Analysis of Ground Effect Conditions on the Aerodynamic Efficiency of a Bat-inspired Robotic Wing, ‘Chiropter’

Abstract

For decades, aircraft have leveraged ground effect—a phenomenon that increases lift and decreases drag on surfaces near the ground—to enhance flight efficiency. However, its influence on low-Reynolds-number flight, relevant for small unmanned aerial vehicles (sUAVs), is vastly underexplored. Notably, small organisms that frequently hunt, cruise, and forage near the ground appear to illustrate a rich source of inspiration for driving better sUAV design in this regime. This paper thus investigates the performance of a bat-inspired wing, “Chiropter”, modeled after the well-studied species Cynopterus brachyotis, when subjected to an engineered ground effect boundary. A comparative study between a conventional NACA 2414 airfoil and bat-inspired planforms was conducted in Princeton University’s wind tunnel. Force measurements were acquired in the wind tunnel’s test section (1.2 x 1.2 m cross section, 1 m long) in the low Reynolds number regime while the wings were subjected to an engineered ground effect boundary. From testing three bat bioinspired planforms, including a rigid wing, a flexible wing, and an actuated flexible wing for downstroke-upstroke cycles, it was determined that flexible wings generally trade efficiency and stability for enhanced lift. Through the trials, ground effect was observed to raise the lift and alter the drag progression for the wings, such that more flexible wings may observe greater lift at small angles of attack and large moments at large angles of attack. Furthermore, for unsteady flapping flight, ground effect vastly alters the surrounding airflow, restricting downwash development and the overall deflection of the wing, while still roughly preserving aerodynamic efficiency during the powered downstroke. The results of this study not only improve the understanding of how bats leverage ground effect to alter their performance, but they also may be used to inform how engineers can design more efficient small-scale aerial vehicles for a variety of use cases, including environmental monitoring, search & rescue, and agricultural applications.