Whole-body kinematics of a fruit bat reveal the influence of wing inertia on body accelerations

Abstract

SUMMARYThe center of mass (COM) of a flying animal accelerates through space because of aerodynamic and gravitational forces. For vertebrates, changes in the position of a landmark on the body have been widely used to estimate net aerodynamic forces. The flapping of relatively massive wings, however, might induce inertial forces that cause markers on the body to move independently of the COM, thus making them unreliable indicators of aerodynamic force. We used high-speed three-dimensional kinematics from wind tunnel flights of four lesser dog-faced fruit bats, Cynopterus brachyotis, at speeds ranging from 2.4 to 7.8ms -1 to construct a time-varying model of the mass distribution of the bats and to estimate changes in the position of their COM through time. We compared accelerations calculated by markers on the trunk with accelerations calculated from the estimated COM and we found significant inertial effects on both horizontal and vertical accelerations. We discuss the effect of these inertial accelerations on the long-held idea that, during slow flights, bats accelerate their COM forward during 'tip-reversal upstrokes', whereby the distal portion of the wing moves upward and backward with respect to still air. This idea has been supported by the observation that markers placed on the body accelerate forward during tip-reversal upstrokes. As in previously published studies, we observed that markers on the trunk accelerated forward during the tip-reversal upstrokes. When removing inertial effects, however, we found that the COM accelerated forward primarily during the downstroke. These results highlight the crucial importance of the incorporation of inertial effects of wing motion in the analysis of flapping flight. Supplementary material available online at

Upstroke wing flexion and the inertial cost of bat flight

“…Because the wings themselves have mass, the total metabolic energy required to fly includes both the energy imparted to the fluid and the additional cost of accelerating and decelerating the wings during the flapping cycle [1]. The cost of accelerating the wings, the inertial work, can exceed the cost of moving the wings through the air, the aerodynamic work [2,3], so understanding the mechanics of animal flight requires an accurate understanding of its inertial cost [3].…”

“…In the subsequent upstroke, the joints of the wings are flexed, adducted and, especially in birds, supinated, to varying degrees. These motions together have the effect of producing a folded-wing posture [2,[8][9][10][11], reducing J w on the upstroke, which should in turn reduce the overall inertial cost of the wingbeat cycle compared with what the cost would be if wing posture was not changed. Inertial costs might therefore be one factor underlying upstroke wing flexion and adduction, hereafter termed folding [5,6,12,13].…”

Inertial attitude control of a bat-like morphing-wing air vehicle

“…In bats, there is biological evidence that the inertial forces produced by the wings have a significant contribution to the attitude movements of the animal, even more significant than aerodynamic forces [13], [14]. In fact, bats perform complex aerial rotations by solely modulating wing inertia [20], [21].…”

Design and Control of Flapping Wing Micro Air Vehicles

“…The wing folding and expanding are controlled through the contraction and extension at elbow, and abduction and adduction at wrist. Combining those movements of shoulder, characteristic traces of wing tips will be emerged in 3D space (134,135,136). The remaining joints within the digits could change the dynamic shape of the wing in camber.…”

“…Bat wings consist of more than two dozens of independently controlled joints, which provide them with the capability to positively adjust the wing morphing according to the aerodynamic demands in flight (27). Additionally, the high accelerations generated by inertial forces of massive wings can enhance their own maneuverability (184,185,186). Taking advantage of such forces, bats can perform roll and pitch maneuvers by solely controlling wings geometry, without the need of appendices that need aerodynamic loads (i.e.…”