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Bungee catapult ... where do I put this build thread???

Hai-Lee

Old and Bold RC PILOT
Mentor
#5
This should be a pretty short build thread. After all it's mainly some plastic water pipe and some bungee rubber!
Here's my very (very very very) rough sketch..
View attachment 130453
Just a few things to be aware of!

The ramp pieces will bow downwards under the tension of the bungee at launch so you end up with a "Ski jump" type profile during launch and with very small planes and too much tension it is possible to damage the wing underside or even break them due to the sudden increase in upwards angle at the end of the ramp..

As the release point/treadle, and bungee cord, (under tension), are not in line with the slope of the ramp. On release the initial pull on the aircraft will have a very large downwards component.

If you do not coat the ramp pieces in something very low in friction you will scar the wing undersides and after a few launches start sawing through the wings!

If you are open to suggestions I would suggest that you drop the ramp angle to 5 degrees and reduce the overall height so that the bungee does not have such a severe angle when it snaps to full tension on the towed/still stationary plane.

Finally ensure that the string(?) to the treadle is considerably longer than the string to the launch hook. If there is not such a difference it is possible, (happened here quite often until the message got through), the released treadle string is whipped upwards as the bungee tension changes its focus form the treadle to the tow hook and can strike and become entangles in the aircraft rearmost control surfaces.

Good luck with it all!

Have fun!
 

CarolineTyler

Well-known member
#6
Just a few things to be aware of!

The ramp pieces will bow downwards under the tension of the bungee at launch so you end up with a "Ski jump" type profile during launch and with very small planes and too much tension it is possible to damage the wing underside or even break them due to the sudden increase in upwards angle at the end of the ramp..

As the release point/treadle, and bungee cord, (under tension), are not in line with the slope of the ramp. On release the initial pull on the aircraft will have a very large downwards component.

If you do not coat the ramp pieces in something very low in friction you will scar the wing undersides and after a few launches start sawing through the wings!

If you are open to suggestions I would suggest that you drop the ramp angle to 5 degrees and reduce the overall height so that the bungee does not have such a severe angle when it snaps to full tension on the towed/still stationary plane.

Finally ensure that the string(?) to the treadle is considerably longer than the string to the launch hook. If there is not such a difference it is possible, (happened here quite often until the message got through), the released treadle string is whipped upwards as the bungee tension changes its focus form the treadle to the tow hook and can strike and become entangles in the aircraft rearmost control surfaces.

Good luck with it all!

Have fun!
I will lower the overall height was thinking about that too. The string I drew was incorrect in arrangement and I make that a bit clearer tomorrow and I've seen some add a small rubber ball on it to reduce that snapping effect. Not sure about the reduction of the angle - seen several simular systems where the initial 5-7 degrees was increased to around 15, which is what this was 'designed?' to be. It will be easy to make less/more anyway.
What do you think of adding some of that smooth pipe insulation along the runners? seen that a few times on other bungee catapults. What would you suggest to use?
Thanks for the input :)
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#7
I will lower the overall height was thinking about that too. The string I drew was incorrect in arrangement and I make that a bit clearer tomorrow and I've seen some add a small rubber ball on it to reduce that snapping effect. Not sure about the reduction of the angle - seen several simular systems where the initial 5-7 degrees was increased to around 15, which is what this was 'designed?' to be. It will be easy to make less/more anyway.
What do you think of adding some of that smooth pipe insulation along the runners? seen that a few times on other bungee catapults. What would you suggest to use?
Thanks for the input :)
What we use here which is the result of efforts of most of the club members is actually a simpler system than you currently have envisaged.
We make the ramp out of a length of 100mm, (or larger), diameter PVC pipe around 1200mm long which is cut in half lengthwise.

On each of the cut edges of the half pipe we have a length of 20mm PVC pipe with a lengthwise split so that they fit over the cut edge of the half pipe. At one end there is a hole drilled in the end of the half pipe for the ground anchor, (a tent peg). The other end of the half pipe has a block of wood fitted for the appropriate elevation.

The treadle is separate and the bungee cord runs down through the centre of the half pipe. For the aircraft there is a loop tied in the cord and the free end of the cord is quite long and is tied through a large mudguard washer which is then held by the treadle prior to launch.

Also remember to fit a safety mechanism so that the bungee cannot be released accidentally as the stored energy can cause significant injury or damage.

Have fun!
 

Wildthing

Well-known member
#8
I have never built one but anyone done one or tried with a sliding bed that the aircraft sits on? Bungee cord is now attached to the bed so no downforce on the aircraft so then it doesn't matter what angle you have it at. I tend to make things more difficult then they need to be and 50% of the time it doesn't work anyways. :D
 

Arcfyre

Well-known member
#9
I wonder if you could do this even more simply? Why not a high-start type setup with a single bungee attached to a string. Anchor one side of the bungee to the ground, tie the other end to the string, and the final end of the string gets hooked on the aircraft. Walk backwards to stretch the bungee and then release when the tension is right. I've been wanting to try something like this but I don't have the appropriate bungee material.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#10
I have never built one but anyone done one or tried with a sliding bed that the aircraft sits on? Bungee cord is now attached to the bed so no downforce on the aircraft so then it doesn't matter what angle you have it at. I tend to make things more difficult then they need to be and 50% of the time it doesn't work anyways. :D
You are correct that a launch "Dolly" could be used but like all things as you increase complexity you introduce other concerns and issues.
The Dolly has mass and so the bungee "Pull" must be proportionally higher.
The dolly has mass and can accelerate rapidly after the plane lifts off.
The sudden slowing aircraft after lift off and the rapidly accelerating dolly can cause the "Tail" cord of the bungee to whip up and strike the aircraft in mid air.
The dolly can actually bounce up off of the ground and end up in the flight path of the launched aircraft.
The dolly can travel a far greater distance than just the bungee cord would after launch.
A tethered dolly or an arrested dolly can also kick up in the flight path of the launched aircraft.
An aircraft launched on a dolly which is free of the bungee will still leave the end of the ramp and continue bouncing along the ground possibly in an erratic manner and can even veer to one side or the other.

Just a few thoughts on the issues!

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#11
I wonder if you could do this even more simply? Why not a high-start type setup with a single bungee attached to a string. Anchor one side of the bungee to the ground, tie the other end to the string, and the final end of the string gets hooked on the aircraft. Walk backwards to stretch the bungee and then release when the tension is right. I've been wanting to try something like this but I don't have the appropriate bungee material.
I raised a thread on something similar. It was a bungee system for ground launching aircraft, (NO RAMP), and it works very well still even on the large EDFs, (greater than 3Kg). Whilst it still used a treadle the use of the treadle is purely optional and without it it becomes a cheap histart system!

See: https://forum.flitetest.com/index.php?threads/the-best-20-i-invested.53587/

Have fun!
 

Arcfyre

Well-known member
#12
I raised a thread on something similar. It was a bungee system for ground launching aircraft, (NO RAMP), and it works very well still even on the large EDFs, (greater than 3Kg). Whilst it still used a treadle the use of the treadle is purely optional and without it it becomes a cheap histart system!

See: https://forum.flitetest.com/index.php?threads/the-best-20-i-invested.53587/

Have fun!
Thanks for the link, I must have missed your original post. If I can scrounge up the materials I may give this a shot.

I'm done stealing @CarolineTyler's thread though, back to you!
 

kilroy07

Well-known member
#19
Of course I have already checked out those links and the foot release is a shameless rip off
Yea, it was just all nice and "laser cutable"... ;)

I've had mine stored under the workbench for quite awhile now... after I see how your build goes, I might just have to go out and get some more PVC pice myself!