ABC, X2 Cruise
In fact, Sikorsky's earlier S-69 ABC didn't even need a gearbox for the rotors during cruise. Unintentionally, the rotors were driven aerodynamically like a gyrocopter.
Dave, perhaps I misunderstood, but if the "unintentionally" wasn't there, we could have agreed. The S-61F work led the way on that particular rotor behavior question, along with one or two others, the most attention getting being the tip mach related rotor stability issue.
Thanks,
John Dixson