Energetic and Control Trade-offs in Spring-Wing Systems

TL;DR

This study investigates how elastic resonance in spring-wing systems affects energy efficiency, agility, and stability, revealing inherent trade-offs that impact insect flight performance and design considerations.

Contribution

It introduces the Weis-Fogh number as a key parameter to analyze perturbation effects on resonant spring-wing systems, highlighting the trade-offs between energy efficiency and maneuverability.

Findings

Higher Weis-Fogh numbers improve energetic efficiency.

Increased Weis-Fogh number reduces wing stability.

Elastic resonance trade-offs limit insect flight agility.

Abstract

Flying insects are thought to achieve energy-efficient flapping flight by storing and releasing elastic energy in their muscles, tendons, and thorax. However, flight systems consisting elastic elements coupled to nonlinear, unsteady aerodynamic forces also present possible challenges to generating steady and responsive wing motions. In previous work, we examined the resonance properties of a dynamically-scaled robophysical system consisting of a rigid wing actuated by a motor in series with a spring, which we call a spring-wing system \cite{Lynch2021-ri}. In this paper, we seek to better understand the effects of perturbations on resonant systems via a non-dimensional parameter, the Weis-Fogh number. We drive a spring-wing system at a fixed resonant frequency and study the response to an internal control perturbation and an external aerodynamic perturbation with varying Weis-Fogh number. In our first experiments, we provide a step change in the input forcing amplitude and study the wing motion response. In our second experiments we provide an external fluid flow directed at the flapping wing and study the perturbed steady-state wing motion. We evaluate results across the Weis-Fogh number, which describes the ratio of inertial and aerodynamic forces and the potential energetic benefits of elastic resonance. The results suggest that spring-wing systems designed for maximum energetic efficiency also experience trade-offs in agility and stability as the Weis-Fogh number increases. Our results demonstrate that energetic efficiency and wing maneuverability are in conflict in resonant spring-wing systems suggesting that mechanical resonance presents tradeoffs in insect flight.