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Parts of a Bungee...

Old 04-21-2004, 07:15 PM
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Virgin_Blue
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Default Parts of a Bungee...

Hi,
I've been looking at some Bungee (Hi-start) options for my Spirit 2M glider...

Why do Bungee setups have a section of Bungee (Surgical Rubber) and a section of non-elastic cord (i.e. Fishing Line)???

Why isn't the full length Bungee Cord??

Is it to reduce weight that is on the Glider during the Start?
Is it to reduce cost and allow reasonable altitudes to be reached??
Is it to reduce the amount of launch force on the glider???

Thanks

--
VB
Old 04-22-2004, 01:01 AM
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FHHuber
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Default RE: Parts of a Bungee...

The actual "bungee cord" (or surgical tube) provides the power. You don't need 500 ft of surgical tube to supply the power to get a 500 ft launch height.

The tube is MUCH heavier than the line.. and lifting 500 ft of tube would be impossible for a 2 meter sailplane.

The tubing is expensive. ($1.40 a ft for the type I use..) and no one wants to pay $600 for a bungee launcher.

You could actually use LESS force if you used longer tube. The tube could store the same power in less percentage stretch and that would mean less pull against the model. You control the pull felt by the model by selecting the tube diameter and length for the desired launch height based on the model. (Ideally we would use a different piece of tube for each different sailplane design and desired launch height... but its impractical.)

The use of nylon monofilament fishing line is a BAD choice! Monofilament is an elastic material. you can store a LOT of power in the line. I have seen 8 lb fishing line snap and whip back slashing a friend's arm open... requireing 18 stitches!

You can not guarantee WHERE the fishing line would snap without using a shore section of lesser strength. The worst place for it to snap is appx 20 to 40 ft from where it is attached to the model, as it has the distance to have stored substantial energy, and can whip back at high speed, and the end will not slow down appreciably before it hits you.

So even though the monofilament is intuitively a good thing for launching the models (stores more energy within the line itself... light, inexpensive) its too dangerous to use it.

The new stranded low stretch fishing lines on the other hand... are still very strong, light and not overly expensive. You lose a bit of power storage potential... but add 5 ft to your surgical tubing and you more than make up for it. And its SAFER!
Old 04-23-2004, 05:35 PM
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aeajr
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Default RE: Parts of a Bungee...

That was a very good explaination of why elastic and line is used.

I fly a 2M Spirit among my sailplanes. Great flyer!

Here is another resource you might find interesting or useful.

http://www.labyrinth.net.au/~learmont/HiStart.html
Old 04-23-2004, 05:49 PM
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michpittsman
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Default RE: Parts of a Bungee...

Low stretch monofilament is available, but the problem is its susceptibility to abrasion damage from dragging it across the ground. Also, if you had 500' of surgical, you would have to stretch it about 1/2 MILE to get a decent pull, and probably 2/3 of it would be laying on the ground at launch release. Braided dacron fishing line in 80# is my choice; no stretch, modestly priced...JIM LSF796

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