Rubber

jander14indoor
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Re: Rubber

Postby jander14indoor » November 24th, 2009, 10:23 am

After as much looking as I have time for I couldn't find the original disussion where the details were covered. On the comments I could find, Jan 09 and Jun 09 were mentioned as particularly good batches of TAN Super Sport. The discussion I remember said to expect continued better performance due to recent formulation changes, but I haven't refound that string. Arrrggghhhh.

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Livonia, MI

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Re: Rubber

Postby calgoddard » November 24th, 2009, 12:35 pm

June 2009 and September 2009 batches of TSS were sold out when I checked last week.

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Re: Rubber

Postby blue cobra » January 28th, 2010, 5:00 pm

I'd like to use a torque meter, and the best plans I saw seem to be these. So, if I am correct, a torque meter can really just be a length of steel music wire fixed at one end with a hook at the other so you can twist it. If this is the case, the real challenge in building a torque meter is making the standard to which you will measure the movement of the needle. I assume this calculator is meant to assist in this. (Click the link then: Utilities>Torque Meter Calculator...). So...

What range of torque would our motors need?

How do you use that calculator?

Could I use my .020 music wire?

Could you put on enough torque to make the music wire permanently twisted, so that the needle does not return to its original location?
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Re: Rubber

Postby jander14indoor » January 29th, 2010, 7:50 am

Yep, torque meters are as simple as that. Several plans available, which is best depends on tools available to you and your skill set. I don't mind soldering, so I used a different design, but that will work.

I find 2 in-oz for the full 360 range of the dial to be good for SO. Smaller motors tend to break around 1-1.2 in-oz, larger around 1.8 in-oz. For a good plane, launch torque will probably be in the 0.4 to 0.8 in-oz range, depending on ceiling height.

A 0.020 diameter for the torque sensing element of the design (the free straight length between the anchor and the pointer). That calculator can be used to iterate the length to a desired range. I set the units at 2, diameter at 0.020, and varied the length till 2 in-oz was as close to 360 degrees as I could get it. Around 9.3 inches.

Note, there are a lot of assumptions under that tool, if you want to understand it better, go to the referenced site for more details. Example, it assumes a steel wire for the stiffness coefficient. It assumes that you pick a wire size and max torque that does not permanently deform the metal (exceed its elastic limit) at 360 degrees twist. (0.020 in diameter wire for a 2 in-oz full scale range is OK)

Be warned though, I've found that torque meter hooks made from wire as fine as 0.020 tend to bend/open with use, sometimes releasing a tightly wound motor at a very inconvenient time. Two possible solutions I can think of. One, double the hook part back on itself to the pointer and bind it togehter with fine copper at the pointer. The other is to solder a thicker piece (say 0.032) onto the sensing element at the end, making the hook from the thicker wire. That's what I did. Make sure the free length of the sensing element is the desired amount between the anchor and where you solder it as that 0.032 wire won't twist at all compared to the sensing element.

Comment on calibration. You can find procedures on the web, I think one is described on the Free Flight Fantasies site. But frankly I'm not sure I'd bother other than as a demonstration of procedure to the students. I've found the calculated scale accurate enough, and careful building has allowed my meters to be comparable to each other within usable accuracies. You don't need much beyond two to maybe three digits of precision for these things to be invaluable. Of course calibration DOES let you compare to others. Which is a key element in science.

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Livonia, MI

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Re: Rubber

Postby blue cobra » January 29th, 2010, 10:10 am

Thanks, I think I'm understanding what you're saying. If the distance from the right angle at the end to the needle is 9-11/32 (9.34375) with .020 music wire every 90 degrees would be approximately 0.5 in/oz of torque. So, I could draw a circle, mark every 10 degrees or so, and attach it behind the pointer to have a scale close enough for my purposes. Then I could make my hook/pointer like below and solder along the grey line.

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Re: Rubber

Postby jander14indoor » January 29th, 2010, 3:04 pm

Oooooo, pretty pictures, I'm jealous.

That said, you got my word description exactly right. Though you don't need to go as heavy as a 0.0625 hook for this size torque meter. 0.032 is more than adequate. Its WAY stiffer than 0.020 (surprisingly so for such a seemingly small change) and won't affect the readings if used as drawn.

And you got the calibration part right too. I drew up the dial on a simple drawing program with marks at every 0.1 in-oz and ticks at the 0.05 marks between. More than adequate to the purpose. Looked pretty spiffy too.

I used to make batches and give them away when I did more hands on coaching, and never found more than about 0.05 in-oz variation between my meters at full scale. Again, more than adequate for this purpose.

Just use the most accurate length measure you can lay hands on while building, and build careful.

I also set up a process where I soldered my anchor and hook onto the sensing element before I assembled it into the holding frame. That let me check and correct the element as needed before burying it in an assembly.

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Livonia, MI

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Re: Rubber

Postby blue cobra » January 29th, 2010, 10:50 pm

Ok, one more question and I think I'll be set to build. The .020 music wire, at the end opposite the hook, it will be bent at 90 degrees and sandwiched between two pieces of wood. I need it to be 9-11/32. Would that be measured from the bend to the needle or from where the wire exits the wood to the needle?
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Re: Rubber

Postby jander14indoor » January 30th, 2010, 5:16 am

If it were VERY thoroughly glued into the wood so it couldn't turn you'd measure from where it left the wood. Since its not likely to be glued that tight, you should probably actually cut a small groove along the straight section as it exits so its completely free and measure from the bend.

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Re: Rubber

Postby mg » January 31st, 2010, 3:53 pm

Could someone explain half motors? Do they use the same toruqe either way? If I used a half motor, and it went 20 feet, would a whole motor also go 20 feet? Also what would you use as an extention piece?
Thanks, MG

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Re: Rubber

Postby gh » February 1st, 2010, 1:53 pm

Half-motors are wound up to the same torque, but contain exactly half the mass of rubber than a full-length motor would. For a given torque, a half-motor will deliver approximately half the flying time and half the maximum height that full motor would, making it an useful and quick way to practice for a higher site and/or to trim without having to make full-length flights.

I believe the extension spacer needs to be half as long as your hook-to-hook distance (so that the half length of rubber doesn't get stretched and exert more torque) and of course needs to be half the mass of a full motor. It also needs to withstand the torque, so I think a piece of basswood plus additional adjustment mass (glue, clay, etc.) is pretty common.
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