Kinematics of flight and the relationship to the vortex wake of a Pallas' long tongued bat (Glossophaga soricina)

Abstract

To obtain a full understanding of the aerodynamics of animal flight, the movement of the wings, the kinematics, needs to be connected to the wake left behind the animal. Here the detailed 3D wingbeat kinematics of bats, Glossophaga soricina, flying in a wind tunnel over a range of flight speeds (1-7 m s(-1)) was determined from high-speed video. The results were compared with the wake geometry and quantitative wake measurements obtained simultaneously to the kinematics. The wingbeat kinematics varied gradually with flight speed and reflected the changes observed in the wake of the bats. In particular, several of the kinematic parameters reflected the differences in the function of the upstroke at low and high flight speeds. At lower flight speeds the bats use a pitch-up rotation to produce a backward flick which creates thrust and some weight support. At higher speeds this mechanism disappears and the upstroke generates weight support but no thrust. This is reflected by the changes in e.g. angle of attack, span ratio, camber and downstroke ratio. We also determined how different parameters vary throughout a wingbeat over the flight speeds studied. Both the camber and the angle of attack varied over the wingbeat differently at different speeds, suggesting active control of these parameters to adjust to the changing aerodynamic conditions. This study of the kinematics strongly indicates that the flight of bats is governed by an unsteady high-lift mechanism at low flight speeds and points to differences between birds and bats.