A Bi-Stable System for Rotor Span Extension in Rotary-Wing Micro Aerial Vehicles

Previous studies have shown that rotary-wing micro air vehicles have limited range and endurance due to the high power loading associated with small rotors. A span morphing rotor could provide performance improvements associated with a larger rotor diameter while maintaining the small vehicle footprint in constrained environments. This paper presents a concept where increase in centrifugal force with increasing rotor RPM causes snap-through of a bi-stable mechanism within the blade, resulting in rotor span extension. The design and analysis of such a system is presented and initial rotor tests showed asynchronous extension of the two blades due to dissimilarity. A cable-pulley mechanism was designed and integrated into the prototype to ensure synchronous extension of the two blades. Using high-speed camera footage for measurement of rotor span extension and RPM, the prototype rotor was observed to snap-through at about 950 RPM, over about ¼ rotor revolution (0.015 sec). The snap-through RPM calculated from the high-speed camera was verified using data from a Hall Effect sensor on the rotor hub. Model simulation results of rotor extension versus RPM showed excellent correlation with measurement.