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Author Topic: Indoor rubber testing methods for Dummies?  (Read 652 times)
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spr
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« on: January 22, 2018, 08:42:29 AM »

Are there any quick and easy methods to test and compare different rubber batches for indoor duration  (F1D, F1M) use? 

I have several batches of Super Sport 2015-2017 and have also located some old  of Tan II - batches unknown -  and would like to know how they compare with each other.   Creating exact same size (grams/meter)  loops  and testing them by winding and recording dewind torques is a  lot of work - especially if the mystery rubber turns out to be crap.

Simo
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piecost
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« Reply #1 on: January 22, 2018, 04:44:22 PM »

My own attempts at testing have been tedous and frustrating. Multiple samples of each batch being required due to breaking at high turns and checking repeatability. I found that the scatter within results from the same batch is similar to the differences between batches!

I recorded unwinding torque and turns for a nominal 5inch loop of 1.28g/m rubber. I visually compared the curves. Mainly concerned with the torque at half turns. I also calculated the specific energy but this introduced more uncertainty. For example, how close each windinf test is to breaking effects the energy. So is it better to wind to a constant torque? But some batches may wind to a higher torque....

Differences in mass and mass per meter can be corrected by adopting non dimensional torque and turns coefficents.   Perhaps not for resolving small differences between batches.

I found testing most useful in understanding the deficiencies in my winding technique. I was getting bunching which reduced the maximum turns available. This caused large differences in the turns to reach target torque within each batch.

I wonder if stretch testing could be used as an alternative.
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Hepcat
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« Reply #2 on: January 22, 2018, 09:53:25 PM »

 Simo.
In reply to your opening words: “Are there any quick and easy methods …”, the answer is ‘NO’!  Piecost’s answer is honest and wise.
I did quite a bit of testing long ago but stopped when I realized I could not afford to not use a box of rubber because testing said it was not the best.  Nowadays I may just test a short loop out of interest to see if the extension when it breaks is somewhere near the magic 10x. I also think that tests done under careful conditions of temperature, winding rate, and so on do not necessarily help much outdoors on a cold/warm day, with a quickly tied knot, wind blowing dust and so on.
I notice you mention using the rubber for indoor classes and there I think circumstances are somewhat different.  It is still good, of course, if the rubber has good stretch and energy figures but it appears to me that flyers make up several motors and fly with them. Obviously torque, turns and flight times will be recorded but the ‘best’ motor is not necessarily the one with the highest time so far.  How hard they have wound and how long they have rested can influence which motor might be best for the next flight.
John
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cglynn
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« Reply #3 on: January 23, 2018, 08:20:38 AM »

Check this thread out for a good look at rubber testing methods and batches of rubber that have tested well.

http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=21752.0
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Skymon
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« Reply #4 on: January 23, 2018, 10:42:55 AM »

If you have access to an accurate force tester machine (pull tester) you can just cut a length and pull it till it snaps and get a reasonably representative force curve for comparative purposes.

You can get digital output pull gauges on ebay for under £100.

This would be the quickest and easiest way to compare the materials you have.

https://www.ebay.co.uk/itm/Digital-Push-Pull-Gauge-Gage-HF-500N-Force-Gauge-Tester-Meter-Brand-New-/171436680359


Si
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Olbill
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« Reply #5 on: January 23, 2018, 12:02:23 PM »

When I'm preparing for a contest I make motors from several different batches of rubber. I won't go into all the details b/c some people don't believe in my results. But basically I wind each motor to .9 times the calculated breaking torque, back off to what I think is a correct launch torque and then unwind the motor in 200 turn increments, putting down the torque at each step. Then I add up all the torque numbers, apply a correction factor based on the weight of the motor and rank the motors according to their scores. I've been doing this for a number of years.

I could maybe say that my results back up the method but probably the main factor is that I fly on the best rubber I can obtain. I don't have to save the good stuff for F1D since I don't fly F1D.
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Tapio Linkosalo
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« Reply #6 on: January 24, 2018, 04:45:01 AM »

I'm afraid there is no simple and precise method for evaluating data. Moreover, it all depends on what precisely you want to find out. The stretch method has its advantages: it does not wear rubber as winding does, and also there is no errors in the results due to motor bunching. So it is a good means to determine the maximum energy return of each rubber batch. But if you are interested in indoor models, then I do not think stretching is of much use, as also the energy return pattern (curve) has a major impact on how "good" the rubber is.

For example, I recently did a winding experiment on two motors, Tan2 99/07 batch that I use for F1D and F1M flying, and more recent (2016) SuperSport. The aim was to find out, how to adjust VP props when using the latter rubber, as I expect the 99/07 (as well as all Tan") to go too brittle to use shortly. The attached graph shows the unwinding torque curves (recorded at 10 turns intervals) for the two batches. As expected, the SS had much higher max torque peak. Interestingly, on similar length motors, most of the cruise torque was similar, so end of the flight should be rather similar for both motors. Which in practice turned out to be. I expected that I should back off my SS motor to similar max torque as T2, but with that, the model did not climb at all. Turned out that with SS the torque drops so fast that energy is not sufficient for climb, whereas with T2 my model obviously climbed with the "knee" of the torque curve. I had to launch the model with higher torque when using SS (did some back-off though) and had to reduce the maximum prop pitch to get the model climb to ceiling! Then there is the issue that SS takes fewer turns, so eventually I should adjust my prop so that I can use full torque of the SS to get decent flight times.

Summing up the area underneath the curve (i.e. integrating for total energy) suggests that my SS motor has 95% of the energy return of the T2. However, if I start to calculate the SS energy from the same torque as I use for T2 (i.e. simulating back-off), I only get 86% of the energy. So when using SS, it would be very important to be able to utilize the maximum torque of the motor!
   
(blue line for Super Sport, red for Tan2)
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Re: Indoor rubber testing methods for Dummies?
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cglynn
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« Reply #7 on: January 24, 2018, 11:23:53 AM »

I think its been discussed before, but if you are flying a model optimized for May 99 rubber, and for whatever reason have to use 2016 Super Sport, the trick is to use a motor of the same weight, but about 12% (iirc) longer, so essentially a thinner cross section (or less mass/length if you prefer).  That allows you to get the turns needed for duration, and utilize the climb torque.  I am going to be playing with the 2016 rubber almost exclusively, as my supply of 3/02 and 10/97 have diminished significantly, so it will be interesting to see how the longer, thinner 2016 motors stack up to the 3/02 motors I am accustomed to.
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spr
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« Reply #8 on: January 24, 2018, 12:21:09 PM »

My flight tests do not support the theory of using 12 % thinner SS motors than Tan 5/99. Motors that thin simply cannot keep the plane in cruise long enough and I end up with short flights and lots of turns left. Using thinner motors requires smaller props with lower pitch and  bigger rpm, thus wasting the extra turns you can pack in to a thinner  rubber.

OK, I am just an indoor novice and still learning a lot. Therefore  I'd love to hear that I am wrong, if someone has opposite experience.

Yesterday I quickly did some unscientific stretch tests. The unlabeled TAN II samples had a stretch factor around 7,5x and broke easily in stretching. 2014-17 SS batches could be stretched to about 9-9,5. Guess I won't yet trade my house to these TANs.
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