Forward flight of swallowtail butterfly with simple flapping motion

Among the various types of flying insects, swallowtail butterflies have unique morphological features. Their wing area is very large relative to their body mass, and their flapping frequency is low. ...... Another feature of swallowtail butterflies is the small degree of freedom of the wing motion. The fore wing partly overlaps the hind wing and they flap as one large wing with little feathering. The feathering is structurally restricted by the wing connection. This means that the ability of butterflies to actively control the aerodynamic force of their wings is limited and the undulating body motion is produced passively by simple flapping.

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To clarify the passive body motion in butterfly-type flapping flight, we fabricated a tailless ornithopter having the same mass and wing shape as an actual swallowtail butterfly. This ornithopter enabled us to observe the passive body motion caused by simple mechanical flapping in free flight. We also clarified the effect of the wing stiffness on the passive body motion by changing the wing venation of the ornithopter. Our experiments demonstrated that the forward flight of a swallowtail butterfly is realized by simple flapping without feedback control of the wing motion and that passive deformation of the wing significantly affects the passive body motion and resultant aerodynamic coefficients.

Simple flapping motion was achieved by means of a rubber-band-driven crank mechanism that made the wings flap vertically relative to the body at 10 Hz. 

A butterfly wing consists of thin membranes supported by wing veins extending from the wing base to the outer edge. The wing stiffness depends on the vein pattern(Wootton 1993).

To emulate the stiffness distribution of an actual wing, we fabricated the artificial wings with plastic veins mimicking those of an actual swallowtail butterfly (figure 2(a)). A mold for the veins was fabricated by plasma dry etching, and the veins were molded on a thin polymer film.
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The longitudinal position of the center of gravity was adjusted by changing the longitudinal position of a 0.04 g balance weight attached to the body of the ornithopter.....

The passive flight motion must depend on the design of the wings. In particular, the wing deformation determined by the wing stiffness presumably affects flight performance (Wootton

1993). A butterfly wing consists of thin membranes supported by wing veins extending from the wing base to the outer edge, and the wing stiffness depends on the vein pattern. 

4. Conclusion

Using the butterfly-type ornithopter, we demonstrated that the undulating body motion caused by simple flapping of swallowtail butterflies in forward flight has the effect of enhancing the lift coefficient during downstroke. The enhanced lift coefficient at the beginning of downstroke in

free flight exceeded 3.0, which is more than four times that in a steady flow. Though the drag coefficient also increased, the lift coefficient was large enough to keep the thrust coefficient

positive. This lift and drag enhancement was attributed to large angle of attack caused by the passive up-down body motion. The body motion was greatly affected by the wing deformation

depending on the wing stiffness and resultant aerodynamic coefficients in free flight. Wing veins are needed to prevent feathering deformation and produce a large up–down body motion.

Since butterfly-type flapping flight can be realized with simple flapping motion without feedback control, butterfly aerodynamics can be applied to future aerodynamic systems