I recently was reading a thread about someone wanting 60 minutes flight time on a large octocopter and other saying they wanted to power their quad with a combustion engine.
Neither of them knew how to get what they wanted then i had a great idea (i think its great)
Rather than make devastatingly complex gears and clutches +custom electronics for a combustion copter or have a giant battery for long flight, i found a great middleground
A mini on board Generator.
Basically my idea is as follows
Buy a standard mini Nitro engine and a beefy DC motor or make a custom generator coil.
Hook the shaft of the nitro motor to the generator motor. hook up two motors or bigger motors for more current.
more revvs = higher voltage or current.
Hook the generator to a power regulator set at whatever voltage required eg: 12v
then power everything off the regulator using standard electronics.
You can have vastly longer flight times and easy refueling requiring no spare batteries
and you can have a backup battery if the motor should stall.