Flow visualization and force measurements on a hovering flapping-wing MAV ‘DelFly II’

Abstract

The development of micro aerial vehicles is motivated by the need for autonomous unmanned aerial vehicles to observe barely accessible areas. Insect flight has been a source of inspiration during the development of DelFly, a flapping wing MAV at Delft University of Technology. In order to further decrease the overall size of DelFly, preserving the excellent flight performance, a thorough understanding of the aerodynamics is necessary. Compared to steady airfoils, flapping wings are able to generate high lift coefficients due to unsteady aerodynamics. It is expected that the occurrence of a leading edge vortex at wing rotation plays an important role in unsteady lift generation. Due to the biplane wing configuration of DelFly a wing-wing interaction, called the clap and peel mechanism, is caused. This mechanism is assumed to further enhance the lift generation by expelling air downwards, omitting the need for a start and stop vortex to be shed and strengthening the leading edge vortex. The influence of wing flexibility and the resulting wing deformation during isolated rotation, called the flex motion, supposedly postpone the shedding of the stopping vortex and strengthen the leading edge vortex. The motivation of the current investigation is to visualize the flow field around a flapping wing MAV with proven flight performance. Complex wing kinematics, highly threedimensional flow and fluid-structure interaction are thus included without simplifications. Particle image velocimetry measurements and simultaneous force measurements are performed on the DelFly II flapping-wing MAV, to investigate the flow-field behaviour and the aerodynamic forces generated. The high flexibility causes the wings to peel and flex during the rotation at the minimal and maximal amplitude respectively. This passive wing deformation largely influences the flow field behaviour. The wings start peeling apart before the clap has been completed. During the peel, the PIV analysis shows a strong influx between the wings and a conical vortex structure above the leading edges. This peel mechanism contributes significantly to the lift, as revealed by the force measurements. During the clap, the presence of a mirror wing causes the air to be expelled downwards and to roll up into vortices of the same direction as the bound vorticity of the subsequent stroke. The jet generated by the clap is stronger on the inner half of the wings, but exceptionally weak on the outer half. This is attributed to the high flexibility of the wing foil. The strong and chaotic starting vortex shed at the beginning of the outstroke, is responsible for a strong decline in lift force during the subsequent translational phase. The clap mechanism does rather attenuate than enhance lift. The occurrence of a leading edge vortex during the flex can not be assessed from the PIV images due to optical obstruction, but is likely to appear since the wing flexing is accompanied with a large increase in lift. An additional augmentation in lift could also be attained by the delayed shedding of a combined starting and stopping vortex. Although this flap phase is accompanied by an increased lift, it does not contribute to the total lift as much as the peel mechanism. The PIV visualization first suggests the occurrence of a leading edge vortex at the onset of the subsequent translational stroke. Especially during the flex the high deflection of the wings achieved at a higher flapping frequency is favourable. The force plot shows a relatively higher and broader peak in lift force. The aerodynamic mechanism underlying this phenomenon could not be identified.