[spoonjim]

A fuel powered drone uses collective pitch propellers. The blades all spin at fixed RPM but the angle of the blades on each propeller can be adjusted individually with a servo. It actually far outperforms electric motors in maneuverability.

[toss1]

there are a few examples of those, but they are pretty much one-offs. This is mostly because one blade pitch mechanism is messy enough with a helicopter, 4-8 with a multi-rotor is just too many critical moving parts to be effective.

Most of the fuel powered drones will be basically an internal combustion engine powering a generator, which then feeds into the regular battery & ESC circuits (that's the type of machine I was working on).

EDIT: Also important is that much of the yaw control of multirotor drones is done by changing individual rotor speeds to create a net yaw reaction from the acceleration/deceleration of the rotors. With constant-RPM motors, that goes away, along with a lot of yaw control authority.