PERFORMANCE OF A BUTTERFLY IN TAKE-OFF FLIGHT

The flight of a butterfly, Pieris melete. was observed in the take-off phase and was analyzed theoretically from aerodynamic and kinetic vie wpoints. A vortex method, which was recently developed by the present authors, was used in this analysis. During the downstroke, the butterf ly generates mainly a vertical force. The acceleration of the butterfl y's body during the first half of the downstroke is especially large, and this acceleration is mainly caused by a large unsteady pressure dr ag acting on the wings. This large unsteady pressure drag is generated by the vortices shed into the flow from the outer edges of each wing of a pair; it is increased by the interference effect between a pair o f wings when the opening angle is small. This force can be estimated b y the previous quasi-steady analysis when the force coefficient is cha nged to 4. In addition to the unsteady pressure drag, an aerodynamic f orce due to added mass is generated and this is also increased by the interference effect between a pair of wings. During the upstroke the b utterfly generates mainly a horizontal force. The change of direction of the forces during the down- and upstrokes is controlled by variatio n in the inclination of the stroke plane. The moment, which is created by the aerodynamic force acting on the wings and by abdominal motion, changes the thoracic angle, that is the inclination of the stroke pla ne.