Flight dynamics of a butterfly-type ornithopter

We developed a very small and light butterfly-type ornithopter (BTO) to investigate butterfly flight. Its weight is 0.4 g, wing span is 140 mm and flapping frequency is 10 Hz. It flies forward passively. The flights were recorded with a highspeed video camera and the longitudinal motion of the BTO was analyzed. When the center of gravity was appropriate, the angle of attack of the body varied cyclically. We found that the cyclical change of the angle of attack produced total upward aerodynamic force. Then we visualized air flow around the BTO both in tethered flight and in free flight. We revealed that free body motion caused a stable attachment of leading edge vortex. We also found that the free body motion helped the wings´ smooth upstroke, which was not found in tethered flight.