Effects of the Butterfly Forewing Flap-and-twist Motion on the Generation of Thrust and Lift

—A butterfly is a unique flying insect that can fly at a low flapping frequency of 10-15 Hz. Therefore, it consumes little energy while flying. However, the mechanism of low-frequency wing beat has not been thoroughly explained. In this work, it was found that the synchronized flap-and-twist motion enhances the positive lift during both upstroke and downstroke. Models of butterfly forewings were made and tested to investigate the effects of flapping and twisting motions on the generation of thrust and lift. The active flapping and passive twisting mechanisms are proposed. Different ranges of flapping and twisting angles of the wings were investigated. The experimental result shows that the large symmetric twist angle [-75°, 75°] has a unique 3-cycle repetition of flapping force, which generates positive lift in a range of 0-0.06 N most of the time, with strong thrust fluctuations in a range of ±0.10 N. This synchronized flapping and twisting motion with positive lift generation is one explanation for butterfly flight in nature and reveals how butterflies can lift themselves with such a low flapping frequency.