I don't know much about rotory aerodynamics. but......
Would'nt a single axis counter rotating rotor eliminate the retreating blade stall ,limiting higher speeds in helicopters?.
RB if you think about it, both rotors would still have retreating blades (all helos have and always will) and thus there will always be the problem. The only way to eliminate it, rotate the rotors a la Osprey and AB609.
