Leading-Edge Vortex Improves Lift in Slow-Flying Bats

Abstract

Staying aloft when hovering and flying slowly is demanding. According to quasi-steady-state aerodynamic theory, slow-flying vertebrates should not be able to generate enough lift to remain aloft. Therefore, unsteady aerodynamic mechanisms to enhance lift production have been proposed. Using digital particle image velocimetry, we showed that a small nectar-feeding bat is able to increase lift by as much as 40% using attached leading-edge vortices (LEVs) during slow forward flight, resulting in a maximum lift coefficient of 4.8. The airflow passing over the LEV reattaches behind the LEV smoothly to the wing, despite the exceptionally large local angles of attack and wing camber. Our results show that the use of unsteady aerodynamic mechanisms in flapping flight is not limited to insects but is also used by larger and heavier animals.

Leading edge vortex in a slow-flying passerine

“…1. For the on-wing measurements where an attached vortex structure (AVS) was present on top of the wing, and its view was not blocked by the wing, we estimated the AVS circulation following Muijres et al [8]. During the on-wing sessions the bird was wearing goggles for eye protection (figure 1).…”

“…Using a vertical transverse plane (y-z) close behind the animal, we measured tip-vortex dynamics throughout the downstroke following Muijres et al [5], for both individuals (28 wingbeats). Using an onwing set-up with vertical streamwise plane (x-z), we measure the airflow dynamics near the wing surface and the corresponding wing kinematics following Muijres et al [8], for flycatcher no. 1.…”

“…The airflow around the birds was measured using particle image velocimetry in two planes (electronic supplementary material, figure S1) [5,8]. Using a vertical transverse plane (y-z) close behind the animal, we measured tip-vortex dynamics throughout the downstroke following Muijres et al [5], for both individuals (28 wingbeats).…”

“…From the wing kinematics data we determined the downstroke average effective angle-of-attack a eff and the downstroke average effective wing speed U eff ¼ jfu þ U 1 ; wgj, following Muijres et al [8], for each wing section separately as well as for the complete wingspan (u and w are the horizontal and vertical wing downstroke flapping velocities, respectively). Downstroke average velocities were estimated as the average of sine functions fitted through the U(t) distributions (MATLAB, fit), while a eff is the average of all a eff (t) measurements.…”

“…Although LEVs have been found in a swift model [10] and a goose flapper [11] in cruising flight, LEVs have primarily been studied in slow and hovering flight [7][8][9]. In this case, the wing typically operates at high angles-of-attack (greater than 208), which causes the airflow to separate at the wing's leading edge [12].…”