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Author Topic: Question - Estimating Max Turns  (Read 549 times)
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ghcrash
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« on: April 12, 2017, 01:31:39 AM »


Is there a simple formula or rule of thumb for estimating the maximum turns, without breaking, for a rubber motor?   Especially one that is applicable to the size of rubber normally used on EZB and Ministick planes.

I found one formula but it was for winding multiple strands of 1/8" rubber but it didn't seem to give reasonable numbers for 1/16" and smaller rubber.

Thanks in advance.

       George
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cglynn
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« Reply #1 on: April 12, 2017, 09:40:02 AM »

Personally, I like to make up a small loop, 2inches or so, of the motor being tested, put it on a torque meter, and winding it until it breaks.  You can than calculate turns per inch, and know the breaking torque of the motor.  The formula (which I don't know off the top of my head) works well, but assumes uniform rubber, stored properly, and all that.  I like to know how the exact piece of rubber I am using is going to behave, and I have found that testing small samples is the best way for me.

Chris
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Tapio Linkosalo
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« Reply #2 on: April 12, 2017, 10:11:04 AM »


Torque meter is the thing to use. Always wind the given size on motor to the same torque (determined by trial and error). Only when wound to that torque, check the turn to find out if there is enough of them. If not, discard the motor and try another one!
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ghcrash
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« Reply #3 on: April 12, 2017, 01:09:40 PM »


Thanks guys.  Unfortunately I do not have a torque meter.  I know that I could do a wind-to-failure test but I would like to determine beforehand at about what point that failure will occur.

   George
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ram
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« Reply #4 on: April 12, 2017, 01:29:43 PM »

See:

http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=21658.msg203679#msg203679

Rey
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jakepF1D
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« Reply #5 on: April 12, 2017, 03:19:36 PM »

As ram pointed out, formulas exist for various batches of rubber that can get you in the ballpark.  To really maximize turns though, you'll need to make or buy a torque meter.  You could make a simple torque meter with a few bits you can get from a hobby shop, and I've seen articles about calibrating based on wire diameter and length.  Even if it wasn't calibrated it would give you a visual representation of when the torque begins spiking which indicates that you're approaching the break point.
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Hepcat
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« Reply #6 on: April 12, 2017, 07:49:21 PM »

George,
Below is a formula I devised many years ago and it has served me well. I almost always use it in the simplified form with the constant 44 at the start because I have never seen a different value for the density and I don't often do stretch testing. If you do do stretch testing the reults can be incorporated which is not the case with any other fomula I have seen. Also there is no messing with number and size of strands all you need is the motor length and its weight. As others have mentioned no formula can cover things such as rubber age, the knot, temperature, slight variations in manufacture, winding technique and perhaps many other things but it should be a useful starting point.
John
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Re: Question - Estimating Max Turns
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ghcrash
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« Reply #7 on: April 13, 2017, 08:11:10 AM »



Thanks for all the input guys.   I now just have to put what I have learned into practice.

Thanks Hepcat for the formula.   I noticed that it had a stretch factor in it.  From the formula it looks like the more you stretch the rubber, the more turns you can put into it.  (I'm a notorious under stretcher.)

Ram, you were a great help too.  The link you gave provided more information than just a formula for max turns.

From both Hepcat's and ram's formulas, it appears that I have been winding to a point way short of the breaking point.

Again, thanks to everyone that replied to my question.
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calgoddard
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« Reply #8 on: April 13, 2017, 09:36:44 AM »

ghcrash -

Since you apparently fly EZB and Ministick airplanes, you are probably interested in maximizing the flight duration of your models.

In low ceiling heights, e.g. 20 - 24 feet, it is essential to wind your rubber motors with a torque meter if you want to maximize duration.  You don't want your model to stop climbing 10 feet below the ceiling.  You also don't want it to rocket to the ceiling, get one or two lucky bounces, and then hang up on a light fixture.

What follows is a simple explanation that has been repeated elsewhere on HPA many times.

For a given motor size, prop, air frame, trim, etc., the torque on your rubber motor at the time of launch will largely determine how high your model airplane climbs.  I like to find out, by trial and error, the launch torque that will give me the perfect "no touch" flight in a Cat I site (low ceiling). This flight is characterized by a long, slow climb to a peak height just below the ceiling, beams or other obstructions, followed by a long cruise (maintaining peak altitude) followed by a long descent.  The model should land with unused turns roughly equal to the back-off turns (described below).

