Kinematics and inertial effects in locust flapping wings

Instantaneous aerodynamic forces on flapping wings are influenced by inertia of the moving masses of the wings. In the present study, the inertial forces and kinematics of wings of the locust Schistocerca americana were investigated experimentally. The developed experimental setup includes freshly extracted hindwings and forewings, mechanical transmission producing active pitching and flapping, a vacuum chamber, and a high-speed video system. Masses and locations of mass centers were measured and averaged based on data collected for four locusts. Videos were taken and time-resolved displacements of a set of point markers on the surface of flapping-pitching wings were obtained. Flapping angle amplitudes, determined at the middle section, are practically the same in air and in vacuum in both forewings and hindwings. Pitching amplitudes at the root and midsection of hindwing differ by up to 50% due to torsional deformation or twist. In the air, the average twist angles are 3 degrees and 18 degrees in forewings and hindwings, respectively. This is due to higher torsional stiffness of the forewings as compared to hindwings. Substantial increase of the twist deformations of the forewings in vacuum was observed and attributed to the removal of aerodynamic damping. Also, in air, the high frequency components are damped out, indicating increased aerodynamic damping at higher flow accelerations. Inertial forces are calculated based on a rigid-body model of a wing with prescribed displacements. The segmentation of a wing into meshes of different topology affects the results of calculations influenced by wing's flexibility. For the same forewing and hindwing tested in air and in vacuum, amplitudes of inertial forces in the air are substantially lower due to the air damping effects reducing wing oscillations.