I searched this forum and I couldn't find this variable diameter rotor concept referenced anywhere. Mods, please move or delete if redundant.
Interesting video from 2007:
Helicopter Blade Technology - Variable Length Rotor - Video - Breakthrough Awards - Popular Mechanics
This concept relies on centrifugal forces to move the blade out at higher RPMs, and a spring mechanism to bring it back in at lower RPMs. Wonder what advantage that is given that most rotor systems operate at a more or less constant RPM (within green arc, of course).