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Author Topic: The Art of Rubber for Indoor Scale?  (Read 1917 times)
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Jack Plane
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« on: November 21, 2015, 08:56:39 AM »

I understand that power or torque is function of rubber width ('thickness' if you like) and duration a function of length, but I'm still trying to fully understand the art of rubber motors for indoor scale, so a couple of questions or so for any maestros willing to spill the beans...  Smiley

1./  Is there any reason for using two 'thin' loops of rubber rather than one 'thick' loop of the same loop-length (e.g. 2 x 1/16" rather than 1 x 1/8" or the suchlike)?  Is there an 'energy storage/release' reason behind this?

2./  A longer motor will absorb more turns (i.e. give greater duration) but is the relationship linear - e.g. will a loop of 2 x peg to nose length give a full 1/3rd increase in duration compared to a loop of 1.5 length?  Also, although power comes out of cross-sectional area, does an increased loop-length have any impact on the torque available during the different flight phases of take-off, cruise and land?

3./  What is the theory behind 'braiding' a motor, and is this ever used for indoor scale?

4./  Finally, why do some models have a cunningly-bent winding eye on the wire-shaft forward of the prop whereas others don't?   And, in the latter case, I've also noticed that some people use a small ring bound to the rubber, which is then attached to the prop-shaft hook.  Are these all just personal preferences?

Hope my questions aren't too silly!  Smiley

Jon

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DaveWC
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« Reply #1 on: November 21, 2015, 10:45:31 AM »

I can help with a couple of your questions.
Motor length I calculate by turns per inch, 1/8" gives me 80 turns per inch, 10" loop= 800 turns, 20"=1600 and so on. I wind short test motors until they break and back off about 20%.
Brading a motor makes it shorter so it won't bunch up in a ball, I don't use it on indoor models since they land with winds left in them.
The different hook ends are to try and stop the rubber from climbing up the hook and messing up the works.

Just my take on things FWIW
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DavidJP
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« Reply #2 on: November 21, 2015, 10:48:15 AM »

Very relative questions Jon - and ones in respect of which I am still trying to understand and get answers too.

In my relatively short experience I have found that indoor models - particularly scale - are fickle in the extreme. So here are my comments on your questions (rather than answers) and I am doing this partially was an exercise in finding out how much I think I know!! Roll Eyes

1.  Yes - sometimes. For example look at the chaps who fly F1B - large rubber duration models.  Some of their motors are seemingly 'hundreds' of strands of 1/16th. But I think it does depend upon the type of model and how well you can gauge what it needs against how it performs.  A friend of mine has a PP J3 Cub which is so sensitive a couple of "thou" in the motor thickness seems to make a difference.  He found that out simply by trial and error!  I don't think any off the Boffins will be able to come up with a specific answer like for that model use X and for that model use Y.  So far though as a rough guide with most small models that are fairly lightly loaded more thin strands seem to work better than fewer thick ones.  Recently I have been starting off with a loop of 3/32nd about 11/2[/size] times the 'ook to peg distance.

2.  In principle it appears the answer is 'Yes' it will (or should) give an increase in duration but I don't think in any degree of precision as you ask - 1/3rd for example.  I believe the prop is a large influence here.  You need to check the model is not "overpropped".  As to the "impact" probably only noticeably at the run down end rather than start up. But you can achieve a slow initial run as the model has to accelerate.

3 Braiding is useful where the model will continue to glide on after the turns are all used up  as it holds the motor together and thus the c.g is not affected by a slack motor flopping about in the fuselage. In my limited experience indoor models will land with some turns still on the motor so it is in effect 'braided'. There are I believe other virtues but not really relative to small indoor stuff.

4 The bent 'ook is a "S" hook ("Z") if viewed from the rear which stops the motor climbing over the 'ook and bunching and possibly stopping the prop from turning.  You can buy these from John at Flite hook and he also does nice little ones in aluminium called "Croket Hooks".  You can bend them as part of the whole prop shaft of course but some folk and it easier to have the S hook separate.  The idea is that you hook the loops equally over the two parts of the 'ook.  I take it my picture shows the one you mean?

