Relaxing from the conventional regarding of the rigid flapping mechanism, in this review paper the author introduced flexible wing frames for micro-air-vehicles (MAVs) with the wing span of 20 cm at Tamkang University. The constructed flappingMAVGolden Snitch with a smallest body mass of 5.9 g created a successful 107 s flight record with a four-bar linkage driving mechanism in 2008. Augmented by the precision injection molding (PIM) manufacture, the almost polymer-made MAV with the modified driving mechanism increases the flight endurance up to 480 s in 2010. Via high speed photography, the author has ever found the wing-tip trajectory as an oblique figure-of-8 which composes the original up-and-down flapping and the induced coherent streamwise vibration while 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 corresponding influence. This figure-of-8 was done by the aero-elastic interactive nature as well as the symmetry-breaking of a simple flapping system. The bifurcation (duality) phenomenon of the oblique figure-of-8 was shown. How the rigidity of the flexible wing frame influences the flapping appearance was also addressed qualitatively. The flexible MAVs exhibited the peculiar figure-of-8 away from the conventional domain of MAVs by the perspective of scaling laws. Some remaining technical issues or future works of the figure-of-8 flapping were summarized finally.
