Bird-inspired tendon coupling improves paddling efficiency by shortening phase transition times

TL;DR

This paper introduces a bioinspired tendon coupling mechanism that significantly enhances paddling efficiency in drag-based swimming devices by reducing transition times between phases.

Contribution

It presents a novel tendon coupling design inspired by aquatic birds that shortens phase transition times, improving efficiency over passive paddles.

Findings

Efficiency increased by 2.0 to 2.4 times with the new mechanism.

Distal leg joint clutching had minimal impact on swimming efficiency.

The mechanism offers a new principle for designing efficient drag-based swimming devices.

Abstract

Drag-based swimming with rowing appendages, fins, and webbed feet is a widely adapted locomotion form in aquatic animals. To develop effective underwater and swimming vehicles, a wide range of bioinspired drag-based paddles have been proposed, often faced with a trade-off between propulsive efficiency and versatility. Webbed feet provide an effective propulsive force in the power phase, are light weight and robust, and can even be partially folded away in the recovery phase. However, during the transition between recovery and power phase, much time is lost folding and unfolding, leading to drag and reducing efficiency. In this work, we took inspiration from the coupling tendons of aquatic birds and utilized tendon coupling mechanisms to shorten the transition time between recovery and power phase. Results from our hardware experiments show that the proposed mechanisms improve propulsive efficiency by 2.0 and 2.4 times compared to a design without extensor tendons or based on passive paddle, respectively. We further report that distal leg joint clutching, which has been shown to improve efficiency in terrestrial walking, did not play an major role in swimming locomotion. In sum, we describe a new principle for an efficient, drag-based leg and paddle design, with potential relevance for the swimming mechanics in aquatic birds.