I hope this helps a bit -but hopefully a Boffin will be long and put it all out for us.  I can only add that many years ago I asked a well known household name in this field very similar questions and they were met by much Hmm... well.... depends..... and then he said that he really did not like to recommend motor sizes as there were so many influences - not least the "weather"!!!  Which may explain the erratic behaviour of my chums Piper Cub?

For me the first hurdle is to get the model to fly reasonably steadily for long enough for me to see what it is doing. Then I can attend to any oddities in the flight pattern. Once that is mastered then you can turn to duration.  But altering the motor - particularly increasing the "grunt" then presents new problems.

PS Daves reply appeared whilst I was typing this - there you go - it is true - I talk too much!!
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Re: The Art of Rubber for Indoor Scale?
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calgoddard
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« Reply #3 on: November 21, 2015, 12:02:46 PM »

I'd like to weigh in on these questions.

1.  Some outdoor fliers believe you will get better performance by having more strands for the same weight and cross-section of rubber motor, e.g. a 10 gram motor for a P-30 made up of 12 strands of 1/16 inch rubber will take more turns and out-perform a 10 gram motor made of 6 strands of 1/8 inch rubber, assuming that both motors are made from the same batch of rubber.  Judging from what I have seen, particularly the experience of world class F1B fliers, I think there is some truth to this belief.  I have most of a 5 pound box of 1/16 inch rubber sitting in one of my refrigerators.  Every time I try to make up rubber motors for outdoor airplanes larger than an Embryo using this 1/16 inch rubber I seem to end up with a tangled mess, even though I have a jig for making up rubber motors (spools movable on large metal ruler). I make all my rubber motors for outdoor flying from either 1/8 inch rubber or 3/16 inch rubber.

For indoor, I never see multi-loop motors used by any flier, even world class fliers (e.g. Kang Lee, the current world F1D champion).  That's enough to dissuade me from the hassle of making up such motors.

2. I am not sure if increased loop length changes the torque versus turns curve for the same cross-section.  I would think any such relationship would be negligible in regard to the duration achieved.

On the other hand, the weight of the rubber motor has an enormous effect on the duration of indoor models, in particular.  There is a sweet spot for a given model depending on its type.  Some eperienced fliers go by rubber motor weight as a percentage of model weight. For example, a Penny Plane might fly for the longest duration with a rubber motor that is as much as 80% of the weight of the airplane (excluding the weight of the rubber).  In this example the rubber motor could weigh 2.4 grams.  Of course matching prop to motor after optimum trim has been achieved is the ticket to the longest duration, assuming the airplane has been built to the minimum allowed weight.  

Lew Gitlow has a nice graph in his book that shows the optimum rubber motor weight for most classes of indoor models.  

3. Braiding a rubber motor allows you to shorten the motor while maintaining the same cross-section.  This minimizes the chances that the motor will bunch up, particularly in the rear of the fuselage, and shift the CG, causing a stall.  I have not seen braiding used on rubber motors used in indoor flying. The maximum duration will normally be achieved when the model lands with turns remaining on the motor.

4. Reverse-S hooks are used on the prop shafts of outdoor models to prevent the rubber motor from climbing off the hook.  I don't recall seeing them used in indoor models.  O-rings are used at each end of a rubber motor for flying indoor stick models to aid in removing the rubber motor from the winder and the torque meter and hooking it up to the prop shaft and rear hook of the model.  With indoor cabin models such as Embryos and Bostonians, the rear of the rubber motor is retained by a motor peg, and the motor is wound inside the airplane, hopefully with a tiny blast tube surrounding the rubber motor.  

I see now that your questions are related to indoor scale. Sorry for digressing to indoor stick models.  Gitlow's graph recommends 36 - 40% for indoor scale models.  While the graph and accompanying text are silent on this aspect, I assume he must be including the weight of the airplane in recommending this optimum range.  Even with that assumption, that range seems on the high side to me.  You'll need to experiment with different rubber motors for each indoor scale model.  As the late, great Cezar Banks used to say "the stop watch doesn't lie."  
« Last Edit: November 21, 2015, 12:53:17 PM by calgoddard » Logged
DavidJP
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« Reply #4 on: November 21, 2015, 12:28:54 PM »

Calgoddard,

I agree with you about single loop motors for F1D etc. but the question was directed at scale. Maybe it is something "local" but quite a few people use multi strand here and also S hooks as well as O rings but not that many seem to use the latter on the small stuff. They do seem effective otherwise.

