[duskwuff]

Balancing and maneuvering a multirotor requires fast, precise control of power output. It's difficult to get that from a gasoline engine.

[RealityVoid]

Simple, add a variable pitch propeller. There are people that did it.

[6nf]

The main reason multiprop electric drones are so cheap and popular is because you didn't need varipitch props any more. Petrol powered drones existed for decades but they were always more expensive and prone to failure than the new electric fixed prop multirotors.

[photogrammetry]

If you think adding variable pitch propellers to a multirotor is "simple," you're sorely mistaken :-)

[baddox]

There's not much point in debating what "simple" means, but it has certainly been done. There are collective pitch multirotors that are mass produced, marketed and sold to recreational customers, and quite stable. Granted, most of these still use electric motors. The point is for 3D flying, not the potential longevity of a combustion engine.

The first commercially available one I know of (no longer produced):

http://www.curtisyoungblood.com/legacy-product-support-curti...

A knock-off of the Stingray:

http://www.hobbyking.com/hobbyking/store/__77122__Assault_Re...

A big exception: this thing is massive and works with nitro and 2-stroke engines:

http://www.curtisyoungblood.com/legacy-product-support-curti...