WARRICK, Douglas R.; TOBALSKE, Bret W.; POWERS, Don R.; Oregon State University; University of Portland; George Fox University: Kinematic Symmetry and Aerodynamic Asymmetry in the Hovering Flight of Hummingbirds

Hummingbirds sustain hovering using kinematics unique among birds. Because of the apparent symmetry of their upstroke and downstroke, it has long been assumed that during hovering their upstroke is aerodynamically equivalent to their downstroke. To test this assumption, we examined the 3D wing kinematics (using 500 Hz video), and near-field airflow (using DPIV) during the hovering flight of rufous hummingbirds (Selasphorus rufus, body mass 3.3 g, n = 4). Kinematics suggested that downstroke and upstroke were similar in function, but such functional similarity was not supported by the DPIV data. Span ratio was 98 + 3%, approximately half the wingbeat cycle was spent in upstroke, and the wings were supinated during upstroke to nearly 100&deg relative to the midline of the body. Including local airflow, the angle of incidence of the wing varied continuously during the wingbeat from 80&deg at the start of downstroke to -80&deg at the start of upstroke, and was similar at mid-downstroke and mid-upstroke (25&deg and -25&deg respectively). In contrast, wake structure was highly asymmetric. Absolute values for peak vorticity were 2109 + 608 s^-1during downstroke and 1253 + 445 s^-1during upstroke. Absolute downstroke circulation (measured at tip vortices) was 0.10 + 0.01 m² s^-1, five times that of absolute upstroke circulation (0.02 + 0.01 m² s^-1). Induced velocity in the middle of the momentum jet, averaged over multiple wingbeats, was 2.4 + 0.4 ms^-1. We attribute the surprising disparity between upstroke and downstroke to the hummingbird’s cambered airfoil that is inverted during upstroke. NSF IBN-0327380