your data, highest turn count is usually the 2nd or 3rd winding; or maybe the 4th.

Good job finding the formula and working through the testing.

Brian T

I wish to apologize for hastily posting yesterday without adequately checking my work. In computing the volume of rubber I used the 32" motor length instead of total rubber length of 65.5". For thickness and width, I used a micrometer, holding the rubber up to the jaws and being and juuuust not letting any light through (reproduceable). I figure there's no point in measuring new rubber, since I want to know how many turns I can put into rubber that is fully broken in. Therefore, I used a spent motor for the dimensions as well as for the breakage test.

So, the formula I used is:
N=√L³/w . √d . (x-1)√x/√2π

And the numbers I have are:
L is motor length=32"
w is weight=2.75g
d is density=weight of rubber /volume of rubber=2.75g / 65.5"x0.060"x0.040"= 17.49g/in3
x is breaking stretch ratio = 9.5

Which gives me N=4800, 90% of which is 4300. That's amazingly reflective of my empiric data and reassures me that we have decent rubber and technique, pending opinion(s) of the forum. The 4300 is initial and would need to be wound back to 4100 to get 0.3 in.oz launch torque. That doesn't seem like much unwinding and makes me cautiously optimistic that our rubber, plane, and prop are pretty well matched.

Since we've been launching at maximum 3750 turns, it looks like we can add another 350 turns. At 2400 rpm we can hope for another 8 seconds, from 1'27" to 1'35", which frankly doesn't seem like much of an improvement

With the small wing, heavy weight, and small prop, the times will be quite compressed (people are reporting 1-1.5 minutes so far). Therefor, an 8-second improvement is actually quite significant (10% or so).

Coach Chuck

I'm curious to see how folks will get past 1'40" this year. Last year we flew 1'15" at the last local meet and we were happy... but we got 4th! We saw the Senior Flyer on Youtube fly about 2'20", and when we got our design to fly 2'17" over the Summer, we figured "we're done!"

Then later we saw a SO 2019 plane flying 4'25"! I'd never have believed it if I didn't see the video. After all the great work we did, we were only about half of that. Bottom line, I'm not assuming anything this year, and I'm anxious to see what the best flyers will do.

Question: is it better to make all motors the exact same weight with slight fluctuations in length, or is it better to have identical length motors with slightly different weights. I can see arguments for both.

I'm noticing a 32" ready-to-fly motor can be anywhere from 2.8g to 2.9 g.

When discussing times, you need to look at ceiling height too.

We were at 3:35 in a 24 foot gym for State, but was at over 5000 for elevation. Nationals was about 38 feet to the lights, so times were better. We scored 5:08 raw, there is video out there of that as well. We were hurt a little by air currents due to the crowd, we were expecting closer to 5:30.

This year will be far less.

As far as motor size, this had no longer a maximum motor mass event. Therefore, I suspect as you get closer, the optimum curve may be fairly flat. Your range is only a few percent on mass. I would expect that keeping your optimal cross sectional area of rubber (g/in) would be most important. But experiment. Try more or less total mass, and optimize cross section (or loop length) for each mass, and see where the stopwatch leads.

Coach Chuck