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

Application of a novel deep learning-based 3D videography workflow to bat flight

Studying the detailed biomechanics of flying animals requires accurate three-dimensional coordinates for key anatomical landmarks. Traditionally, this relies on manually digitizing animal videos, a labor-intensive task that scales poorly with increasing framerates and numbers of cameras. Here, we present a workflow that combines deep learning-powered automatic digitization with filtering and correction of mislabeled points using quality metrics from deep learning and 3D reconstruction. We tested our workflow using a particularly challenging scenario: bat flight. First, we documented four bats flying steadily in a 2 m3 wind tunnel test section. Wing kinematic parameters resulting from manually digitizing bats with markers applied to anatomical landmarks were not significantly different from those resulting from applying our workflow to the same bats without markers for five out of six parameters. Second, we compared coordinates from manual digitization against those yielded via our workflow for bats flying freely in a 344 m3 enclosure. Average distance between coordinates from our workflow and those from manual digitization was less than a millimeter larger than the average human-to-human coordinate distance. The improved efficiency of our workflow has the potential to increase the scalability of studies on animal flight biomechanics.

Maneuvering Characteristics of Bilateral Amplitude-Asymmetric Flapping Motion Based on a Bat-Inspired Flexible Wing

Flapping-wing micro air vehicles (FWMAVs) have gained much attention from researchers due to their exceptional performance at low Reynolds numbers. However, the limited understanding of active aerodynamic modulation in flying creatures has hindered their maneuverability from reaching that of their biological counterparts. In this article, experimental investigations were conducted to examine the effect of the bilateral amplitude asymmetry of flexible flapping wings. A reduced bionic model featuring bat-like wings is built, and a dimensionless number ΔΦ* is introduced to scale the degree of bilateral amplitude asymmetry in flapping motion. The experimental results suggest that the bilateral amplitude-asymmetric flapping motion primarily induces maneuvering control forces of coupling roll moment and yaw moment. Also, roll moment and yaw moment have a good linear relationship. To achieve more efficient maneuvers based on this asymmetric motion, it is advisable to maintain ΔΦ* within the range of 0 to 0.4. The magnitude of passive pitching deformation during the downstroke is significantly greater than that during the upstroke. The phase of the peak of the passive pitching angle advances with the increase in flapping amplitude, while the valleys lag. And the proportion of pronation and supination in passive pitching motion cannot be adjusted by changing the flapping amplitude. These findings have important practical relevance for regulating turning maneuvers based on amplitude asymmetry and help to understand the active aerodynamic modulation mechanism through asymmetric wing kinematics.

Stealth echolocation in aerial hawking bats reflects a substrate gleaning ancestry

Predator-prey co-evolution can escalate into an evolutionary arms race.1 Examples of insect countermeasures to bat echolocation are well-known,2 but presumptive direct counter strategies in bats to insect anti-bat tactics are rare. The emission of very low-intensity calls by the hawking Barbastella barbastellus to circumvent high-frequency moth hearing is the most convincing countermeasure known.2,3 However, we demonstrate that stealth echolocation did not evolve through a high-intensity aerial hawking ancestor becoming quiet as previously hypothesized2,3,4 but from a gleaning ancestor transitioning into an obligate aerial hawker. Our ancestral state reconstructions show that the Plecotini ancestor likely gleaned prey using low-intensity calls typical of gleaning bats and that this ability-and associated traits-was subsequently lost in the barbastelle lineage. Barbastelles did not, however, revert to the oral, high-intensity call emission that other hawking bats use but retained the low-intensity nasal emission of closely related gleaning plecotines despite an extremely limited echolocation range. We further show that barbastelles continue to emit low-intensity calls even under adverse noise conditions and do not broaden the echolocation beam during the terminal buzz, unlike other vespertilionids attacking airborne prey.5,6 Together, our results suggest that barbastelles' echolocation is subject to morphological constraints prohibiting higher call amplitudes and beam broadening in the terminal buzz. We suggest that an abundance of eared prey allowed the co-opting and maintenance of low-intensity, nasal echolocation in today's obligate hawking barbastelle and that this unique foraging behavior7 persists because barbastelles remain a rare, acoustically inconspicuous predator to eared moths. VIDEO ABSTRACT.

Turning-ascending flight of a Hipposideros pratti bat

Bats exhibit a high degree of agility and provide an excellent model system for bioinspired flight. The current study investigates an ascending right turn of a Hipposideros pratti bat and elucidates on the kinematic features and aerodynamic mechanisms used to effectuate the manoeuvre. The wing kinematics captured by a three-dimensional motion capture system is used as the boundary condition for the aerodynamic simulations featuring immersed boundary method. Results indicate that the bat uses roll and yaw rotations of the body to different extents synergistically to generate the centripetal force to initiate and sustain the turn. The turning moments are generated by drawing the wing inside the turn closer to the body, by introducing phase lags in force generation between the wings and redirecting force production to the outer part of the wing outside of the turn. Deceleration in flight speed, an increase in flapping frequency, shortening of the upstroke and thrust generation at the end of the upstroke were observed during the ascending manoeuvre. The bat consumes about 0.67 W power to execute the turning-ascending manoeuvre, which is approximately two times the power consumed by similar bats during level flight. Upon comparison with a similar manoeuvre by a Hipposideros armiger bat (Windes et al. 2020 Bioinspir. Biomim. 16, abb78d. (doi:10.1088/1748-3190/abb78d)), some commonalities, as well as differences, were observed in the detailed wing kinematics and aerodynamics.

