A fused deposition modeling (FDM) 3D printing method is applied to develop the flapping mechanisms with bevel gears to achieve the leading-edge twisting. The purpose of the current study is to emphasize the characteristics and phenomena of leading-edge twisting in flapping wing vehicles. Some remaining technical issues or future works of the figure-of-8 flapping were summarized finally. The flexible MAVs exhibited the peculiar figure-of-8 away from the conventional domain of MAVs by the perspective of scaling laws. How the rigidity of the flexible wing frame influences the flapping appearance was also addressed qualitatively. The bifurcation (duality) phenomenon of the oblique figure-of-8 was shown. This figure-of-8 was done by the aero-elastic interactive nature as well as the symmetry-breaking of a simple flapping system. The time-averaged lift, thrust coefficients and the structure aging of MAVs have been investigated to mention the corresponding influence. 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. 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. The constructed flapping MAV Golden 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. 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.
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