Nice to see though that broadly your views and mine are not that far apart.
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piecost
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« Reply #5 on: November 21, 2015, 12:31:11 PM »

These are some good questions and I will attempt to answer. I am by no means an expert and look forward to other poeples views.

1. For some outdoor models the total thickness of rubber needed exceeds that which can be achieved by a single loop of the widest rubber available (1/4" ?), so multiple loops are used. The flyer may vary the rubber width and number of strands to acieve the thickness and length needed (known from experience). For competition duration classes the maximum mass of rubber is fixed and so the flyer has to choose between a short-fat motor and a long-thin one of the same weight.

The maximum theoretical turns and wind-down torque versus turns should not vary with the use of multiple loops of narrower rubber of the same total cross section as a single thicker loop. In practice, more loops of thinner rubber might be preferred as they allow a finer tuning of total motor width and a strand breaking is less catastrophic/repairable.

Indoor models tend to need rubber in widths the same or lower than the standard available so a single loop is mostly used. Rubber strippers can be used to fine-tune the motor to obtain intermediate widths (say between 1/8" and 3/16") or less than 1/8". There is no point in using more than 2 loops.

2. The torque available is only related to the cross section and number of strands, not the length. It can be thought of as the power of the engine. The length can be thought of as the amount of fuel. Lengthening the motor will allow a longer motor run, but makes the model heavier and climb less. The indoor scale experts are able to get a model to gently just climb from a hand launch or RoG with a smooth progression from climb to descent and land. They may be winding to less than 80% maximum turns to avoid the rapid climb associated with the power-burst of a fully wound motor. Their motors are long enough for good duration. I think that there is a big benifit from having a light model. It requires a thinner motor to fly and more of the motor mass may be used in addition length which results in a longer duration.

As for motor length, perhaps 150% of the prop to peg length is a starting point. I found it impossble to wind a motor of more than 225% without terrible bunching, but have tried a motor of up to 400% in a model with a fat fuselage, where the bunches did not touch the internal structure. However, this motor was so heavy that an increase in duration was not acieved and cause trim changes when the bunches wound off. As to motor weight; an indoor scale model might use a motor 15% to 25% of the airframe weight. What do others think?

3. I think braiding adds a bit of tension, by shortening, to a multi-looped motor to avoid the nose block falling out when the turns runout in flight. Indoor models typically land with turns remaining and use single loops so it is not appropriate.

4. I think that non-climbing hooks were only really needed on indoor models which tend to have longer motors than outdoor models. I could be wrong.

Another thing is using an appropriate propeller. The motor has to be matched to the propeller and the model. It is allways worth looking at plans and trimming instructions for similar models as a guide.


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Pete Fardell
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« Reply #6 on: November 21, 2015, 12:45:11 PM »

3. Braiding a rubber motor allows you to shorten the motor while maintaining the same cross-section.  This minimizes the chances that the motor will bunch up, particularly in the rear of the fuselage, and shift the CG, causing a stall.  Braiding is not used on rubber motors in indoor flying because the maximum duration will normally be achieved when the model lands with turns remaining on the motor.

This question came along at a good time. Although it is true that motors in indoor scale models don't seem to be braided, do you think a braided motor would be less prone to bunching at the back end? I ask because my Antoinette's motor was getting bunched at the rear peg and I could really do without this problem so as to maybe eke out a few more seconds of smooth flight time.
Alternatively, if a reverse S is used at the rear, to hook onto the peg, would that stop bunching in the same way as it does when used at the front?
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calgoddard
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« Reply #7 on: November 21, 2015, 01:07:09 PM »

Pete -

In outdoor flying, another technique can be used alone, or in conjunction with braiding, to address the same CG shifting problem when using longer rubber motors. It has become known as the “wobbly motor peg” or the “rotating motor peg.”  Use of a wobbly motor peg can supposedly allow use of a rubber motor in an outdoor scale model that is about 4 to 5 times the hook to motor peg length.  I have never seen a wobbly motor peg used on an indoor scale model, but I have not seen that many indoor scale models fly.

