Vortex Ring and Dynamic Lift Generated by Downstroke of Elastic Butterfly Wing

The shape of airfoil contributes to the performance of the airfoil itself. Elastic deformation during unsteady motion not only generates vortices with larger vorticity, but also enables the generation of sustained dynamic lift than rigid moving airfoil. Membrane airfoils exhibit the high performance by elastic deformation with unsteady motion. Butterfly wing, which is an example of application of membrane airfoil, have been shown to generate a pair of vortex rings and dynamic lift through flapping motion. However, it has not been clarified sufficiently how the wing shape responds to changes in dynamic lift and vortex rings. In this study, authors also visualized the vortex ring, simultaneously measured the dynamic lift, and measured the wing shape. When the wing pass through the horizontal position (when the flapping angle φ = 0 deg.), the dynamic lift is at its maximum, and the vortex ring becomes more developed. As a result, the wing chord direction gradually curves, and when the flapping angle φ = -30 deg., the entire wing chord becomes camber shape. Since no deformation occurred in the wingspan direction at any flapping angle in this measurement range, it can be said that camber in the wing chord direction is necessary to generate the dynamic lift and vortex ring.