Howdy Ya'll. BHT recommends a 97% rotor RPM setting for cruise, it reduces vibes and provides a more comfortable ride. Also, beeping RPM down to 97%, mast TQ is reduced, and by adding collective pitch to re-establish cruise setting (more or less 80%), there is additional A/S to gain but no particular savings in fuel economy.
For all those whom are familiar with the 205/212/412 family tree, the XMSN remain largely unchanged. The 6600 RPM refers to the output from either the Lycoming T-53 or the PT-6 Twin Pack's combining gearbox to the main rotor XMSN.
This shaft (also known as long shaft) rotates at 6600 RPM at 100% setting and 6402RPM at......................97%.
So, in the end it's all and the same.