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

Abstract

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.

Turning-ascending flight of a Hipposideros pratti bat

“…However, the morphological parameters of the current study are in the same range as that of Windes et al . [ 49 , 50 ] in spite of the bat being from a different species. The mass (1% heavier) and span (3% larger) are almost identical, but the H. pratti bat in the current study has a wing area larger by 20%, and thus a 15% larger planform area.…”

“…[60][61][62]). For further details about the computational set-up used for the current paper, please refer to prior work on bat flight aerodynamics [35,39,49,50]. The computational domain extends from 8 chord lengths upstream to 24 chord lengths downstream in the x g -direction with a cross-section of 16 × 16 chord lengths in the y g -and z gdirections, respectively, representing the tunnel cross-section.…”

“…Thus, the current study follows a computational approach to deconstruct the detailed kinematic-aerodynamic nuances of a manoeuvring bat flight. This approach has been used in the literature mostly for level flight [31,35,39] and recently for manoeuvring flight by Windes et al [49,50]. Using measured kinematics and aerodynamic simulations, Windes et al [49] investigated a right ascending sweeping turn of a large insectivorous bat (Hipposideros armiger) and found simultaneous and synergistic banking and yawing to be the fundamental turning mechanism.…”

“…This approach has been used in the literature mostly for level flight [ 31 , 35 , 39 ] and recently for manoeuvring flight by Windes et al . [ 49 , 50 ]. Using measured kinematics and aerodynamic simulations, Windes et al .…”

“…Using measured kinematics and aerodynamic simulations, Windes et al [49] investigated a right ascending sweeping turn of a large insectivorous bat (Hipposideros armiger) and found simultaneous and synergistic banking and yawing to be the fundamental turning mechanism. Later, an analysis of a U-turn of the same bat revealed that active control of the velocity along with the body rotations allows the bat to achieve the centripetal force for the 180°turn [50].…”