George White summed up the problem pretty well in his article entitled Taming the Rear Motor Peg in which he stated:  “As the motor unwinds it wants to squirm about and flex up and down at the rear peg. The use of a solid peg interferes with that and the motor has a gathering at the tail.”

A wobbly motor peg is essentially an outer motor peg that rotates around a fixed inner motor peg.  

Tom Arnold explained how the wobbly motor peg works in his article entitled An Article About Very Long Rubber Motors in which he stated:  “The explanation of how it works . . . is that by allowing the motor to shift, roll, and twist at the peg as it unwinds it prevents that big knot from forming around the peg. For some reason it works itself out of a jam and lets the full winds come out. Now you still have a long sloppy motor to slide back and forth but you have gotten every turn out of it you could before then.”  

Some fliers prefer the inside diameter of the outer tube to be considerably larger than the outside diameter of the inside tube to achieve a certain degree of desired wobbling. Do a search on HPA or one on GOOGLE and you will find various articles on how to make rotating motor pegs and wobbly motor pegs.  In outdoor flying, some wobbly motor pegs take the form of bobbins that slip over a conventional motor peg.

There is no reason you cannot use braiding and/or a wobbly motor peg in an indoor model that I can think of.  Keep the area around the motor peg large and clear of obstructions to prevent knots from hanging up. Adding weight to the tail in the form of dual motor pegs will shift the CG rearward so you'll need to take this aspect into account in building and/or trimming your indoor scale models.  

P.S. - Your Antionette is beautiful and its flight at the contest which I viewed was gorgeous.  Congratulations on a fine showing!  
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piecost
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« Reply #8 on: November 21, 2015, 01:23:30 PM »

Some great posts!

I tried a reverse S-hook for the rear peg of a NoCal Cassutt. I think that it helped prevent the motor wanting to climb off, but it still bunched and was really difficult to remove with turns still on the motor.

I tried a bobbin on the rear peg of a peanut model. I thought that the loose fit for the bobbin was only to prevent jamming with peg bending; I didn't realise that a wobble was important. I may try a wobbly-bobbin.
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Art356A
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« Reply #9 on: November 21, 2015, 01:29:59 PM »

Jack's question 4 questioned the winding loop on the forward side of the prop hub. I think the general use of blast tubes obsoleted them.

The best anti bunching hook seems to be the Czech hook and its variations, like the SFA hooks. The "Z", or reverse "S", is just as good and more 0-ring friendly.

As for the wobbly peg, sewing machine bobbins have a 1/4 inch, or maybe 6mm, bore and run loose on a 3/16 tube.

a.
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Jack Plane
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« Reply #10 on: November 21, 2015, 01:33:21 PM »

I agree - great posts - thanks so much for the replies to my questions - and fascinating also to learn about rubber techniques for other disciplines, both indoors and out - valuable stuff!



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billdennis747
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« Reply #11 on: November 21, 2015, 01:35:55 PM »

At Interscale, I used a longish motor in a short distance, with a Gizmogeezer device in my Kit Scale model. The motor ran down with a few turns left on, and perfectly-distributed knots.
Many years ago, Dave Hipperson wrote (about Open Rubber duration) that distribution of knots depended on how, and how much, the motor is wound. Thus, even for low-turns trimming flights he would wind to max and then let it run down before launching.
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Pete Fardell
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« Reply #12 on: November 21, 2015, 01:39:35 PM »