Downstroke and upstroke conflict during banked turns in butterflies

For all flyers, aeroplanes or animals, making banked turns involve a rolling motion which, due to higher induced drag on the outer than the inner wing, results in a yawing torque opposite to the turn. This adverse yaw torque can be counteracted using a tail, but how animals that lack tail, e.g. all insects, handle this problem is not fully understood. Here, we quantify the performance of turning take-off flights in butterflies and find that they use force vectoring during banked turns without fully compensating for adverse yaw. This lowers their turning performance, increasing turn radius, since thrust becomes misaligned with the flight path. The separation of function between downstroke (lift production) and upstroke (thrust production) in our butterflies, in combination with a more pronounced adverse yaw during the upstroke increases the misalignment of the thrust. This may be a cost the butterflies pay for the efficient thrust-generating upstroke clap, but also other insects fail to rectify adverse yaw during escape manoeuvres, suggesting a general feature in functionally two-winged insect flight. When lacking tail and left with costly approaches to counteract adverse yaw, costs of flying with adverse yaw may be outweighed by the benefits of maintaining thrust and flight speed.

Analysis of a 180-degree U-turn maneuver executed by a hipposiderid bat

Bats possess wings comprised of a flexible membrane and a jointed skeletal structure allowing them to execute complex flight maneuvers such as rapid tight turns. The extent of a bat's tight turning capability can be explored by analyzing a 180-degree U-turn. Prior studies have investigated more subtle flight maneuvers, but the kinematic and aerodynamic mechanisms of a U-turn have not been characterized. In this work, we use 3D optical motion capture and aerodynamic simulations to investigate a U-turn maneuver executed by a great roundleaf bat (Hipposideros armiger: mass = 55 g, span = 51 cm). The bat was observed to decrease its flight velocity and gain approximately 20 cm of altitude entering the U-turn. By lowering its velocity from 2.0 m/s to 0.5 m/s, the centripetal force requirement to execute a tight turn was substantially reduced. Centripetal force was generated by tilting the lift force vector laterally through banking. During the initiation of the U-turn, the bank angle increased from 20 degrees to 40 degrees. During the initiation and persisting throughout the U-turn, the flap amplitude of the right wing (inside of the turn) increased relative to the left wing. In addition, the right wing moved more laterally closer to the centerline of the body during the end of the downstroke and the beginning of the upstroke compared to the left wing. Reorientation of the body into the turn happened prior to a change in the flight path of the bat. Once the bat entered the U-turn and the bank angle increased, the change in flight path of the bat began to change rapidly as the bat negotiated the apex of the turn. During this phase of the turn, the minimum radius of curvature of the bat was 5.5 cm. During the egress of the turn, the bat accelerated and expended stored potential energy by descending. The cycle averaged total power expenditure by the bat during the six wingbeat cycle U-turn maneuver was 0.51 W which was approximately 40% above the power expenditure calculated for a nominally straight flight by the same bat. Future work on the topic of bat maneuverability may investigate a broader array of maneuvering flights characterizing the commonalities and differences across flights. In addition, the interplay between aerodynamic moments and inertial moments are of interest in order to more robustly characterize maneuvering mechanisms.

Determination of spatial fidelity required to accurately mimic the flight dynamics of a bat

Bats possess unique flight capabilities enabled by their wing morphology. While the articulated bone structure and flexible membrane constituting the wing are known to play a critical role in aerodynamic performance, the relationship has never been robustly quantified. Characterization of the sensitivity between precise wing contour and aerodynamic performance is important when designing a biomimetic flight vehicle based on experimentally measured wing kinematics. 3D optical motion capture, a standard method for obtaining wing kinematic measurements, discretely samples the smooth surface of a bat wing during flight. If the constellation of tracked 3D points is too sparse, a loss of critical information occurs. Here, we have explored the relationship between the density of wing surface points and several aerodynamic metrics, specifically, wing surface area variation, aerodynamic loads, and power expenditure. Loads and power were calculated using an incompressible Navier-Stokes solver. Of the metrics examined, aerodynamic power was found to be most sensitive to the spatial fidelity of the wing-the normalized root mean squared difference (NRMSD) between the 10- and 238-point cases was 35%. Load calculations varied slightly less with a peak NRMSD of 24% between the highest and lowest fidelity cases. Lastly, the wing surface area was least sensitive to the spatial fidelity of the wing kinematics, with a maximum NRMSD surface area of 8%. Close similarity in aerodynamic behavior was observed when using either a 120- and 238-point surface representation, establishing a bound to the sensitivity between wing shape and aerodynamics. The results from the 10- and 22-point configurations demonstrate that sparse representation of a wing surface can lead to a loss of information. The characterization of kinematic complexity of the wings both informs how many degrees of freedom are important to measure and also informs how many degrees of freedom are required to robotically reproduce the flapping flight.

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

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.