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) CF analysis at Reynolds number of 2500. The calculated efficiency data show that both efficiencies (propulsive efficiency-qt and efficiency for producing lift-P-f) of the wing are optimized at Strouhal number (St) between Oland 0.5 for a range of wing pitch angles (upstroke angle of attack alpha(u) less than 45 degrees); the St for high P-f (St = 0.1 similar to 0.3) is generally lower than for high n(p) (St = 0.2 similar to 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-f 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.