I built a largish fixed-wing UAV (Hugin 2.6m, roughly 8 kg MTOW if I recall). It's definitely too large to hand launch, and it needs a surprisingly long takeoff run (especially on grass) to get airborne and clear of trees etc. This is fine when I have access to a private road or something that can be used as an airstrip, but when that's not available, things become difficult.
I'm toying with the idea of improvising a cheap catapult that could get it airborne in a much smaller space, either using a spring-loaded or gravity driven arm or even just a stretched bungee cord of the right length and thickness. Before I experiment too much, I thought I'd ask whether anyone else has come up with a good solution.