You need to wind the rubber motor to the point where it is almost ready to break, and then back off turns (unwind) to the desired launch torque to maximize flight duration in a low ceiling height.

Here is an example.  The numbers are not actual data points for a real model airplane, but merely made up to illustrate my point.

Let's say you determine that your model airplane will climb to a height that is a foot below the ceiling when launched at 0.3 inch-ounces of torque.  Let's further say that your torque meter shows this level of torque when you wind your rubber motor directly to 1600 turns.  You do not remove your rubber motor from your winder and torque meter and mount it on your model airplane at this point.  Instead, you continue winding to 1800 turns and 0.6 inch-ounces of torque which is the absolute limit you have previously determined this size of rubber motor will take without breaking.  Then you back off turns (wind backwards) so that the rubber motor has 1700 turns and 0.3 inch-ounces of torque. Notice that the rubber motor now has 100 more turns for the same 0.3 inch-ounces of torque than it had when wound directly to 1600 turns. Those extra 100 turns will usually mean a longer no touch flight.

Here are several caveats.

First, some fliers like to gamble and bounce around the ceiling to maximize their flight time.

Second, variations in temperature change the way energy is stored and dissipated in rubber motors, and change aerodynamic drag.  So in my example, 0.3 inch ounces of torque may give you the perfect no touch flight one day, and cause the same airplane with the same trim, same rubber motor, wound the same way, to bang into the ceiling on another day when it is warmer in your flying site.

Third, try to stretch the rubber motor at least 6 - 8 times its relaxed length before you start winding.  Put in half of your anticipated max turns and then step in gradually as you put in the remaining half.

Early in my modeling career I had the privilege of flying with the late, great Cezar Banks.  He told me that if you are not breaking rubber motors, you are not winding enough.

I hope this helps.  Others - please chime in if I have misstated anything here. My understanding is that this is pretty standard theory and practice, with minor variations among experienced fliers.  I am still learning.    

P.S. - It is not essential that your torque meter dial be accurately calibrated to inch-ounces of torque.  This is only necessary if you want to compare data with other fliers.  You just need a set of equally spaced numbered graduations on the perimeter of the disc-shaped dial or face of your torque meter to serve as reference points.

Sizing your rubber motor to achieve max flight duration is a whole different subject covered elsewhere on HPA. I learned a lot from this web site from many experts, including Hepcat.

« Last Edit: April 13, 2017, 09:58:25 AM by calgoddard » Logged
USch
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« Reply #9 on: April 13, 2017, 10:23:21 AM »

In the plan gallery you can find a spread sheet from Fred Rash named:

Turns, Torque Calculator for Super Sport Rubber (Revised 2014-10-05)

To my humble opinion it is the perfect tool for indoor motors to estimate max turns and torque. Of course you use it as a starting point as every batch of rubber is slightly different from others. But you get realistic numbers to start with.

Urs
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Tapio Linkosalo
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« Reply #10 on: April 14, 2017, 12:00:30 AM »

In low ceiling heights, e.g. 20 - 24 feet, it is essential to wind your rubber motors with a torque meter if you want to maximize duration.  You don't want your model to stop climbing 10 feet below the ceiling.  You also don't want it to rocket to the ceiling, get one or two lucky bounces, and then hang up on a light fixture.

Exactly. And to add one more spice to the stew: the rubber motors break in on every wind, so even if you manage to find the number of turns that is correct to climb just to the ceiling, not leave short not rocket to the ceiling, for the next flight on the same motor the correct number of turns is a few tens of turns more. On the other hand, torque (and in low ceilings winding to max torque and then backing off to launch torque) is very accurate indicator of the maximum altitude the model will reach. So torque meter is essential for flying indoor.

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calgoddard
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« Reply #11 on: April 14, 2017, 09:23:22 AM »

Tapio -

Thank you for pointing out a detail I left out in Reply #8.  Each time you wind a rubber motor to near max it will stretch a bit more, so in successive winds, you will need more turns at launch in order to achieve similar results, all other conditions being equal.  I have found that after the second or third wind to max, the performance of the rubber motor in terms of the duration that can be achieved usually begins to drop off.  Of course when and how much depends on the batch of rubber.  Like many other fliers, I have gotten my best times indoors with May 99 Tan II.  I would never fly with it outdoors because too much rubber would be needed for an F1G motor, for example, and other factors diminish the importance of the rubber outdoors, at least in my experience.  You still need to fly with good rubber outdoors, it just does not need to be precious May 99 Tan II.

