Bat-Inspired Wing Aerodynamics and Optimization

Abstract

A S ENGINEERS search to make micro air vehicles more stable, maneuverable, and efficient, many are turning toward biology for inspiration. Bats adapt to their environment by morphing their wings in flight. Depending on their niche, certain species soar and glide [1], whereas others perform barrel rolls in nature [2] and can pull up to 4.5g in obstacle courses [3].Morphological changes afford bats great agility at low speeds, and they are able tomaintain stability and control at low Reynolds numbers, in which viscous effects and leading edge laminar separation bubbles cause nonlinearities in lift [4–6]. Bats achieve these feats with fingerlike jointed bone structures and flexible wing membranes. These unique traits allow them to change camber and twist in flight, unlike avian span changes. Recent studies investigate replicating flapping bat flight [7–9], whereas others focus specifically on flexible, membrane wing benefits at low Reynolds numbers for improvements in gust alleviation and delayed stall characteristics [10–12]. These wings are passive elements, and it is difficult to attach control surfaces to them for flight authority. By actively controlling and morphing flexible wings, conventional controllers can be replaced. Aircraft morphing allows single vehicles to have multiple functions, ideally with continuous lifting surfaces to alleviate drag and vibration and increase efficiency. This is the focus of Garcia et al. [13], which proposes a nonflapping wing with twist capabilities. Morphing enables tailoring of wing shapes to multiple flight regimes, from takeoff through cruise to landing [14–16]. Many mechanisms have been proposed for morphing, such as the smart joint, an active rigidity composite suited to actuating a batlike membrane wing in flight [17]. This low-profile device can be embedded at joints in the fingerlike skeletal wing structure as a bimorph actuator [18]. Whereas this work focuses on static wing configurations rather than morphing behavior, results help specify actuator requirements used for a variable camber and twist wing. In this work, two key features of bat flight are studied, uniquely evolved bat wing planforms adapted to environments, and capabilities afforded through variable camber and twist, and are applied to rigid, fixed wing designs for small man-made craft. The goal of this study is not to mimic natural bat flight, but to understand how certain aspects of bat flight apply to the engineering problem of wing design for micro air vehicles. II. Background and Motivation