The S-76D brochure features a claim of dual speed rotor. Can somebody expand on this for me? Thanks guys.
Among the S-76D helicopter’s features are all-composite, flawtolerant
main rotor blades; an advanced Thales avionics system and
autopilot; dual speed rotor with active vibration control; powerful Pratt
& Whitney Canada PW210S engines; a quiet mode for enhanced public
acceptance; and an optional Rotor Ice Protection System (RIPS) for
all-weather capability.
http://www.sikorsky.com/StaticFiles/...Links_1109.pdf