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Old Jun 17, 2012, 02:17 PM
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It may be that the rubber is too strong for the plane you are using. 8mm is around 5/16 in diamater. That would be good for a 2M if it were latex rubber. But I have no experience with silocone rubber so I can't compare it to latex rubber which is what the rest of us are using.

Buy or borrow a fish scale and do a pull test.

If the rubber is too strong you can't get enough stretch. So the energy is released explosively, like a fabric bungee. Latex rubber stretches to about 3X its rested length and contracts at a slower rate giving you a longer launch time turning into more altitude with less stress on you and the plane.
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Old Jun 17, 2012, 02:37 PM
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Quote:
Originally Posted by aeajr View Post
One of our members got a Hosemonster 4M hi-start. Man is that thing way too strong. I think we measure it at about 40 pounds pull at about 2.5X stretch and that was about all I was going to pull.
I use the 4M Hosemonster to launch my supra pro. It really rockets up with a 3x stretch. At first I found my arms getting first tired and then sore after several launches. So I have been working out my arms at the gym for the last two months and I can now launch many times a day without much soreness. I am 68 and tall and skinny so arm strength is not my forte, nonetheless, I can do it and it and the highstart works well.
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Old Jun 17, 2012, 07:53 PM
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Old Jun 17, 2012, 09:25 PM
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As you said, it depends on how you use it.

If you are looking for an F3J kind of launch and have a plane that can stand up to that the a short, super powerful launcher is excellent. But that is not what is typically meant by a hi-start. Those are typically called zip starts for their short length and their super powerful rubber that really "zips" the plane up. But don't use it with a Sprit or a Gentle Lady, you will snap the wings.

This thread is about hi-starts and those are typically 100 feet of rubber and 300 to 400 feet of line providing a pull of 12 to 24 pounds at 3X pull.
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Old Jun 17, 2012, 09:37 PM
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Old Jun 18, 2012, 07:13 AM
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The topic is learning to use a hi-start, so the people reading and responding here are typically people who have no experience with hi-starts and, often have little experience with thermal soaring.

For the most part I set up my hi-starts for maximum height launch.
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Old Jun 19, 2012, 02:14 AM
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I had another go with my HK hi-start last night... using the same generic 2m model.

My mate tensioned the line prior to clipping-on the model.

The "tension" equated to an extension of 300 feet. In other words, we walked back about 300 feet from the slack position. Of course, some of that distance results from the considerable stretching of the monofilament line, so I'm not sure by how much the rubber extended.

Anyway, the performance was much better, as you might expect. I'm sure that there is still a lot of "friction" from the long grass, though. On one occasion, the rubber/monofilament junction snagged in the grass and couldn't be tugged loose. We had to run down from the launch point and phyically clear the obstruction.

Keeps you fit, mind!

Next job will be to increase the extension to about 400 feet; that is, 400 feet of stretch. Will also try a decent model, like a Sig Riser.

I reckon that the length of our system was about 800 feet, when stretched. You need a lot of room.
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Old Jun 19, 2012, 07:16 AM
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If you have 100 feet, about 30 meters, of rubber and 400 feet of line then, yes, at full pull you will be about 800 feet.

I typically use mason's line which doesn't have much stretch so I can't judge the effect of the mono on the launch.
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Old Aug 13, 2012, 06:16 PM
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Thankyou all very much for this thread.

Hi,

Just finished over a dozen successful launches with my Hi Start. Without this thread I would not have been able to achieve this so thank you to all for the wonderful input and excellent comments to make this a success and keep my new molded beauty intact!

I purchased a hi start 3 meter from aerofoam which is a convertible model.
The field I used today is my neighbors, it is 12 acres and only enough room for 150 feet of rubber and 250 of line. I have started very conservatively and only stretched the cord out 80 feet for the first launch after a few hand tosses to set up camber and trim for launch. She went straight up the and flew of the hook at about one hundred feet above ground.

I am now stretching the line out to about 150' she is going up straight and flying straight off at 240 feet above ground.

Have not measured tension with a fish scale at 150' of stretch but it is easy to hold the plane ahead of the wing and javelin throw her with about 20 deg of pitch up as many on this thread have noted. After the toss she rotates and goes straight up.

My plane is a NAN Shadow 3.7 X Tail. CG is 109 and tow hook is at 105 behind leading edge.

Attached is PDF of the launches, Green line is altitude, you can see the hand tosses to set trim and the gradual build up in line tension as I get more relaxed with comfortable with having a rubber band pull my plane up.

Tomorrow I will add another 50' of stretch and try again

To all reading this thread, you can do it. If I can you can.....

I had it go sideways once at about 100' up the line a little rudder brought it right back very quickly, the guys on this are right on. It only took a little oppisite rudder and she was back on track

I love this new bungee thing..

Nick
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Old Aug 13, 2012, 08:53 PM
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That is a great report. And that is a wondeful glider you have there.

Thanks for sharing your success with us.
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Old Oct 03, 2012, 08:26 PM
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I have a question, maybe it's in the thread, but I missed it. What's the purpose of the very long line attached to the bungee? It it purely to give the plane a longer "arc" to climb on, or does it serve some other less obvious purpose? Thanks for the info.
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Old Oct 03, 2012, 09:13 PM
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That's about it, Jeff....
If you launch into the wind, the longer the string, the higher the launch, within reason.
In no wind situations, the rubber length becomes more important.

R,
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Old Oct 03, 2012, 10:22 PM
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Quote:
Originally Posted by Bugman Jeff View Post
I have a question, maybe it's in the thread, but I missed it. What's the purpose of the very long line attached to the bungee? It it purely to give the plane a longer "arc" to climb on, or does it serve some other less obvious purpose? Thanks for the info.
Like the string on a kite, it allows for a larger arch and a higher launch.
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Old Oct 03, 2012, 10:25 PM
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Got it, thanks
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Old Oct 04, 2012, 08:33 PM
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Quote:
Originally Posted by Bugman Jeff View Post
I have a question, maybe it's in the thread, but I missed it. What's the purpose of the very long line attached to the bungee? It it purely to give the plane a longer "arc" to climb on, or does it serve some other less obvious purpose? Thanks for the info.
Others have pointed out why a long line helps. An analytical answer is given in:

http://charlesriverrc.org/articles/d...artphysics.htm
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