Role of vortex in free-flight of insects

It is well-known that insects utilize vortices generated by flapping, by which they generate larger lift than that evaluated by the ordinary aerodynamic theory. However, the relative position between the vortices and the wing is affected by the motion of the center of gravity(CG). Such effect has not been well understood. To clarify the effect, we numerically analyzed a simple model considering the coupling between the vertical CG motion and the separation vortices generated by horizontally-flapping wing[1]. As the result, despite a single flapping motion, the model exhibits two types of flapping flight: a steady flight and a wandering flight. In a steady flight, an ordered vortex array of the inverted Karman vortex type is generated, and the CG velocity oscillates regularly. In a wandering flight, vortices are distributed randomly, and the CG velocity varies irregularly. These two types of flights are selected by the initial conditions even when all the parameters are the same. The possible flights are analyzed in terms of bifurcation, and it was shown that the bifurcation structure is qualitatively similar to another flapping model with completely different flapping motion[2]. These results illustrate the role of vortex in the free-flight problem. References: [1] M. Iima, A two-dimensional aerodynamic model of freely flying insects, to appear in Journal of Theoretical Biology(2007), doi: 10.1016/j.jtbi.2007.03.012. [2] M. Iima and T. Yanagita, A transition from ascending flight to vertical hovering: a study of a symmetric flapping model, Europhys. Lett., 74(2006) 55–61.