A few weeks ago Brett Binnekade put a catapult launch system http://diydrones.com/profiles/blogs/skwalker-x8-catapult on here for his X8 that worked pretty well. I value my fingers and arms and decided to make something along the same lines. I give full credit to the orginal design.to Brett.
I used the same general layout and tried several times to get a joint to work reliably to enable the ramp to brake down into 2 sections so it will fit in a normal car. The method I use now works very well.
The other problem was the stiffness of the trolley, the design I now use is much stiffer and reliable.
I used more elastic which is now over pulleys to get a longer pull and a stopping system for the trolley to avoid smashing the trolley into the end stops at the top of the ramp.
The final result can be seen in operation at http://youtu.be/Eobl_iSa_cs The ramp has operated more than 100 times with no failures and the launches are very repeatable and positive..The catapult and cradle can be modified for most aircraft up to 6-7kg.