Bioinspired wingtip devices: a pathway to improve aerodynamic performance during low Reynolds number flight

Abstract

Birds are highly capable and maneuverable fliers, traits not currently shared with current small unmanned aerial vehicles. They are able to achieve these flight capabilities by adapting the shape of their wings during flight in a variety of complex manners. One feature of bird wings, the primary feathers, separate to form wingtip gaps at the distal end of the wing. This paper presents bio-inspired wingtip devices with varying wingtip gap sizes, defined as the chordwise distance between wingtip devices, for operation in low Reynolds number conditions of Re = 100 000, where many bird species operate. Lift and drag data was measured for planar and nonplanar wingtip devices with the total wingtip gap size ranging from 0% to 40% of the wing's mean chord. For a planar wing with a gap size of 20%, the mean coefficient of lift in the pre-stall region is increased by 7.25%, and the maximum coefficient of lift is increased by 5.6% compared to a configuration with no gaps. The nonplanar wingtip device was shown to reduce the induced drag. The effect of wingtip gap sizes is shown to be independent of the planarity/nonplanarity of the wingtip device, thereby allowing designers to decouple the wingtip parameters to tune the desired lift and drag produced.

A Brief Review on Aerodynamic Performance of Wingtip Slots and Research Prospect

“…24 C L versus C D for the planar configuration demon-strating the effect of gap size. γ = 20% produces the greatest lift while incurring small drag penalties for the wing-wingtip system [33] Fig. 25 C Di versus α b for planar and nonplanar configurations for pre-stall angles of attack showing a reduction in induced drag when switching to a nonplanar configuration [33] showed that the tip sails have observable efficiency in reducing induced drag.…”

“…γ = 20% produces the greatest lift while incurring small drag penalties for the wing-wingtip system [33] Fig. 25 C Di versus α b for planar and nonplanar configurations for pre-stall angles of attack showing a reduction in induced drag when switching to a nonplanar configuration [33] showed that the tip sails have observable efficiency in reducing induced drag. To study the potential use of adaptive wingtip sails for the reduction of induced drag through variations of cant angles, a total of 55 configurations were chosen with different cant angles by Cerón-Muñoz et al [47,48] and tested their performance in a wind tunnel (Figs.…”

“…To figure out the influence of cant angle, slot width, and angle of attack of wingtip on the aerodynamic properties of wings, Lynch et al [33] conducted wind tunnel experiments on wings with plenty of wingtip arrangements (Fig. 23).…”

“…Chen and Qi [45,46] optimized the wingtip sails for a transport aircraft with rectangular wings, and the results Fig. 23 Experimental setup with slot width γ = 40% and (0, 0, 0) configuration for angle of attack of wingtip [33] Fig. 24 C L versus C D for the planar configuration demon-strating the effect of gap size.…”

Impact of wing-tip vibration on the development of a wing-tip vortex

“…Due to the rapid emergence of unmanned air vehicles, the aerodynamic characteristics of wing-tip vortices of small scale aircraft have extensively been investigated. Lynch et al (2018) tested the aerodynamic performance of a bioinspired wing-tip, which is a wing-tip with gaps mimicking a wing of a bald eagle. They reported improvements in lift and drag characteristics versus the conventional wing-tip.…”