With the amount of equipment in my X8 is on the increase, so is the weight. Its becoming very difficult and somewhat dangerous to hand launch.
Iv decided to build a catapult. I dont like the idea of a bungee launcher because of the amount of flat space required. some places I fly dont have this.(eg: off a cliff =) ) I really wanted something self-contained and foldable.
The APM flies the X8 really well BTW =)
If anyone wants to build one I have posted all the info and pics here
Comments
Hi Bret, I am using standard hardware store bungee chord bought on a per metre basis. I originally used this on the pulley system. About 1.25 metres attached to the base of the rail, up and around the pulley, and back down onto the carrier. This was each side. However this did not give enough pull to launch the plane. Even when I stretched it so much that there was no more elasticity. Therefore i went out and bought the bungee, and swapped with the tubing. There was still not enough pull.
So i ditched the pulley system, and did what you have done and had two loops, which means 4 chords going to the carrier, this was the only way to achieve close to the amount of pull needed for a successful launch. But still wasn't enough.
I am going to go for a hybrid approach, as mentioned a few posts back, and use bungee on the pulley system, and also some loops as you have done. This will ensure a good pull all the way through the rail.
I would have thought that the force measured with a fish scale when the bungee was at tension would give an idea of the pull on the bungee? Therefore 10kg will give 5G on a 2kg plane?
I sorted the issue with the carriage coming off the rail, I simply added a second thick plate beneath the rail to ensure the bolts do not spring out. I think this is absolutely necessary, given the position of my bumpstop.
I'd like to be corrected on any of the above as you have already gone through this pain!
Cheers, Rich
What bungee cord are you using. I used latex tubing from a local medical supplier. It is capable of storing a lot more energy than standard bungee. The Force you measure while the bungee is stretched is NOT a reading of the stored energy. Bungee cord is designed to absorb energy and not store energy. For that reason,I recommend you use latex tubing.
I believe that is main problem with your launcher Richard otherwise it looks well made.
Did you sort out the issue of the carriage coming off the rail?
@brett can you please make list of all materials you bought and how you used them? That would be really helpful.
Hi Bret, That is exactly what I am doing! I am going to re-install the two full length bungees, then add either one or two loops of bungee to give it the initial speed. It is clear from the video the carrier slows down half way along. Possibly resulting in the prop striking the metal. Fortunately the prop broke perfectly enabling me to do a circuit and land!
I'm wondering what bungees Bret used originally to launch a near 4kg x8. As mine is less than 2kg, and still struggling!
Richard: Regarding the need for the carriage to pull through all the way to the end (to avoid the slow down at the half way mark) I wonder what would happen if you took a hybrid approach to the bungee/tubing? What I mean by this is, how would it work if you kept the two double bungees from one end to the other, but also added two more bungees that stretched from the back end around the pulleys and back to the carriage? This will give you a firm thrust from the release and the two that stretched around the pulleys wil have enough tension to continue the carriage travel to the end of the track. Just an idea...
Bret C
Yeah I agree, I've slowed the video down to show what happens to the plane after leaving the catapult, there is a significant drop before it starts to fly. The carrier also slows down about half way up the rail, as the bungees only pull for the first 50%. The pulley system didn't work as even at max tension, just two bungees isn't enough. Two loops just about got it airborne, but i wanted to use the pulley system to get power the entire way down the rail.
Slow motion video...
Looks like it needs at least double the power...
Tried it out for the first time today - the prop kept catching when it exits the catapult - even though it has at least 3 inches of clearance. Here'e the launch (It took an inch of either end of the prop!)
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