Relaxing from the conventional regarding of the "rigid" flapping mechanism, in this work we proposed using "flexible" carbon-fibers and parylene (poly-para-xylylene) films as the wing frames for palm-size micro aerial vehicles (MAVs) with the wingspan of 21.6 cm. We constructed the new flapping MAV with a small body mass of 5.9 g to set a successful 47s flight record without the delicate multiple degree-of-freedom (DOF) motion previously formulated from the mechanism design. Via a high speed

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... era, we firstly reported the wing-tip trajectory of the MAV as a "figure-of-eight" which composes the original simple (four-bar linkage) flapping and the induced coherent streamwise vibration whereas the wingbeat frequency being about 10-25 Hz. The time-averaged lift, thrust coefficients and the structure aging of MAVs have been investigated to mention the influence of this figure-ofeight flapping. The experiment results preliminarily show that this biomimetic figure-ofeight is done by the very nature of the aero-elastic interaction as well as the symmetrybreaking of a simple flapping system. The bifurcation (duality) phenomenon of the MAV with figure-of-eight was shown accordingly. How the rigidity of the flexible wing frame influences the appearance of figure-of-eight flapping was addressed experimentally and empirically. Our flexible MAVs exhibit the peculiar characteristic away from the conventional domain of MAVs, and own the similar behavior to hummingbirds from the perspective of scaling laws in wingspan and wingbeat frequency (versus body mass). 2 I. Introduction NE persistent obstacle in the research of natural flyers and their unsteady flow mechanisms is the difficulty in directly measuring the aerodynamic forces produced by a flapping insect or a bird (mentioned by Dickinson 1 .) A free-flying butterfly has ever been attracted into a wind-tunnel, and an unconventional lift-generating mechanism has then been found in-situ by Srygley 2 . In addition, a MEMS micro force sensor using capacitive detection was developed for characterizing the flight behavior of a tethered fruit fly Drosophila melanogaster by Sun 3 . Recently Hedenström et al. 4 analyzed the images of the wake vortices of bats Glossophaga soricina by means of a digital particle image velocimetry (DPIV) method. Generally speaking, the researchers of the above works studied the unsteady flow mechanisms by observing living natural flyers. Although the prior flight information is firsthand and precious, two shortcomings are still inevitable. First, living natural flyers are hard to control. Constrained by the man-made environment with limited fidelity, the measured gestures of the living animals are even questioned with their reality. Second, these experimental data from the same species of animals are usually without good repeatability. Therefore the corresponding summary of the flight behaviors or mechanisms is hard to conclude from the massive experimental data of natural flyers. Several groups developed their flapping micro aerial vehicles (MAVs) with different configurations and actuation principles in recent years. Caltech's "Micro-Bat" created a 6-min flight record of flapping MAVs using MEMS process and the titanium-parylene(poly-para-xylylene) material system (reported by Pornsin-sirirak 5 .) TU Delft's MAV "Delfly" composed of a pair of dragonfly-like flexible wings recently announced their successful hovering (reported by Barrett 6 .) Otherwise, a fixed-wing type MAV with a scissor-like clapping tail thruster made by Jones et al. 7 of Naval Graduate School. They showed the long flight endurance over 20 min. MEMS group in Tamkang University also presented a MAV capable of exporting the on-site lift information through its PVDFparylene composite wing (reported by Yang 8 .) Anyhow, the unsteady flow mechanisms of these successful artifacts were still controversial, and they deserve people to investigate their aerodynamic forces and kinematic motion, like the research way of living animals just mentioned before. The advantageous merits of this approach are not only the precise control on the dimension but also the adjustability of flight parameters (for instance, freestream velocity, inclined angle of flapping plane, wingbeat frequency and so on.) By this way a biomimetic figure-of-eight flapping induced by the flexible wing frames of the MAVs in this work were found accordingly. The flight information obtained from a MAV with successful flight record can be as a reference compared to natural flapping animals, and give a guideline to the development of next generation MAV in an empirical manner. The figure-of-eight stroke of hovering hummingbirds is the ideal trajectory which many researchers of flapping MAVs hope to pursue. Several sophisticated mechanisms of flapping wings were claimed to fit this natural maneuver motion in the conceptual design, for instance, the Banala et al. 9 in Delaware University employed a 5-bar mechanism for generating a prescribed wing motion taken from hawkmoth kinematic flight data. They additionally designed a mechanism for biaxial rotation of a wing for a hovering MAV (reported by McIntosh 10 .) Meanwhile, an insect-like flapping wing mechanism was proposed by Żbikowski et al. 11, 12 of Cranfield University through the novel idea of a double spherical Scotch yoke. All the above designs have still not availably been applied to the 20cm size MAV and there exists scarcely successful examples of this kind artificial palm-size MAVs manufactured by conventional machining or even MEMS process.