Dave,
Do you have any section drawings available for X2 blades? Just email me direct if you do. My heli performance program can now flap a single rotor to equality, so I'd like to see how X2 lift/downwash looks with cyclic trimmed for forward flight. It's early days so i am still developing the algorithms to handle inflow roll and counterotating configs.
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Nick,
When you comment about S-69 retreating blades going into deep stall at 2.0g at 25'000ft how was this noticed? Was there increased vibration from tip vortex shedding (multiples times per pass) or did the drive torque go up (or autorotating collective down). If rolling to a bank was there a need to monitor the pedals to stop any uncommanded yaw?