A Bio-Inspired Hybrid Flapping Wing Rotor for High-Efficiency Micro Rotorcraft

Enhancing propulsive efficiency at micro aerial vehicle (MAV) scale remains challenging because low Reyn-olds 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.