There is no reason you cannot use braiding and/or a wobbly motor peg in an indoor model that I can think of.  Keep the area around the motor peg large and clear of obstructions to prevent knots from hanging up. Adding weight to the tail in the form of dual motor pegs will shift the CG rearward so you'll need to take this aspect into account in building and/or trimming your indoor scale models.  
P.S. - Your Antionette is beautiful and its flight at the contest which I viewed was gorgeous.  Congratulations on a fine showing!
Thank you! I could do with another 5 seconds flight time though to save my nerves in future competitions and I do think less bunching at the rear would help. The problem with the Antoinette is that there is so little space around the rear peg. The peg itself is permanent at the moment and I've been attaching the motor by means of a hook (not an s hook, just a normal one). After every flight the rubber has needed extracting from the peg after getting itself all bunched and knotted into a ball around this hook. Not ideal! I could possibly get a wobbly tube over the rear peg and just thread the motor round it without any hook. Really I think there is too much rubber (3 loops of 1/8) in too small a space, which made me wonder if braiding might help.

(Thanks for asking the original question, Jon, and sorry for hijacking it a bit!)
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Jack Plane
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« Reply #13 on: November 21, 2015, 01:45:28 PM »

...and sorry for hijacking it a bit!

Pete

No worries at all!  As I said a few moments ago, I'm glad I asked the questions and am finding every contribution to this thread quite fascinating.

Grin
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Pete Fardell
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« Reply #14 on: November 21, 2015, 01:50:01 PM »

Does a longer stretch when winding help with more even knot distribution? I'm still a bit of a chicken when it comes to hauling the stuff right out.
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RalphS
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« Reply #15 on: November 21, 2015, 02:46:04 PM »

Ref Pete's bunching querey at #6 - bunching tends to happen when the person winding "comes in" too fast.  Usual advice is put on half the intended number of turns then start to come in so that you finish winding on or about the final turn.  Bill referred to Dave Hipperson's views on this as above.  'Nuff said.

Bill's reply#11 mentioned winding to max then allow the prop to run off before launching.  The same effect can be had by winding to the max then unwind a few turns while the motor is still on the winder.  This is usually called "backing off" in the UK.  This takes off some of the very high torque levels when winding "skittish" models that may not like full power but can benefit from a lot of turns to get maximum power run.  I wouldn't think that winding to near maximum turns is desirable for typical scale indoor models.

To get to grips with both of these problems can be best learned from experience.
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Pete Fardell
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« Reply #16 on: November 21, 2015, 02:54:14 PM »

Thanks, Ralph. I do try to follow the 'start coming in at half turns' method but am still not sure if I go out far enough in the first place.
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Jack Plane
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« Reply #17 on: November 21, 2015, 03:27:30 PM »

... As to motor weight; an indoor scale model might use a motor 15% to 25% of the airframe weight. What do others think?

I no expert in the slightest, but I've just completed an 18" Keil Kraft Chipmunk, which I'll be indoor flying for the first time tomorrow, and which weighs 32.5g excluding motor.  I've prepared two 1.5 single loop lengths of rubber, one at 1/8" which I fear might be a bit underpowered for the weight/wing-loading, and so the other is 3/8", i.e. 50% thicker, just in case.

The loop of 1/8" rubber weights 2g = 6% of airframe weight.  The 3/8" is 3g = 9% of airframe weight.

However, my weight of 32.5g is a bit heavy and gives a wing loading of 0.6g/sq ft (but this is indeed my first rubber model in 35 years!).  I understand that an expert builder would be aiming for say 0.4g/sq ft, so a weight of about 20g before motor, thus the weights of the two rubber motors above would work out at 10% and 15% of airframe weight at most.  (Whether the thinner motor would yet be sufficient for a proper indoor flight at this more delicate weight I wouldn't yet know!)

Jon
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RalphS
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« Reply #18 on: November 21, 2015, 03:35:01 PM »

Thanks, Ralph. I do try to follow the 'start coming in at half turns' method but am still not sure if I go out far enough in the first place.

The rubber that we use is generally accepted to have a maximum stretch of 10x.  Any more and it may break.  For indoor models being wound to about 80% or less maximum turns I would guess that you could stretch 6x the motor length for a good wind.  So a 12" loop can in theory be safely stretched to 6'.  Whether you need that stretch depends on the duration that you need and the ability of the model to take that number of turns.  I hope that you have a little book with you when you fly so that motor size and weight, number of turns, number of winds and back-off, amount of stretch, flight time, etc., can be recorded and referred to the next time that you fly. Grin
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« Reply #19 on: November 21, 2015, 04:49:06 PM »

Pete -

Here is a technique that you might try to overcome the motor bunching problem in your Antionette as currently constructed.

