According to Lissaman and Shollenberger’s article published in Science in 1970, birds fly in formation to reduce the overall energy expenditure for flight. Thus, this work adopted the same principle of flight formation to ornithopters to save energy. Two experiments and one numerical simulation try are described herein. In the first experiment, a rigid frame housing three ornithopters was made and a tethered flight of it around a fixed suspension point was performed to evaluate the cruise performance of the ornithopter formation and check for energy saving by monitoring the endurance against a known reference value. In the second experiment, a jig housing three ornithopters was subject to a wind tunnel test at the Wind Engineering Center of Tamkang University. The aerodynamic force evaluation of the formation was done. From the preliminary examination, it is found that 3 m/s, 10° angle of attack was the best case for ornithopter formation subject to 14 Hz flapping to generate largest lift and thrust. In the unsteady numerical simulation using COMSOL-Multiphysics, the simple case of two-flapping wings dogging streamwisely was demonstrated. The upstream condition of 1 m/s and 20° angle of attack was formulated and the output instantaneous resultant force on the flapping wings reveal the obvious difference from the single-flapping case. When compared to the real birds flying in a formation, the energy saving of ornithopters may be further improved by replicating the dynamic adjustments of frequency, phase change, and separation among neighboring ornithopters, to obtain best energy-saving results in the future.
