Quantification of the benefits of a compliant foil for underwater flapping wing propulsion

This paper presents results detailing the performance of a flexible wing for use on a vehicle capable of both aerial and aquatic modes of locomotion, with primary focus on the aquatic substrate. The motivation for the research stems from the ability of avian species within the natural world demonstrating this multi-modal capability, utilsing a flapping mechanism as a means of propulsive generation. The fundamental aim is to capture the beneficial traits of a flexible wing and quantify any potential improvements in performance. We present a simplified numerical model which acts as an initial design tool prior to the fabrication of a flexible wing. This model aids in wing geometry selection so that under key kinematic parameters the wing passively deforms during aquatic operations in a beneficial manner, in an attempt to increase the maximum lift coefficient of the foil. Using the model we have fabricated a flexible wing and experimentally evaluated its performance in a range of tests, varying kinematic parameters relating to the flapping motion and forward velocities and compared this with a rigid wing model to investigate if the passive chord-wise flexibility leads to an increase in propulsive efficiency. We present the initial data set making this comparison, showing that the flexible wing was found to exhibit higher propulsive efficiencies at specific kinematic parameter sets. This modeling and experimental study will provide a foundation for the design of future vehicles capable of swimming and aerial locomotion, and help quantify the benefits of passively compliant structures in flapping wing propulsion.