"The prototype is designed with no clutch between the main rotors and propulsor, which requires the pilot to increase forward speed through the variable pitch control on the six-bladed rear propeller. Once in the 180kt realm, the X2's computer will automatically slow the main rotors and increase collective pitch to prevent tip speeds from entering high-drag transonic region, with Bredenbeck correspondingly increasing propulsor pitch to increase the X2's speed as the propulsor also slows."
I'd be interested to see a schematic of the X2 driveline. If there is no clutching (ie. gear shifting) then the only way to reduce the speed of both the main rotors and pusher prop at the same time is by reducing engine speed, which would be rather inefficient for the turbine engine. If both main rotors and pusher prop are mechanically coupled through the drivetrain at a fixed ratio, then changing pitch of one/both will only change load, and not relative speeds.
A change in relative speeds between the main rotors and pusher prop (one speeds up while the other slows down) could possibly be achieved
without gear shifting through the use of a single input/dual output differential gearset. Where each of the two output drive speeds are a function of the relative rotor and prop loads. Such a drivetrain configuration could change the relative output speeds with blade pitch (load) changes.
Or maybe Sikorsky convinced Honeywell to add a second power turbine spool to their engine so that the main rotors and pusher prop can be driven independently. Hey, just because it's never been tried before doesn't mean it won't work!
Maybe someone can shed some light on how the X2 drivetrain changes rotor speeds.
riff_raff