May I enquire:
What is the factor that limits the X2 configuration to only upto FVL-M? Is it due to the difficulty in having rigid rotors beyond this size?
Rigid rotor tip path plane divergence at larger rotor diameters.
The rotor scales essentially infinitely aerodynamically on paper. Building sufficiently light and rigid blades is the issue.
You can increase spacing to allow for blade flapping (particularly in maneuvers), but then drag increases exponentially as ~40% of helicopter drag is from the hub.
Note the published tip clearance from the 6000lb, 26 foot rotor diameter X2 was only ever measured in level flight, and was reduced by nearly 50% between 120 and 240 kt
