Experimental Study on Forewing–Hindwing Phasing in Hovering and Forward Flapping Flight

The effects of the phase difference between fore- and hindwings are experimentally investigated on the aerodynamic force, aerodynamic efficiency, and longitudinal control forces for tandem flapping wings in hovering and forward flight. The aerodynamic force and power are measured using a dynamically scaled mechanical model in a water tunnel at a Reynolds number of 7660-11,400. The experimental results indicate that a tandem configuration, except in-phase motion, is detrimental to the generation of vertical and horizontal aerodynamic forces in hovering and slow forward flight; whereas the vertical force is enhanced in out-of-phase motion when the hindwing leads the forewing in faster forward flight. In-phase motion has the largest aerodynamic efficiency of all the phase differences in hovering and forward flight, which does not correspond to the phase difference that dragonflies use in hovering and forward flight. In contrast, a shift in the phase difference in the range of 45-180 deg (hindwing leads forewing) does not change the total vertical or horizontal force but largely and monotonically changes the pitching moment about the body caused by the force difference between the fore- and hindwings in hovering and forward flight, which corresponds to the fact that dragonflies use antiphase motion in hovering and out-of-phase motion of 50-130 deg in forward flight.