Fig. 3. Sketches of the reference frames and wing motion. o₁x₁y₁z₁ is an inertial frame, with the x₁,y₁ plane in the horizontal plane. o'x'y'z' is another inertial frame, with the x'y' plane in the stroke plane. , positional angle of the wing; _min and _max, minimum and maximum positional angle, respectively; , angle of attack of the wing; ß, stroke plane angle; R, wing length. C_L,w and C_T,w, coefficients of vertical and thrust of wing, respectively.