TOBALSKE, B.W.*; WARRICK, D.R.; POWERS, D.R.; Univ. of Portland; Oregon State Univ.; George Fox Univ.: Dynamics of the hummingbird wake during forward flight over a wide range of speeds.
Hummingbirds fly with their wings almost fully extended during their entire wingbeat. This pattern is associated with having proportionally short humeral bones and long distal wing elements. Previous workers have predicted that an extended wing posture during upstroke should produce negative thrust during forward flight, thereby presenting a functional limit locomotor performance. To test this, we flew rufous hummingbirds (Selasphorus rufus, 3.3 g, n = 4) in a variable-speed wind tunnel (0 – 12 m s-1). We measured 3D wing kinematics using high-speed video (500 Hz), and wake structure and dynamics using digital particle image velocimetry (DPIV). Span ratio declined with increasing flight speed, but it was always greater than 90%, consistent with an extended wing during upstroke. Our flow-visualization data supported the prediction that upstroke forces present a cost, rather than a benefit, during forward flight in hummingbirds. From 4 to 12 m s-1, mid and late upstroke induced an upward velocity to the air, opposing weight support. At mid-upstroke, a counter-rotating pair of vortices (doublet) was shed into the wake. We hypothesize that this doublet results from pressure drag on the wing. Alternatively, it may be generated by long-axis rotation of the wing. Absolute peak vorticity in the upstroke doublet was 843 + 200 s-1 and absolute circulation was 0.08 + 0.03 m2 s-1. These values were similar to absolute vorticity and circulation during downstroke (800 + 150 s-1 and 0.08 + 0.04 m2 s-1, respectively). Our results provide new insight into the function of a morphologically distinct, kinematically “stiff” wing. NSF IBN-0327380
