Experimental study of wing flexibility effect on flapping-wing aerodynamics

Abstract

In this study, the unsteady aerodynamic characteristics of a flexible flapping wing undergoing hovering flight was studied. Although a rectangular wing planform was used, the wing could exhibit similar wing deformation features as natural fliers

negative wing twist and positive camber. This wing deformation was possible by designing the wing root to have an offset angle termed the slack angle. A dynamically scaled-up robotic wing model equipped with a six-axis sensor was immersed a 3-ton water tank to measure the time-varying aerodynamic forces and moments. The digital particle image velocimetry (DPIV) technique was deployed to observe the vortical structures around the wing. The rigid wing was included throughout this study to help understand the contribution of wing deformation in unsteady aerodynamics. First, the aerodynamic load characteristics of the flexible wing revealed that the presence of wing deformation caused the pressure forces to act in the tangential and normal directions. This effect resulted in the effective angle of attack range to be beyond the 90° for the flexible wing case. To this end, the existing aerodynamic model, which was built for the rigid wing, was revised to account for wing flexibility. The newly-extracted force coefficients were well-fitted with a cubed-sine function. The model was rigorously validated with various wing kinematics, giving a good estimation of the experimental results. The estimated error was less than 5%, 6%, and 8% for the lift, drag, and moment, respectively, considering fast to moderate wing kinematics. Based on the revised model, the optimum angle of attack for maximum lift generation was experimentally obtained for each change in slack angle. An increase in slack angle tend to increase the optimum angle of attack for maximum lift generation. However, the practical angle of attack for application in flapping wing micro-air vehicles (FWMAVs) was found to be from 75° to 85°. Generally, the flexible wing generated less drag, consumed less power, and was more efficient than the rigid wing. This was an indication that the presence of wing flexibility requires natural fliers and FWMAVs to undertake less pitching motion in order to reduce the mechanical power and increase the efficiency of their wings. In addition, there was a conspicuous phase delay between the aerodynamic characteristics of the rigid and flexible wings. The delay was found to be very sensitive to wing kinematics. In order unravel the effects of wing kinematics on the unsteady aerodynamic characteristics, experiments were conducted with two main wing kinematic parameters, sweep duration and timing of wing rotation. This study found that the conspicuous phase delay in the flexible wing was more sensitive to the change in sweep duration than the timing of wing rotation. The transient negative lift associated with rigid wings undergoing delayed and advanced wing rotations were observed to have totally disappeared in the flexible wing case. In general, the flexible wing with symmetric and delayed wing rotations generated the highest wing efficiency. The corresponding net force vectors were observed to be tilted in an almost vertical direction for the flexible wing in comparison to the rigid wing. The vorticity distribution at the middle of stroke revealed a slight difference in the vortical structures surrounding the rigid and flexible wings in terms of proximity to the shed trailing-edge vortices (TEVs). The linearly twisted nature of the flexible wing caused the TEVs at the outboard section of the wing to be closer to the surface of the wing. The wing twist again caused the coherent leading-edge vortex (LEV) to be stable along the wingspan, and caused the vortex lift to be sustained at the outboard section of the flexible wing regardless the aspect ratio AR. The finding in this study shows that the wing twist of natural fliers could help generate and sustain sufficient amount of lift at the outboard wing sections. Thus, the spanwise deformation of their wings could be an essential wing feature for maintaining the LEV stability across the wingspan even with high AR wing.