Passive Rotation Joint Design Considerations for Lift and Thrust Generation for a Biomimetic Flapping Wing

Abstract

Maximizing the available lift and thrust force is important for designing efficient flapping wing micro air vehicles. Research to date showed the passive rotation joint between the wing and four-bar linkage is an important design aspect. Two key hinge parameters are the angle of attack stop and passive rotation joint stiffness. In this work these design parameters were independently varied. Their impact on lift and thrust force generation, and the ratio of the first and second system resonance frequencies were measured and compared through experiments utilizing prototype hardware of varying design. The prototype hardware and flapping wing controller is based on previous work, focused on using biomimetic wings combined with a design that only requires two piezoelectric actuators, and will be briefly reviewed. The angle of attack stops tested were 30˚, 40˚, 45˚, 50˚, and 60˚. Five different passive rotation joints were tested of varying stiffness. Optimal angle of attack stops and passive rotation joint designs were found from the experimental results and combined into a best design, which was tested and compared to the optimal results from the independent designs. Results show that while individual selection of angle stop and passive rotation joint stiffness can be optimized, the intersection between the two precludes simply choosing the best of both as the best combined. φ Stroke angle or phase angle ω Flapping frequency Subscript c Carbon fiber h Passive rotation joint hinge m Maximum value n Natural frequency p Predicted s Angle of attack stop L Left wing R Right wing 1 First resonant mode 2 Second resonant mode 1. BACKGROUND The Air Force Institute of Technology's (AFIT) flapping wing micro air vehicle (FWMAV) research program is focused on creating a minimally actuated, power tethered, bio-mimetic flight test vehicle. Current designs utilize two, single degree of freedom piezoelectric actuators to provide control of five of the FWMAV's six degrees of freedom (DOF). The simplified design requires only two drive signals, φ L (t) and φ R (t) for control. A prescriptive method is used to generate these drive signals in an open loop, requiring no feedback of wing dynamics. [1, 2] Previous work has developed a wing that mimics the structural dynamics of the Manduca sexta. [3, 4] Developed FWMAV production techniques have created FWMAVs that generate useful forces and moments, but have yet to achieve a lift to weight ratio greater than one. [5, 1] Research to date showed the passive rotation joint between the wing and four-bar linkage is an important design aspect. Two key joint parameters are the angle of attack stop, α s , and passive rotation joint stiffness, κ h , with the assumption that α s holds the wing at this angle through a wing stroke half-cycle, and κ h determines the ratio of the system second resonant frequency to the first resonant frequency, ω n 2 /ω n 1. Based on previous results, additional work is required to reduce weight and increase lift and thrust forces to achieve the goal of controlled flig...