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Old Jun 29, 2002, 03:31 PM
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TedLarson's Avatar
Los Altos, CA
Joined Sep 2001
607 Posts
Bungee Launcher Hell

I built an EAM F-18. Good lookin' model! Flew it about 5 times handlaunch. Flies well, except the takeoff's are really difficult when hand launching. So, I decided to build a bungee launcher.

I had an old high-start here, so I cut off about a 25' section of surgical tubing from it, and tied it to about 8 feet of nylon cord. I built a PVC pedal launcher like the ones described on this forum. Went down the park this morning to test it out. I mounted the bungee into the ground using a screw-in dog stake. I figure, it can hold back a 90lb dog....it'll hold the bungee. Using a fish scale I drew back the bungee to about 8lbs of pull. Mounted the plane up on the launcher, and let-er-rip. Plane went out about 5 feet...settled onto the grass and proceeded to be dragged about another 30 feet, until it came to a stop.

So...I figure...not enough pull. I go out to the stake, and move it it out another 10' or so. Now my pull is really strong...about 10lbs. I mount the plane up on the launcher, step on the pedal, and away it goes!! AWESOME! Climbing out at a good 20 degree angle. I am thinking, this is going to be an excellent launch.....NOT!!!

The plane suddenly starts making a fast roll to the left, so I start feeding in a bit of right to compensate.....still not off the bungee yet. The roll continues, although a bit slower....finally...feeding in full right, plane is still spinning slowly to the left, comes off bungee in an uncontrolled spin, smacks the ground, and completely destroys the model.

Any ideas what went wrong?

- Ted

Here is my before and after:
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Old Jun 29, 2002, 03:52 PM
Purple power
gregg f's Avatar
shadow hills,ca
Joined Apr 2000
6,594 Posts
ouch
any twists in the rubber? was the power on?
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Old Jun 29, 2002, 04:11 PM
Guest
California
Joined Jan 2002
859 Posts
I saw the launch today at bayland. I think the rubber got stuck with the hook and hold the F18 back... that might be the cause of what happened to the F18 today
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Old Jun 29, 2002, 04:31 PM
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ScottS's Avatar
Houston,TX
Joined Mar 2000
1,217 Posts
Sorry for the loss. I can't say what really happened, but 10lb isn't that much pull for this size airplane. I would guess I use about 20lb but I haven't measured it.
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Old Jun 29, 2002, 06:23 PM
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dave morris's Avatar
orlando fl
Joined Oct 2000
5,714 Posts
Man that awful, OK sounds like you should have more nylon line, also is it possible the stronger pull jammed the ring really hard into the hook, how was the hook designed. I am no expert on Bungee launch (only done about 10 myself), but I recall the ring would fall off unless there was tension.
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Old Jun 29, 2002, 07:13 PM
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Ed Waldrep's Avatar
Las Vegas, NV
Joined Dec 1996
7,937 Posts
I have two bungees, one with 1/8" inside diameter tubing for planes under 3 1/2 lbs and one with 3/16" inside diameter tubing for heavier airplanes. I built the second one after my heavily modified T-33 ended up weighing 4 1/4 pounds. I got the tubing from the Dynaflight Upstart kits. They come with 33 feet of tubing, but I double it up so that you have about a 16 foot long bungee. I normally stretch it out 10-12 paces. The idea is you get the plane up to speed fast and spend little time on the bungee. I mount the hook about 35 to 40 percent of the distance back from nose to the CG and this give a fairly flat launch of about 10-15 degrees. I only use about two feet of nylon line from the release hook on the launcher to the towhook. You could tie a few inches of ribbon or something for air drag to make the hook come off sooner but with my luanches the hook has never failed to disengage. Don't make the ribbon too long or it will get sucked into the fan. You're probably better leaving it off anyway.

How steeply did the plane climb? You don't want a steep 45 degree sailplane like climb, the stall speed is higher on jets and you'll stall if you try to climb too steeply without enough pull. That may have been what happened to you. The more forward hook location keeps the nose down. I can see in your picture that the hook location looks about right.

Double up the tubing and that will give you a powerfull, quick launch. You may want to add more tubing but two 15 foot strands of 1/8" ID tubing should be enough for a 3 lb airplane.
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Old Jun 29, 2002, 10:24 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
4,508 Posts
My condolences on your loss. Losing a plane as pretty as that must hurt.

This is like locking the barn door after the horse ran away but a 5G tension is what I used when bungee launching my T33, or 12 lbs. What was the weight of yours?.
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Old Jun 30, 2002, 12:34 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
4,572 Posts
Re: Bungee Launcher Hell

Quote:
Originally posted by TedLarson
...The plane suddenly starts making a fast roll to the left, so I start feeding in a bit of right to compensate.....still not off the bungee yet. The roll continues, although a bit slower....finally...feeding in full right, plane is still spinning slowly to the left, comes off bungee in an uncontrolled spin, smacks the ground, and completely destroys the model.

Any ideas what went wrong?
Sorry to read about your mishap.

Did it look anything like the sequence below?

If so, then you might be a victim of the dreaded Bungee Death Roll Syndrome (BDRS)

IMHO, this is caused by applying too much elevator at too slow a speed causing an excessively high angle of attack which leads to a tip stall. Usually to the left as happened in your case and James, who's F/A-18 is shown in the sequence.

