An over-simplified explanation (hinted in post #4 above):
- Cruise speed is driven primarily by profile drag at these velocities
- ~40% of rotorcraft drag is driven by hub area
- Hub area is directly related to rotor spacing on a coax
- Rotor spacing is a function of tip path plane deflection (flapping & coning modes)
- Tip path plane deflection is driven by rotor and blade stiffnesses (in steady level flight...recall even on X2, Sikorsky never published TPP clearances under high speed maneuver loads!)
- An increase in GW to FVL medium size will require a larger rotor radius to maintain disk loading and hover performance
- Rotor radius and GW drives loads exponentially, thereby necessitating a reduction in stiffness with current (even experimental) materials
- Reduction of stiffeness yields an necessitated increase in rotor spacing
I believe the upper bound of the ABC concept, barring the discovery of unheard-of-strong unobtanium composites, is in the area of 12,000 lbs (and none of this is to speak of the dynamics concerns of the higher GW rigid rotor).
My suspicion is that Sikorsky has
not found the magic bullet to allow the scaling of this concept above this size due to some of the political maneuvers they have made in the past few years (i.e. recall ~2005 they used to heavily market very large X2-based machines for JHL and other studies which has all completely ceased). It was always claimed that ABC scaled up...and it does!
Aerodynamically
All that said....I am sure that companies like Lord or Moog are desperately working closely with SAC to find a workaround, perhaps some active mass-shifting mechanisms in the blades to alleviate some of the loads. But I am highly suspect they have come up with anything concrete, or feasable.