The phase change between the forewing and hindwing is a distinct feature that sets dragonfly apart from other insects. In this paper, we investigated the aerodynamic effects of varying forewing-hindwing phase difference with a 60 • inclined stroke plane during hovering flight. Force measurements on a pair of mechanical wing models showed that in-phase flight enhanced the forewing lift by 17% and the hindwing lift was reduced at most phase differences. The total lift of both wings was also reduced at most phase differences and only increased at a phase range around in-phase. The results may explain the commonly observed behavior of the dragonfly where 0 • is employed in acceleration. We further investigated the wing-wing interaction mechanism using the digital particle image velocimetry (PIV) system, and found that the forewing generated a downwash flow which is responsible for the lift reduction on the hindwing. On the other hand, an upwash flow resulted from the leading edge vortex of the hindwing helps to enhance lift on the forewing. The results suggest that the dragonflies alter the phase differences to control timing of the occurrence of flow interactions to achieve certain aerodynamic effects.

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We measured the dynamic damping of a pair of flapping cicada wings mounted on a robotic insect thorax mechanism capable of high frequency flapping. The damping coefficients were derived based on the measurements of the wing-thorax mechanism translating along its body principal axes. The robotic mechanism has a 10cm wingtip-to-wingtip span, flaps up to 65Hz, and weigh 2.86 gram including the motor and wings. To measure the flapping induced damping during translation, we developed a pendulum system mounted with encoder, and attached the flapper at the end and in different orientations such that its motion is along its principle axes. The damping of the flapper is then calculated from the decaying rate of the magnitude of the oscillating pendulum. The damping coefficients calculated from the experiments are very close to those estimated based on our mathematic models using Blade-Element Theory (BET) and quasi-steady aerodynamic models. As expected, the damping linearly increases with the flapping frequency and is most prominent along forward/backward direction.