Aerodynamic efficiency of a bioinspired flapping wing rotor at low Reynolds number

This study investigates the aerodynamic efficiency of a bioinspired flapping wing rotor kinematics which combines an active vertical flapping motion and a passive horizontal rotation induced by aerodynamic thrust. The aerodynamic efficiencies for producing both vertical lift and horizontal thrust of the wing are obtained using a quasi-steady aerodynamic model and two-dimensional (2D) CFD analysis at Reynolds number of 2500. The calculated efficiency data show that both efficiencies (propulsive efficiency-η_p, and efficiency for producing lift-P ) of the wing are optimized at Strouhal number (St) between 0.1 and 0.5 for a range of wing pitch angles (upstroke angle of attack α_u less than 45°); the St for high P (St = 0.1 ∼ 0.3) is generally lower than for high η_p (St = 0.2 ∼ 0.5), while the St for equilibrium rotation states lies between the two. Further systematic calculations show that the natural equilibrium of the passive rotating wing automatically converges to high-efficiency states: above 85% of maximum P can be obtained for a wide range of prescribed wing kinematics. This study provides insight into the aerodynamic efficiency of biological flyers in cruising flight, as well as practical applications for micro air vehicle design.