Aerodynamic efficiency explains flapping strategies used by birds

• 成果类型: Article
• 署名作者: Johansson, L. Christoffer
• 署名单位: Lund University
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-10417
• DOI: 10.1073/pnas.2410048121
• 发表日期: 2024-11-12
• 关键词: vortex wake; wingbeat kinematics; flight kinematics; bat; speeds; range; MODEL

摘要

A faster cruising speed increases drag and thereby the thrust (T) needed to fly, while weight and lift (L) requirement remains constant. Birds can adjust their wingbeat in multiple ways to accommodate this change in aerodynamic force, but the relative costs of different strategies remain largely unknown. To evaluate the efficiency of several kinematic strategies, I used a robotic wing [E. Ajanic, A. Paolini, C. Coster, D. Floreano, C. Johansson, Adv. Intell. Syst. 5 , 2200148 (2023)] and quantitative flow measurements. I found that, among the tested strategies, changing the mean wingbeat elevation provides the most efficient solution to changing thrust- to- lift ratio (T/L), offering insight into why birds tend to beat their wings with a greater ventral than dorsal excursion. I also found that although propulsive efficiency (eta p) may peak at a Strouhal number (St, measure of relative flapping speed) near 0.3, the overall efficiency of generating force decreases with St . This challenges the expectance of a specific optimal St for flapping flight and instead suggest the chosen St depends on T / L . This may explain variation in preferred St among birds and why bats prefer flying at higher St than birds [G. K. Taylor, R. L. Nudds, A. L. Thomas, Nature 425, 707-711 (2003)], since their body shape imposes relatively higher thrust requirements [F. T. Muijres, L. C. Johansson, M. S. Bowlin, Y. Winter, A. Hedenstr & ouml;m, PLoS One 7 , e37335 (2012)]. In addition to explaining flapping strategies used by birds, my results suggest alternative, efficient, flapping motions for drones to explore aiming to extend their flight range.