Inertial effects due to passive wing morphing in Ornithopters

Passive wing morphing has proved to be beneficial to the performance of small flapping wing un-manned air vehicles or ornithopters. Previous work has shown that passive morphing, achieved by inserting a compliant mechanism called a compliant spine into the leading edge wing spar of a test ornithopter, reduced the power consumption by 45% and increased the mean lift by 16% without incurring any thrust penalties during straight and level flight. The focus of this paper is to isolate the inertial and aerodynamic effects that occur due to the presence of the compliant spine. Isolating the inertial and aerodynamics effect enables a better understanding of the reason behind the aforementioned force benefits. In order to isolate the inertial effects from the aerodynamics, the ornithopter was placed inside a 5 foot × 5 foot vacuum chamber at NASA Langley Research Center and it was tested at vacuum (1 Torr) and at ambient pressure (760 Torr). The ornithopter was mounted on a 6 degrees of freedom load cell to measure the lift and thrust forces produced by the ornithopter at various flapping frequencies. Also the pitching moment is measured using the same load cell. During the test, four wing configurations are tested. The first configuration is the ornithopter with a uniform, solid, carbon fiber wing spar. The remaining three configurations tested are the ornithopter with various compliant spine designs, inserted in the leading edge spar at 37% of the wing half-span to mimic the function of an avian wrist. The results presented in this paper compare the effect of the presence of several compliant spine designs on the ornithopter's inertia and aerodynamics. Lift, thrust, and pitching moment are used as the comparison metrics. Results show that all of the benefits observed in the vehicle's performance due to the presence of the compliant spines are due to aerodynamic and not inertial effects.