Fig. 1. Sketches of the model wings, the flapping motion and the reference frames. FW and HW denote fore- and hindwings, respectively. OXYZ is an inertial frame, with the X and Y axes in the horizontal plane; oxyz is another inertial frame, with the x and y axes in the stroke plane; o'x'y'z' is a frame fixed on the wing, with the x' axis along the wing chord and y' axis along the wing span. ß, stroke plane angle; , positional angle; , angle of attack; R, wing length.