A bio-inspired hybrid flapping wing rotor for high-efficiency micro rotorcraft

Abstract Enhancing propulsive efficiency at micro aerial vehicle (MAV) scale remains challenging because low Reynolds number aerodynamics, structural flexibility, and severe power constraints limit the effectiveness of conventional rotor design strategies. This paper investigates a new hybrid flapping-rotary propulsion concept, termed the Hybrid Flapping Wing Rotor (Hybrid FWR), which superposes controlled flapping on a rotating blade to exploit stroke-wise asymmetry while retaining a compact rotorcraft architecture. A unified analytical framework is developed, comprising (i) a kinematic model that captures mechanically constrained flapping and inertia-driven passive pitching with experimentally informed transition coefficients, (ii) a blade-element-based aerodynamic model to estimate stroke-resolved forces, and (iii) an experimentally fitted motor–power model to enforce constant input power while varying the hybridisation ratio. The resulting lift-coefficient evaluation accounts explicitly for unequal upstroke and downstroke durations. Model predictions indicate a consistent optimum hybridisation ratio near 0.7–0.8, where aerodynamic loading in the upstroke is minimised, and lift production is concentrated in the downstroke, maximising the cycle-averaged lift coefficient for a given power. More than 200 bench-top trials using a two-motor prototype corroborate the existence of an optimum near a hybrid ratio of 0.7, demonstrating up to a 2.148-fold improvement in power efficiency relative to pure rotation under comparable lift conditions. The findings clarify the physical mechanism governing the optimum and provide a practical basis for efficiency-oriented design and further high-fidelity refinement.