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):
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...