This video demonstrates the effects chordwise balance and hub rigidity have on blade flutter resistance, as applied to small autogyro rotors.
Flutter is a self exiting vibration created by aerodynamic forces interacting with the flexibility of a system. In the case of a rotor blade, this instability can cause it change pitch and violently shake if moving fast enough.
Due to the rapid pitching, the overall drag of the blade is increased, severely reducing it's performance in auto rotation. To avoid this problem, the blades must be balanced very closely to their aerodynamic center, the 25% chord position.