I have read and/or been told that a rubber motor will regain 95% of its original performance if allowed to "rest" for a week or two.

I have not been able to confirm this because, for one thing, flying conditions are never quite the same on two different days. Regardless, why would I want only 95% of the original performance in a subsequent contest?

Some readers may say that this thread has gotten away from the original topic.  But estimating max turns is just the beginning of the winding process. I felt it would be constructive to point out the reasons for winding with a torque meter when it became clear that the person who asked the original question about estimating max turns was winding without one.
« Last Edit: April 14, 2017, 09:35:39 AM by calgoddard » Logged
ghcrash
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« Reply #12 on: April 14, 2017, 10:36:54 AM »


Some readers may say that this thread has gotten away from the original topic.  But estimating max turns is just the beginning of the winding process. I felt it would be constructive to point out the reasons for winding with a torque meter when it became clear that the person who asked the original question about estimating max turns was winding without one.

I think that we are exactly on target.  I'm just getting into this , rubber powered, hobby.  As yet I have not made up my mind if I will have enough interest in it to warrant obtaining all the tools (winders, strippers, torque meter, ect.) that are almost essential if one plans on staying in the hobby and doing the things the proper way, the way that yields the best performance.   While the original question might have been about estimating max winds, it is good to learn why that is not the best way to do things and why a torque meter is a valuable tool to have. 

I will get a little off topic for just a minute.  As a newcomer to the hobby, I find that a lot of question responses are from people who have been in the  hobby for quite a while and have collected all the necessary or neat gadgets.  And a lot of time these individuals give answers that assume that everyone has the same tools or the same level of interest in the hobby as they do.  It's nice/informative to learn the optimal way to do something but it is also helpful to learn tricks/techniques that a beginner can use when he doesn't have all the tools or knowledge of the long time modeler.


Keep the comments coming.   And thanks for all the information.

   George
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jakepF1D
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« Reply #13 on: April 14, 2017, 11:20:27 AM »

Each time you wind a rubber motor to near max it will stretch a bit more, so in successive winds, you will need more turns at launch in order to achieve similar results, all other conditions being equal.  I have found that after the second or third wind to max, the performance of the rubber motor in terms of the duration that can be achieved usually begins to drop off.

This is one way of thinking.  Another way of thinking is that a motor won't take the maximum number of winds possible until it's been wound hard several times.  I regularly hear people comment that their motor is tired and they need a new one.  This is actually when a motor is entering it's prime if you have it sized correctly.  I won't use a motor until it's been wound hard at least 3 times.  Just my 2 cents.
« Last Edit: April 14, 2017, 12:02:27 PM by jakepF1D » Logged
dslusarc
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« Reply #14 on: April 14, 2017, 11:42:58 AM »

Yeah I do the same thing, I make the motor I want then wind it hard several times then go fly. I want the "whipped' version to be the one I fly on. I knew some guys who use to fly on the first wind only. They blamed the rubber as being good for only one wind. Grin   
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Tapio Linkosalo
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« Reply #15 on: April 15, 2017, 02:21:25 AM »

Re: torque meters - they are actually really simple devices, so should be easy to build one. Especially as they are so fundamentally needed in indoor flying. Most simply, a piece of piano wire supperted at one end, and a freely rotating (nylon prop bearing makes easy support) hook in the other. Add pointer and dial, and you are done. Harlan has detailed instructions: www.indoorspecialties.com/articles/Build%20a%20Simple%20Torque%20Meter.pdf

Re: winding the motor. TO my understanding, rubber has molecule "chains" in multiple directions, and breaking in (or winding) breaks some that are strained most. Thus the motor takes more turns on every consecutive wind, but the energy return (and possibly torque?) does not increase, but may even decrease a bit. However as for indoor flying the number of turns you can pack in is so crucial (especially for classes with maximum motor weight, F1D and F1M), it is common practice to pre-wind the motor or use a motor with a couple of hard winds done for the decisive flights. However I'm not sure if the further winds are actually better than the first. At least risks for breakage are bigger.

 
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