If two (or more) rotors share a common vertical axis (eg, attached to the same motor mast) but rotate on different horizontal planes, are the rotors considered separate rotors?

Rotors are defined in rule 3.e as one or more blades that rotate on a common path around a vertical axis. The rules do not include any requirements for the amount of vertical separation between blades necessary, so blades that are separated to an unlimited extent may still qualify as being part of the same assembly. One blade could be on the top of the helicopter or motorstick and another on the bottom, but still be connected in a single rotor assembly. The "path" referred to in rule 3.e does not indicate that every blade must occupy exactly the same volumetric space on each rotation. Instead, it means that when an assembly is rotated around an axis, each blade also processes around that axis. A key test of whether blades are part of the same rotor are to determine if they are rigidly connected to each other. If one blade cannot rotate independently of another blade, they are on the same rotor and constitute a multi bladed rotor. To test if a rotor is single bladed, the students should demonstrate that it can rotate independent of all other blades on the helicopter without damage to the helicopter.