Sikorsky X2 coaxial heli developments.

[Graviman]

Dave, at the risk of going off topic:

Surely the Kolibri was not ABC because the retreating blades were not actually unloaded? The rotor shafts just were not designed with the intention of supporting fully asymmetrically loaded rotors. From memory of the assembly I saw in Coventry (I sent you photos) the hub was more or less articulated: with the direction of rotation chosen for optimal sideslip-yaw coupling would lead to unloaded retreating blades flapping down to considerable extent, along with flap back if ABC was attempted.

I'm not trying to diminish Flettner's genius, but just saying that high speed was not his design intent (at that stage).