After working diligently on a 'Santa Cruz flyer' for much longer than I had planned, I realized that - the only way I could get there was to drop 3 batteries along the way ($300). Not to mention that there was a small possibility that one of the batteries would fall and hurt someone. And still - if anything went wrong - like an unexpected side or head wind, it STILL might not make it. That raised the possibility that I might lose the whole craft somehow.
Too many unknowns.!
So, I started thinking about something that had a higher energy density than batteries. Like a petroleum product.
A gas engine would certainly provide the required power for a longer time than batteries, but multirotors will not fly if all the props are running the same speed (and therefore have the same lift). That is why - with my partner Stan Weiss we are building a gasoline-driven 'heavy lifter' that uses 6 variable pitch props. The pitch changes in order to keep the craft balanced.
But there is another, intermediate way: Multirotors use approx 80% of their power for lift and 20% to maintain balance. What if you used a gasoline engine to power 2 large belt-driven props running in opposite directions (to cancel out the rotational vector), and 4 smaller, electric-driven ones to provide balance. Since the electric 'balance props' only have to handle 20% of the lift, the circuits that drive them don't need to deal with hundreds of amperes. In fact, they don't have to deal with currents larger than a normal large quadcopter. These parts are available off the shelf. And the flight controller doesn't even need to know that 80% of the lift is being handled by the larger props.
Since the big props are belt-driven, the mechanical losses are low. The gasoline engine does have to run at variable speeds, so that it always maintains approx 80% of the lift, regardless of the throttle position. A servo connected to the motor throttle can handle that with a new output from the flight controller. I can write that code.
I know you are getting ready to ask how the electric motors get their power. Easy - from an alternator. A brushless DC motor can be used as a lightweight, high-efficiency alternator. The only problem is that the output voltage varies with the motor speed, and i- in general, alternators aren't very efficient (approx 45%). But with good electronics, the voltage output can be constant and the efficiency can be greater than 85%.
I did the calculations, and I came up with the following:
Gasoline engine: RCGF 56cc 2-stroke model airplane engine 5.3HP @ 7500 RPM
Alternator: NTM 5060 380KV 2665 Watt brushless outrunner
Props Quan 2 20" X 8" pitch. Quan 4 12" X 6.5" pitch.
Controllers - PixHawk + Rasperry pi + Teensy3.2 + PIC18F8723
A picture of the test setup as it is being built is below: