Now many know that I am a passionate supporter and user of the Bungee Catapult system ... having posted many times my design and videos in use. Mine is not only successful design and many other people have modified / adjusted / created own designs based like me - on others designs ...
It works ... and I do think about a similar version myself for carrying to venues where I cannot drive stakes into the ground. Whether it will work for a larger model - I'm not sure.... but for a small to mod size ... say up to 64mm - why not.
Then we have mine which is similar to thousands of others ...
There are commercial versions available - Jetapult etc. as well as HK selling a ramp affair.
I'm not starting this thread to create a 'this is better than this' debate ... just interested in ideas / modifications / changes people have made to designs and why. There are many out there that have seen my schematic for construction ... seen others ... and I reckon there must be many out there thinking about it ... so lets help them get on and build their launcher .. in what ever form or design they choose.
I posted here on General section and not the usual EDF / Pusher sections as Nightfyyer shows - his ramp works for wheeled prop jobs as well !!
The A-Pult .. self-contained job ... I would look to see if any way to have a release mechanism around the hook point instead of that trailing line. It's ok on my launcher as the line falls away on launch - but there the line stays running along the wood 'runway' ... no good for pushers.
Hobby King does a neat in-line Glider release ... that can be operated by Tx ..
OK ... been thinking and I hope if Nightflyyer is on here ... that he forgives my meddling with his design.
I would remove the trigger line that runs back to the mechanism at rear. This is liable to accidents with models as it cannot fall away like my open rail version can.
It needs a release point that is in line with or just ahead of the hook point.
This can be done by using the Hobby King glider release kit ... set into the ramp bed with a Y line of the bungee.
The release is operated by a small servo and Rx with NiMH pack underneath the ramp bed. Use of the Tx gear switch or any other if you want to retain retracts ... Or even a servo tester style arrangement.
The ramp length .. I feel is a bit on the short side and may benefit from a bit longer ... and I wonder if it could be hinged to fold in half ? The only problem then is the cord needs to be sorted.
The extreme forward pulley - mmmm this is open version and bungee falls away nicely - but then would disappear under the ramp and you need fish it out to re-rig. It needs a deep U guide fitted to stop cord falling way too far and trap the end so you only have to pull and latch ...
Would it be better to have a V of bungee under the ramp instead of a pulley based turns ? The launch bungee being connected to the mid point of the V .. so you stretch the V under the ramp bed ... the launch bungee stretches as well - ...
My post is to try and see if any simplification and improvement of 'pull' can be made ..
I've even considered whether we really need the under-bed bungee ... A V of bungee cord with ends fixed UNDER the forward edge of the ramp bed ... pulled back and model launched by the stretch of the V. The V would fall away to lower than the ramp bed once model reaches end ... clear of any rear prop etc. Basically a Crossbow style.
The "slingshot V" bounces around after it relaxes (releases its energy into the projectile)... no guarantee it will drop out of the way of a prop. It might even bounce up into the prop of a pusher if the line between the anchor points is below.
A crossbow's string is always under tension. So its bouncing is very limited, plus its damped by the bolt rail. The crossbow string brushes against that rail the whole way down. We wax the rail to minimize wear and then wrap the string with a sacrificial "thread" (if you fail to replace that thread in time you need a new bowstring)
This is not going to fall out of the way at all...
I know the level slingshot, crossbow cord would do as such if it was strung at same level as the ramp ... I have a Bow and various at home for Rat shooting in my barn !! along with over-sprung Air-rifles ,,, SHOOSH !!
If the V cord is fixed UNDER and back from the front edge ... the tendency would be for the cord to want to fold under as it releases ... if we attach a metal ring as needed for the hook - this will also aid the drop away - as it does on my laid-out Hi-Start (people said my release line being longer than model line would catch pusher props - but it doesn't - in fact it's not even anywhere near being caught).
It's an idea to promote thoughts on how to ..
The best in my view of all launchers is still the laid out Hi-Start style as the bungee has a long stretch that pulls the model over a distance instead of a short jerk. The line falls away nicely etc. etc.
But there must be a way to improve the self-contained ramp style ...
I want to develop a version as a couple of places I fly - you cannot drive any stakes in for anchors. I need to have a totally independent self-contained unit that will launch my 70mm's as well - which I don't think Nightflyyers can ...
Yep, the thing I like about the dolly is that just like regular landing gear the model only has to lift off when both the pilot and the model is good and ready to lift off. But it has the advantage over regular gear of not having to carry the drag and weight of the landing gear in flight.
The bungee and ramp on the other hand is a 'death or glory' kind of approach, there is no going back once the release lever is pulled.
i need the bungee launcher for the parkjets and funjet. i have become really good at the frizbee left hand toss on the superfly and am begining to think i might be able to toss the hk pkj the same way. i won't try it with the funjet though.
the bungee launch isn't so much the issue of do or die,rather the models nigel and i use the launchers for are make or break launchs do to being way over powered. if i was using the launcher for a tamer plane i would have more success at launchs.
What about building a ramp with a long (10 feet?) arm off the front with a pulley. Maybe a couple legs to support it. Then you could have a much longer bungee cord and yet still be free standing. You could pvc pipe for the arm.