Efficiency and Aerodynamic Performance of Bristled Insect Wings Depending on Reynolds Number in Flapping Flight

Abstract

Insect wings are generally constructed from veins and solid membranes. However, in the case of the smallest flying insects, the wing membrane is often replaced by hair-like bristles. In contrast to large insects, it is possible for both bristled and membranous wings to be simultaneously present in small insect species. There is therefore a continuing debate about the advantages and disadvantages of bristled wings for flight. In this study, we experimentally tested bristled robotic wing models on their ability to generate vertical forces and scored aerodynamic efficiency at Reynolds numbers that are typical for flight in miniature insects. The tested wings ranged from a solid membrane to a few bristles. A generic lift-based wing kinematic pattern moved the wings around their root. The results show that the lift coefficients, power coefficients and Froude efficiency decreased with increasing bristle spacing. Skin friction significantly attenuates lift production, which may even result in negative coefficients at elevated bristle spacing and low Reynolds numbers. The experimental data confirm previous findings from numerical simulations. These had suggested that for small insects, flying with bristled instead of membranous wings involved less change in energetic costs than for large insects. In sum, our findings highlight the aerodynamic changes associated with bristled wing designs and are thus significant for assessing the biological fitness and dispersal of flying insects.

Flow development and leading edge vorticity in bristled insect wings

“…The wing's feathering angle was 45° during the up and down strokes and 0° (vertical position) at the stroke reversals. Mean Reynolds number based on wing length and wing tip velocity was ~ 3.4 in all experiments (Engels et al 2021;O'Callaghan et al 2022) and fluid temperature ~ 24 °C.…”

“…Compared to solid wing aerodynamics, there is relatively little known about aerodynamic performance of bristled wings and the flow structures associated with these morphological modifications. A previously published study on model wings with different bristle spacing has shown that lift coefficients decrease with increasing bristle spacing in a non-linear fashion (O'Callaghan et al 2022). At a relative bristle spacing of 0.08 wing length (solid wing, 1.0), a bristled wing loses the ability to produce positive lift while drag is pronounced, yielding drag coefficients around 3.0 due to viscous friction.…”

“…The simplified wings used in this study are identical to the wings previously used in numerical (Engels et al 2021) and experimental approaches (O'Callaghan et al 2022). Wing design and kinematic pattern are explained in great detail in the latter manuscripts, and we thus refer to the materials and methods section of both studies for further information.…”

“…The consequences of size changes for locomotor behaviour in insects may be dramatic, in particular for flight, owing to the change in Reynolds number. As the body becomes progressively smaller, the relative effect of air viscosity on friction increases, resulting in proportionally higher drag and lower lift production (O'Callaghan et al 2022). This leads to elevated energetic costs for flight, which is the most costly form of animal locomotion.…”

“…We scored the time evolution of the leading edge vortex during wing flapping, studied vortex dissipation and investigated how the magnitude of LEV vorticity changes the ability of a bristled wing to produce vertical force for weight support. The materials and methods used in this study are similar to a previously published study on bristle wing aerodynamics, in which we used bristled robotic wings to estimate lift and drag coefficients, including Froude efficiency of propulsion (O'Callaghan et al 2022).…”