|Robert Armstrong 2||29/07/2014 08:52:23|
|234 forum posts|
I recently bought the Parkzone Ka8 foam glider together with the E-Flite Hi-Start kit. There seems to be a great deal of light cord to attach to the length of bungee supplied. The included instructions just say 'connect it all up' basally, as does the video that can be reached from the web site. Can anyone offer any information on the correct length(s) please?
I found one site on-line that advocated 10x cord to bungee length, but with a significantly shorter bungee. Is this a good rule of thumb?
Any help gratefully received.
|Bob Cotsford||29/07/2014 10:37:06|
8261 forum posts
I seem to remember using more like 3:1 but that was with surgical tube, and 5:1 sounds about right for bungee cord but that was 20 or 30 years ago so it's a bit hazy now. Someone is bound to have more recent experience, that or a better memory!
Just found it in George Stringwell's 'Thermal Soaring' and he recommends 30m of bungee with 120m of line.
So starting at the model end you have a parachute with the attachment ring to the top of the line passing through the canopy, the parachute strings are attached to a swivel, then comes the line which is connected to the bungee. I found it best to use heavy duty key rings or similar at each connection, this makes it easier to replace segments as needed. Look up 'blood knot' - recommended for tying the line.
DO NOT use a screwdriver or similar to anchor the end of the bungy, use a corkcsrew style dog stake as available from pet shops. You really do not want the anchor coming out when you stretch the bungee, it's a potential lethal weopon!
|Robert Armstrong 2||02/08/2014 16:46:58|
|234 forum posts|
883 forum posts
+ 1 for using dog stake to anchor the bungee.
Also check that when the line is under tension the parachute is pulled to the closed position, the 'chute should only be free to open once it has detached from the model.
From memory 4 or 5 to 1 bungee to one length sounds about right but weather will have an effect on how the system performs, a prevailing wind will allow 'kiting' on the launch which will allow you to exploit a greater line length than in flat calm conditions.
I like the look of this model, I hope it performs well for you.
Edited By avtur on 02/08/2014 21:31:35
|Frank Skilbeck||03/08/2014 03:59:22|
4610 forum posts
Robert our bungee is around 20m of bungee (surgical tube) and 80m of line, the Ka8 tows very well and we get +80m regularly. The model id a delight to fly, but I think the recommended c of g is a bit too far back and found it flies better about 10mm forward of the recommended point in the instructions.
It also aerotows very very well.
|2871 forum posts|
If the bungee supplied is not proper surgical tubing but more like cotton covered luggage rack hold-downs, I think you'll be in for a disappointing performance. The cotton covered stuff acts more like a catapult rather than releasing its stored energy in a gradual fashion as with surgical rubber..
883 forum posts
The OP, Robert, said he bought the E Flite hi-start which appears to be tube type (not luggage cord) so he should get good performance from it. Plenty of good stuff on YouTube about it.
11659 forum posts
As with the others i would always go with the George Stringwell recommendations. In essence his book is the distillation of the experience of hundreds of modellers. The pedigree of the experience is also of the highest standards, at the time he was an active member of the BATS club as a competitor.
It is interesting peoples opinions of the various energy materials, we all seem to agree, that cotton covered bungee, is the poorest spring, by a country mile.
There has always been a debate over is surgical tubing or solid extruded rubber better. The benefit from the surgical tubing was that it could be stretched by a bigger ratio over solid rubber. So in conditions of higher wind, the bungee could be stretched more after release, which with a good dive and zoom release, would give a higher launch. That is if the tubing was sufficiently meaty to give the initial pull and then to take the stretch on the climb.
On the other hand a meaty solid rubber section, may not stretch as much, but will give a positive initial launch. Then when near the top, the higher tension can be put back in by either straight forward stretching by kiting or by repeated swerving, to build a very high line tension, to go into the dive and zoom launch sequence.
The downside of both methods, is that overdoing it, will have you chasing either the elastomer and or line section, as the parachute carries it away down wind.
|Robert Armstrong 2||04/08/2014 11:23:37|
|234 forum posts|
Thanks again. Definitely rubber tube, not cotton covered.
OP = Original Post?
883 forum posts
Yes ... sorry I drifted into forum speak!
883 forum posts
Oops double post ... deleted
Edited By avtur on 04/08/2014 14:16:13
|Peter Beeney||04/08/2014 14:30:27|
|1587 forum posts|
Yes indeed, OP does = Original Post!
This thread brings back a few memories, I made a bungee launcher a long time ago, 30 yards of three eighth bore surgical tube, the orange stuff, and 150 yards of 70lb breaking strain nylon fishing line. Held together by reasonably heavy duty fishing swivels and heavy duty split rings. Homemade parachute, fabricated by Management, this lasted for years but did eventually wear out. The dog tie anchor post, too. We had a nice big field to fly in back then, so there was plenty of room.
After a fair amount of practise we found on a good day we could stretch this out about a hundred yards + and in breezy conditions it would catapult a 100 inch glider, such as Neville Mattingly’s superb Phoenix right to the top of a still stretched out line. It worked brilliantly, time after time and for many different models; although there were occasional adjustments needed to the piano wire wing joiners…
But at this sort of launch power it’s vital to make sure all is in order before letting go. I held the model pointing vertical and if there’s anything wrong, say like rudder trim slightly misaligned, it’s uncontrollable, and there is absolutely no chance of stopping it. It just goes up like a rocket on steroids!
We soon had a couple more of these, glider flying was very popular then, but the competition stalwarts wouldn’t use them; they still insisted on measuring their 150 yard tow line, and I spent a lot of time running about towing gliders up to height as well. They didn’t even like practising under false conditions. Towing was ok if there was some wind, but hard work when we were becalmed.
We did increase the length of the line, to the limit of launching distance, this did work well too, but there seem to be a point where the weight and drag of the line begins to limit the height at which the glider will reach, particularly if it’s a bit on the small side.
Also, like all aspects of aeromodelling, and indeed life itself, bungees I think need to be treated with some respect. The stored energy in a well stretched out line is considerable, at the very least a nasty smack in the face. We never had any incidents fortunately, but I like to think I always erred on the right side when making things secure: probably by a factor of at least 10!
|Steve Houghton 1||13/08/2014 15:06:52|
1904 forum posts
I use a high start bungee to launch my X Models 2m Whisper. I made up the bungee myself using 8mm? tubing from Hobby King, which cost about $10 per 10m and dirt cheap compared to some of the stuff available in Europe, and at that price it's cheap enough to replace annually if need be.
I used 30m of tubing and attached it to 90m of 80lb test fishing line and ending up with a parachute. Once connected to the tow hook on the plane, I then walk back 120 paces until there is good tension on the line and I can hold it reasonably easily with one hand.
The biggest problem is finding an open field, without any power lines going across it, or 3ft high foliage and not lined with tall trees. As it happens, I'm going to look at one this evening and if it's any good I'll see if I can contact the owner.
I've considered joining a club and using there field but I feel that un powered gliders are not compatible with powered planes and heli's, for several reasons.
Please login to post a reply.
Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!