Aerodynamics of manoeuvring flight in brown long-eared bats (Plecotus auritus)

Abstract

In this study, we explicitly examine the aerodynamics of manoeuvring flight in animals. We studied brown long-eared bats flying in a wind tunnel while performing basic sideways manoeuvres. We used particle image velocimetry in combination with high-speed filming to link aerodynamics and kinematics to understand the mechanistic basis of manoeuvres. We predicted that the bats would primarily use the downstroke to generate the asymmetries for the manoeuvre since it has been shown previously that the majority of forces are generated during this phase of the wingbeat. We found instead that the bats more often used the upstroke than they used the downstroke for this. We also found that the bats used both drag/thrust-based and lift-based asymmetries to perform the manoeuvre and that they even frequently switch between these within the course of a manoeuvre. We conclude that the bats used three main modes: lift asymmetries during downstroke, thrust/drag asymmetries during downstroke and thrust/drag asymmetries during upstroke. For future studies, we hypothesize that lift asymmetries are used for fast turns and thrust/drag for slow turns and that the choice between up- and downstroke depends on the timing of when the bat needs to generate asymmetries.

Keywords: aerodynamics; bats; kinematics; manoeuvring flight; particle image velocimetry.

Figure 1.

Illustration of the digitized points for kinematic analysis. 1: right wingtip, 2: left wingtip, 3: left shoulder, 4: right shoulder, 5: tail tip, 6: tip of right hand fifth digit, 7: tip of left hand fifth digit, 8: base of right side thumb, 9: base of left side thumb, 10: base of tail, 11: centre of left foot, 12: centre of right foot, 13: centre of head.

Figure 2.

Example sequence of a manoeuvre. The wake is illustrated as the iso-surface of the Q-criterion at Q = 0.002 coloured by Z-vorticity. In this example sequence, the bat makes a leftward turn of about 160 mm lateral displacement over a forward flight distance of approximately 1.8 m.

Figure 3.

Examples of initiation phase of the manoeuvre. Orange lines show right side and blue lines show left side. (a) Downstroke with asymmetries in lift, causing a rolling motion. Differences in how deep the wings are beaten, the difference in lift and the change in roll are encircled. Direction: Left. Bat 2. Net roll moment during peak force: 1.23 mN·m. (b) Downstroke with asymmetries in thrust and drag, causing a yawing motion. Differences in thrust/drag, wing length and the change in yaw are encircled. Direction: Left. Bat 1. Net yaw moment during peak force: 0.31 mN·m. (c) Upstroke with asymmetries in thrust and drag, causing a yawing motion. Differences in thrust/drag, wing and tail angle of attack (AoA) and the change in yaw are encircled. Direction: Right. Bat 1. Net yaw moment during peak force: 0.25 mN·m.

Figure 4.

Examples of the displacement phase of the manoeuvre. Orange lines show right side and blue lines show left side. (a) Downstroke with asymmetries in lift, causing a rolling motion. Differences in lift and the change in roll are encircled. Direction: Left. Bat 2. Net roll moment during peak force: 1.06 mN·m. (b) Upstroke with asymmetries in thrust and drag, causing a yawing motion. Differences in thrust/drag, wing and tail angle of attack (AoA) and the change in yaw are encircled. Direction: Right. Bat 2. Net yaw moment during peak force: 0.29 mN·m.

Figure 5.

Examples of the termination phase of the manoeuvre. Orange lines show right side and blue lines show left side. (a) Downstroke with asymmetries in lift, causing a rolling motion. Differences in lift and the change in roll are encircled. Direction: Left. Bat 2. Net roll moment during peak force: 0.82 mN·m. (b) Downstroke with asymmetries in thrust and drag, causing a yawing motion. Differences in thrust/drag, wing length and the change in yaw are encircled. Direction: Right. Bat 1. Net yaw moment during peak force: 0.69 mN·m. (c) Upstroke with asymmetries in thrust and drag, causing a yawing motion. Differences in thrust/drag, wing and tail angle of attack (AoA) and the change in yaw are encircled. Direction: Left. Bat 1. Net yaw moment during peak force: 0.30 mN·m.