Bill Gowen and others manage the knots on their incredibly long A - 6 motors with a pair of segments of plastic coffee stir stick, each about 1" long.  Both strands of the rubber motor feed through each segment.  One segment is located at the rear end of the rubber motor right next to the rear hook. The other segment is located next to the prop hook.  Bill is the current world record holder in the A-6 class (and other indoor classes).

Be discerning in your choice of coffee stir sticks. They vary in weight, internal diameter, thickness, and color.  Some coffee stir stick segments would add too much tail weight to most indoor models.  Maybe you would not need to manage knots up by the prop hook, but I would rather have something that might potentially serve a functional purpose than simply add clay ballast to the nose.  You'll probably need some extra weight forward if you use a coffee stir stick around the end the your rubber motor in your Antionette.      

This technique might just give you the extra five seconds you are looking for.

I hope this helps.  

But wait, you Brits don't drink coffee as much as us Yanks.  In the States it seems like there are coffee shops in most areas that have commercial stores and restaurants.
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« Reply #20 on: November 21, 2015, 04:57:42 PM »

Pete -

Just re-read Reply # 12.  You won't get three loops of 1/8 inch rubber through any coffee stir stick.  You might want to look into plastic drinking straws.  
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« Reply #21 on: November 21, 2015, 05:49:13 PM »

Before I took to using a T-hook I used plastic drinking straws as sleeving over the rubber to tame bunching of rubber, and still use a short length of same at the peg if the fus. is narrow. When the motor is stretched for winding, a lot of turns will fit into the drinking straw, and then of course they jam right into it when you contract the motor during loading - but the turns jammed into the straw seem to unwind fairly well nevertheless, on most occasions. The whole turns-stuffed straw can flip at an angle to the motor axis sometimes, but that's better than a chaotic clump of turns trying to climb behind the motor peg IMO. I also use drinking straw as a sleeve over the motor peg to allow freedom of movement as detailed above, with a knotted loop of 1/16" rubber round the motor and snugged up to the straw to keep the straw in place. The straw can easily crush or deform under the tension of  the motor, but it's light and replaceable.

I always thought that motor length affects torque - that a short motor wound to x of its breaking point will have more torque than a longer motor of the same rubber wound to x of its breaking point. It certainly feels like this in my experience, but maybe that's just because the ship is lighter with the shorter motor and so acts as if it's got more power?

Jon, 1/8" seems rather feeble for the span and weight you've got there - but I've never flown indoors! I have found that models seem to fly when very underpowered, but act all anæmic and land with acres of turns left on the motor. If the motor weight is 6% of airframe weight and the model's going to land with unused turns, I wouldn't think it would be up for long - but it depends rather on the prop and how draggy the model is.

Stephen.
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DavidJP
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« Reply #22 on: November 22, 2015, 04:52:55 PM »

Very useful and has helped my learning curve very much! 

I like the drinking straw idea and will be trying that.

Recently I have been becoming aware that the prop can be quite an influence too.  With your permission Jon could I ask for views on this from those who have contributed?
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Russ Lister
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« Reply #23 on: November 22, 2015, 05:14:51 PM »

Jon,
You may have noticed by now, but earlier you referred to g/sq. ft. rather than g/sq. in.
Sorry, aeromodelling OCD made me mention it!  Roll Eyes

When I'm not being lazy, I use the 'tube in tube' rear peg method too.

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« Reply #24 on: November 22, 2015, 05:39:08 PM »

Recently I have been becoming aware that the prop can be quite an influence too.  With your permission Jon could I ask for views on this from those who have contributed?

David - no need to request permission, just ask away - its all grist to the mill!   Smiley

Jon,
You may have noticed by now, but earlier you referred to g/sq. ft. rather than g/sq. in.
Sorry, aeromodelling OCD made me mention it!  Roll Eyes

Russ - ah yes, of course - I meant g/sq.in.!!  Grin
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