This comes from that Register article:
One also notes that variable-rotor-revs tech like that of the X2 is quite rare, most whirlycraft spinning their blades at a fixed rate. Boeing's unmanned A160T uses variable-spin, and is
said to be "four times quieter" in the hover than a regular copter - not to mention having advantages in fuel economy.
Apologies if this has already been covered in this thread but this is the first time that I've seen a mention of this variable rrpm. Is this really going to happen or have the Register managed to get their technologies confused?