Fig. 1. Sketches of the reference frames and wing motion. OXYZ is an inertial frame, with the XY plane in the horizontal plane. o'x'y'z' is another inertial frame, with the x'y' plane in stroke plane. oxyz is a frame fixed on the wing, with the x axis along the wing chord, and the y axis along the wing span. , positional angle of the wing; _min and _max, minimum and maximum positional angle, respectively; , angle of attack of the wing; ß, stroke plane angle; V, free-stream velocity; and R, wing length. C_L and C_T, coefficients of lift and thrust, respectively; C_l and C_d, coefficients of wing lift and wing drag, respectively; C_d', x' component of C_d..