Two Blade Teetering Rotor. Many existing small turbines use 3 rigidly mounted blades. When a three bladed turbine yaws, the rotor places large gyroscopic loads on the turbine and supporting tower. In order to resist these loads the structure must be heavily built. This increases the weight and cost of both the turbine and tower. The two blade teetering rotor virtually eliminates these large gyroscopic loads allowing the weight and complexity of the turbine and tower structure to be substantially reduced. The teetering rotor hub allows the turbine to run as smooth and vibration free as any 3 bladed turbine - even smoother in turbulent winds.
One problem with free yaw downwind turbines is that they can occasionally get caught upwind. Aerostar turbines have a teeter hinge mechanism (patent pending) that substantially reduces the possibilty of the turbine being caught upwind. The design of the teeter hinge also provides acoustic isolation of the rotor from the drive train (patent pending) so that noise from the speed increasor is attenuated and not radiated by the rotor blades.
References:
Why Two Bladed Rotors Make Sense
NASA Spinoff Article discussing the Aerodynamics of the Aerostar Rotor.