This is only true for teetering rotor heads (eg: Bell, Robinson) where the fuselage hangs freely by the mast from the centre of the rotor disk. (Under -ve G the rotor and fuselage don't stay nicely aligned, and mast bumping happens - not good).
If there is a rigid/semirigid rotor head like on models, then there is no problem having the center of mass above the rotor. I have sometimes found models to be more stable and perform better when flying inverted.
For soemthing like this 'bike' - the thrust vector rotates with the vehicle - it will stay lined up with the CoG. Like rockets - it may appear unstable to have the motor at the bottom, but it is actually stable.
If there is a rigid/semirigid rotor head like on models, then there is no problem having the center of mass above the rotor. I have sometimes found models to be more stable and perform better when flying inverted.
For soemthing like this 'bike' - the thrust vector rotates with the vehicle - it will stay lined up with the CoG. Like rockets - it may appear unstable to have the motor at the bottom, but it is actually stable.