Aerodynamic characteristics of flexible flapping wings depending on aspect ratio and slack angle

Abstract

Experimental investigations are made for the combined effects of aspect ratio (AR), slack (βS) and pitch angles on the aerodynamic characteristics of flexible flapping wings in hover. βS is introduced as a way to indirectly alter the flexibility of the wing. An optimum AR range of 3 to 5 based on the lift coefficient is observed depending on the flexibility. For a constant AR, the intensity of the leading-edge vortex (LEV) with corresponding circulatory-based lift mitigates as βS increases beyond 2.5{degree sign}. The variation of βS affects the magnitude of the shed trailing-edge vortices (TEVs) but the vorticity core is maintained. We found the shed TEVs to be the key vortical feature of twistable flexible wings in comparison to the rigid (untwisted) cases. More intriguingly, the negative wing twist played a significant role in sustaining the circulatory lift at the outboard section for even high AR cases. The primary LEV trace is found to be an indicator for the effective spanwise limit of the LEV. Although an increase in AR reduces the effective spanwise limit, it is found that wing flexibility further decreases the radial distance. Again, the study reveals that lift enhancement in the rigid wing requires a wider effective downwash area induced by the outward movement of the LEV traces to merge with the tip vortex. Contrarily, the flexible wing requires an elongated downwash area induced by the wing twist to enhance the aerodynamic performance.

Leading-edge curvature effect on aerodynamic performance of flapping wings in hover and forward flight

“…The wings were designed to have the same area of 0.0133 m 2 (AR = 3.01). However, there was a slight increase in r2 from 0.601 to 0.612 for y * = 0-0.15, respectively, as presented in our previous work [26]. This was less than 3.7% increase in the square of r2 , which can be critical for the aerodynamic force normalization.…”

“…The three-ton towing tank experimental measurement facility in the KAIST SSHS laboratory was used to conduct the experiment. The facility comprises of two distinct setups for direct force/moment measurement as shown in figure 1(a) and flow visualization measurement, which are both found in our previous works [16][17][18]. The two-degree-of-freedom (2-DoF) scaled-up robotic model mainly comprised 2 servomotors (XH-430-210W, Robotis) and a gearbox.…”

“…A high-precision six-component force/moment sensor (Nano 17 IP68, ATI Automation) with a measurement range of ±25 N and ±250 N mm was mounted at the wing root (refer to [16][17][18] for more details). Equations ( 5) and ( 6) are the sensor force components along the wing-fixed coordinates (X W , Y W , and Z W ) transformed to lift (−Z B ) and drag (X B ), represented in the body-fixed coordinate (refer to figure 1(a) for the coordinate representations).…”

“…This measured uncertainty stemmed from the sensor, gearbox, 3 servomotors, water density, etc. Detailed calculations and descriptions of the force, DPIV, and uncertainty measurements are found in our previous works [16][17][18].…”

Influence of passive deformation in the lift coefficient of a NACA0012 wing model

“…Several authors [9][10][11] showed how the aerodynamic coefficients (including the slope of the lift coefficient) of flexible micro air vehicle (MAV) wings of low aspect ratio at low Reynolds number can either grow or decrease depending on the geometry of the wing and the settings of flexible parts. Furthermore, the aerodynamic performance of flexible wings under flapping conditions (typical of bio-inspired UAVs) and the influence of their aspect ratio have also attracted attention lately [12][13][14][15][16].…”