TOBALSKE, B.W.**; WARRICK, D.R.; POWERS, D.R.; Univ. of Portland; Oregon State Univ.; George Fox Univ.: Effect of Wing Design on Wake Structure in Small Flying Birds

Two species, rufous hummingbird (Selasphorus rufus , 3 g) and zebra finch (Taeniopygia guttata, 16 g) represent opposites with regard to wing design and kinematics among small birds. The hummingbird has pointed wings of high aspect ratio, and the finch has rounded wings of low aspect ratio. Across all flight speeds, upstroke : downstroke span ratio is greater than 95 % in the hummingbird and less than 20 % in the finch. We used digital particle image velocimetry (DPIV) to investigate the aerodynamic consequences of these interspecific differences in design. We measured circulation in the wake as the animals flew at speeds from 4 to 12 m/s in a variable-speed wind tunnel. Upstroke aerodynamics were distinct between species. At mid-upstroke, hummingbird wings shed a counter rotating vortex pair that was due to the ending and starting of lift production on the distal wing during early upstroke and late upstroke, respectively. The vortex pair indicates thrust production at the expense of weight support. This is a novel mechanism for matching pressure drag during forward flight; previously, it was thought the hummingbird would suffer negative thrust during upstroke. In contrast with the hummingbird wing, the finch wing does not shed substantial vorticity during upstroke. Using body lift, the finch induces downward velocity to the air flowing over the body, and this downwash is continuous throughout the entire wingbeat cycle. In the hummingbird, induced downwash through most of the wingbeat reverses to upwash during late upstroke. Our results confirm that wing design has a dramatic effect upon aerodynamics, and they improve understanding of the predictive limitations of purely kinematic data. NSF IBN-0327380