IDR gave the best description a couple of years back:
The unique and Frontier-patented feature central to the A160 is that the revolutions per minute of its main rotor can be reduced to as little as 40% of its maximum value, reducing drag and fuel consumption by half. This cannot be done with a conventional articulated rotor because it would lead to catastrophic vibration, so the A160 has a fully rigid rotor. The blades change in pitch, but the blades and hub are hingeless and stiff in the flapping plane - the hub is a solid forging from AerMet 100 high-strength steel - so that the aircraft is controlled as much like a fixed-wing aircraft as a helicopter. Roll and pitch moments are transferred directly from the rotor to the vehicle.
Reportedly, tip speeds dip as low as M0.25. The A160 can operate at a range of rotor RPMs: the original three-blade carbon-graphite optimum speed rotor spanned 150-350 rpm, while the new four-blade OSR system runs at 200-400rpm.
The other interesting part of the program is the ultra-efficient 650shp coupled diesel being developed: see
OPOC thread.
I/C