Gravity-assisted Takeoff of Bird-inspired Flapping-Wing Air Vehicle Using Cliff-drop

Abstract Flapping-Wing Air Vehicles (FWAVs) have been developed to pursue the efficient, agile, and quiet flight of flying animals. However, unlike lightweight FWAVs capable of vertical takeoff, relatively heavy FWAVs face challenges in self-takeoff, which refers to taking off without both external device and energy input. In this study, a cliff-drop method is implemented for an independent takeoff of a heavy FWAV, relying solely on gravity. In the takeoff process using the cliff-drop method, the FWAV moves on the ground to a cliff edge using a wheel-driving motor and then descends from the cliff to achieve the necessary speed for flight. To demonstrate the cliff-drop method, the KAIST Robotic Hawk (KRoHawk) with a mass of 740 g and a wingspan of 120 cm is developed. The takeoff tests demonstrate that the KRoHawk, significantly heavier than the vertical-takeoff capable FWAVs, can successfully take off using the gravity-assisted takeoff method. The scalability of cliff-drop method is analyzed through simulations. When drop constraints are absent, the wheel-driving motor mass fraction for cliff-drop method remains negligible even as the vehicle’s weight increases. When drop constraints are set to 4 m, FWAVs heavier than KRoHawk, weighing up to 4.4 kg, can perform the cliff-drop takeoffs with a wheel-driving motor mass fraction of less than 8%.