Meeting Abstract

70.3  Tuesday, Jan. 6  No net thrust on the upstroke: the effect of wing inertia on body accelerations of fruit bats during flight IRIARTE-DIAZ, J*; RISKIN, D. K.; SWARTZ, S. M.; Brown University; Brown University; Brown University jiriarte@uchicago.edu

During slow flight, some bats species produce a tip-reversal upstroke, where the distal portion of the wing is moved upward and backward with respect to still air. Tip-reversal upstroke has been widely hypothesized to produce thrust during upstroke, based on the observation that a bat accelerates its torso forward during upstroke in low speed flights. This forward acceleration, however, could be produced by inertial forces generated by the backward flapping motion of the wings at that part of the wingbeat cycle, rather than acceleration of the center of mass (CoM) resulting from the interaction of the wings with the airflow. To investigate the instantaneous aerodynamic force production during the upstroke and downstroke portions of the wingbeat cycle, we developed a model of the mass distribution of the wing and body of the lesser dog-faced fruit bat, Cynopterus brachyotis during flight at speeds of 3 to 8 m s^-1, based on detailed high-speed, three-dimensional kinematics. The mass model allowed us to determine the position of the CoM and therefore to calculate the accelerations of the CoM independently of torso accelerations. We found that bats used tip-reversal upstroke only during slow flight and that the torso accelerated forward during tip-reversal upstroke. This acceleration, however, was the result of the inertial force produced by the motion of the wings. Inertial forces affected both vertical and forward acceleration measurements at all speeds, but the horizontal inertial component decreased as speed increased while the vertical component remained constant across speeds. Our results highlight the importance of the incorporation of inertial components to the study of acceleration in flapping organisms.