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Butterfly-type flapping flight was defined by a low flapping frequency, low wing loading and large wingspan. On the basis of the definition, we made a butterfly-type ornithopter (BTO). The BTO has no stabilizing device such as a tail fin, however, it flew stably when the position of the center of gravity was appropriate. Flights of the BTO were recorded with a high-speed video camera, and its longitudinal motion was measured. In the stable flight, angle of attack of the body αbody varied coincidentally with a flapping cycle. To reveal the effect of the change of αbody on aerodynamics, we visualized the air flows around the wing of the tethered BTO for several values of αbody. When αbody is positive, leading edge vortex (LEV) was attached to the wing during downstroke, and it separated from the wing when αbody was too large. We also visualized the air flow around the free flying BTO. In this case, αbody was large in the beginning of downstroke, however, LEV was not separated from the wing during downstroke due to the rapid decrease of αbody. This result suggests that butterflies are designed to take advantage of passive change of αbody.

Passive Change of an Angle of Attack in Butterfly-Type Flapping Flight