Dan
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Last edited by DanSavage; Jun 30, 2002 at 12:38 AM.
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Old Jun 30, 2002, 11:10 AM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
2,413 Posts
Gee, thanks for using my experiences

James
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Old Jun 30, 2002, 11:57 AM
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Tim H.'s Avatar
USA, TX, Fort Worth
Joined Nov 2001
4,356 Posts
Been there...Done that!

Same thing. Looked good, started rolling, crunch! After much head scratching and deliberation (see Bungee Launcher Survey thread) http://www.rcgroups.com/forums/showt...threadid=41096

I determined that I had too much rubber (50 feet) and the bug was spending too much time on the rubber. I have since shortened the bungee (25" doubled up) and am getting much cleaner launches.

Tim H.
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Old Jun 30, 2002, 02:38 PM
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dave morris's Avatar
orlando fl
Joined Oct 2000
5,714 Posts
Hmmm, I wonder if my F-16 had the tow hook to far back, at first I thought I had to much up trim, but maybe the tow hook was too far back, pitched up into about a 35 degree climb, I push over to try and correct but too late, air speed gone now I am coming down. was able to get it slightly under control. The tow hook is just behind the chin inlet, dont know how to move it up with out getting in the way of the duct. F-16 guys where do you mount the tow hook?
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Old Jun 30, 2002, 03:29 PM
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TedLarson's Avatar
Los Altos, CA
Joined Sep 2001
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I think spending too much time attached to the bungee was definitely a contributing factor. Althought, I don't feel like I was stuck on the bungee. It did release, however, at that point, I was not in control. I had a 8' piece of nylon cord between the rubber and the plane to prevent the rubber from getting hung up on the plane. Also, I was at full-power right off the ramp. I have lots of experience high-starting thermal gliders, and I knew I was going to have to keep my launch angle low. A jet just doesn't have the lift of a 2m sailplane...:-)

One thing I noticed was that I really had to stretch out the rubber in order to get the pull I was looking for. The stretched out bungee seemed VERY long. For the plane to release, I would have needed to be far enough down range for it to slack up, and come off. I think by the time this point was achieved, the plane had slowed down, and I tip stalled, thus inducing the roll. This would explain why the roll was uncontrolled.

I had about 20' of surgical tubing. To get enough tension, I stretched it to about 80'. As the plane went down range, the bungee seemed to be pulling the plane toward the ground. I made several corrections to get level flight. It seems to me that the release height of the plane is going to be about equivalent to an almost unstretched bungee. So, if i have 20' bungee my release height should be about 20'. Does this sound right??? If so, one might argue that you need a really early release when you are closer to the ground. This means that the fully stretched bungee should be as short as possible, even if you have to double or triple up to get it short. Also, it seems like having a good-sized, length of nylon cord on the front would be helpful in slacking up the line once the plane is up, thus getting off early.

After hearing some of your experiences, I realized I am not the only one to have a bungee launcher wreck my plane. It sucks too, because it seems to me that most bungee accidents are certain to cause alot of damage.

Thanks for all your tips and advice. Now all I all I need to do is fix my plane...


- Ted
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Old Jun 30, 2002, 03:36 PM
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TedLarson's Avatar
Los Altos, CA
Joined Sep 2001
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Quote:
Originally posted by Admiral_Red
Gee, thanks for using my experiences
Dan: Great photo sequence. Mine looked similar, although the roll occured further down range, just before coming off the bungee.

James: Is that your KC bug? What was the end result?

- Ted
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Old Jun 30, 2002, 09:49 PM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Quote:
Originally posted by TedLarson
Dan: Great photo sequence. Mine looked similar, although the roll occured further down range, just before coming off the bungee.
Thanks. It really shows what I believe causes a bungee death roll. IMO, too much pitch on launch. The photo sequence is across, then down.

12
34

If you look in photo #3, you can see the bungee below the model, so James' roll began before it came off the bungee as shown in pic #2, just like where it sounds like yours began.

To answer your question, no, your launch height should not equal the length of the bungee. (20')

Most of the launches I've seen, the model is, at best, about half that high. Most are lower.

If you were trying to climb to 20' then you probably were holding too much elevator on launch. A 20 degree launch angle is too steep. I usually try to just keep the model off the ground until its off the bungee.

We use a 20' length of Hollyday dipped latex rubber and stretch it 300%, so your 400% length doesn't sound too bad considering you're using surgical tubing. You should be looking for a pull of about 5 G's or 5 times the weight of the model you're launching.

We're using about 2-3' of nylon cord on the model end of the bungee. I'm not sure of the exact length, but it's only as long as it needs to be to keep the bungee away from the nose of the model on launch.

Most of the time, the model comes off the bungee right over the stake and the rubber drops into a heap just beyond it.

To me, it still sounds like the model tip stalled on launch because you were holding in too much elevator trying to maintain a 20-degree climb.

Dan
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Old Jun 30, 2002, 10:42 PM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
2,413 Posts
Here is the end result. Laying next to it is my 3M sailplane that got killed the same day Oh...oh, the end result... Too much time on the bungee, and pilot error. Rebuilt it and have flown it again with a much shorter bungee doubled over.

James
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