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Normal Topic Rubber knot, where does it go? (Read 3032 times)
Sky9pilot
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Re: Rubber knot, where does it go?
Reply #4 - Apr 25th, 2014 at 9:21am
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Pete's right on the money with his latest reply!  Thanks Pete!

Czech hooks see picture below.

Reverse S hooks - Chris Boehm's video on how to make a reverse S hook found here on S&T Click Here Reply #2

Tom
  

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koutky_hook_style_part_05.jpg

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staubkorb
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Re: Rubber knot, where does it go?
Reply #3 - Apr 25th, 2014 at 5:39am
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Vibrating motors do absorb a lot of energy.  Causes are a number of things, but I have found that the prop hook style can have big effects.

The nature of the way the rubber "sits" in the wound state also has a relation to the way it will unwind.  This is one of the reasons you'll sometimes see the phrase "smooth out the knots" in some instructions.

The prop hook that I have found to work best for me in almost all cases is the reverse "S" (or "Z") shape in conjunction with an O-ring (plumbing supplies) or the "Tim Gray(Grey?)" loop on the prop end of the rubber.  Very smooth running (even without the Gray loop) because the hook forces the rubber to stay in the shaft plane.  For models with up to four strands of 3/32nd, I've had no issues with rubber "O" rings, but have had them snap on a Cloud Tramp motor (4 strands 3/16th or 6 x 1/8th @ 1300+ turns), so a bit of judgement is required.  The "Czech" and the "SFA" hooks are similar takes on a style to reduce the climbing tendency of the motor.  The SFA one is all wire and DOES need accurate bending skills and the Czech one is easily made from existing round hooks.

Another culprit can be a shaft that has a too long unsupported length which can result in flexing - and the different harmonics going out of sync.  The angle of the rubber run in relation to the prop shaft can also play a role - try to keep the angle as "flat" as possible.

However, one thing is a primary (if not THE primary) requirement for all hooks is that the center of the hook is as close to perfectly aligned to the shaft as possible!

I have never had any semblance of luck keeping a motor centered on the hook with the common (unfortunately) round or diamond styes.  The rubber always wrapped itself off to one side, resulting in vibration making the model act like Mile* C.

I usually place my knot at the rear, behind the peg, my feeling being that there is then a more linear load = less stress on the knot.  I DO see the reasoning for having it along the motor length, in that the sideways pull snugs the primary knot up against the "keeper" knots.  One disadvantage would be in a narrow fuselage.
  

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Sky9pilot
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Re: Rubber knot, where does it go?
Reply #2 - Apr 24th, 2014 at 11:46pm
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I generally put the knot at the anchor end as well.  I've started using neoprene "O" rings to make connection easier. 

For vibrations some use a bobbin at the anchor end.  Some use a tube in a tube setup with end caps so the middle of the anchor tube is able to rotate, I've never used this method myself.  I don't usually build models with the size of motor that would tend to bunch up that much.  But then again bunching isn't limited to big motors.

Tom
  

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bigrip74
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Re: Rubber knot, where does it go?
Reply #1 - Apr 24th, 2014 at 11:34pm
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Terry: l also place the knot at the motor peg for strength and have for as long as l can remember.
  
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terryman
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Rubber knot, where does it go?
Apr 24th, 2014 at 11:30pm
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Where is everyone putting the rubber motors knot in a model and why?  Does it make any difference worth mentioning.

I have been positioning mine at the peg, where it is least likely to cause the motor to vibrate while unwinding in flight but it is not ideal for keeping the tail end light as is desirable for most scale models.

Also, are there methods to deal with a vibrating motor?  I balance the prop (mass) the best I can by scraping the heavy blade and making the prop shaft as straight and true as possible but some motors run "smooth as silk" while others vibrate like crazy for reasons I cannot identify.  It think the vibrations waste a lot of the motors energy and maybe not good for the models aerodynamics.  Is this worth the effort to remedy and how?

Terry
  
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