Aerodynamics of intermittent bounds in flying birds

Flap-bounding is a common flight style in small birds in which flapping phases alternate with flexed-wing bounds. Body lift is predicted to be essential to making this flight style an aerodynamically attractive flight strategy. To elucidate the contributions of the body and tail to lift and drag during the flexed-wing bound phase, we used particle image velocimetry (PIV) and measured properties of the wake of zebra finch (Taeniopygia guttata, N=5), flying at 6-10 m s^-∈1 in a variable speed wind tunnel as well as flow around taxidermically prepared specimens (N=4) mounted on a sting instrumented with force transducers. For the specimens, we varied air velocity from 2 to 12 m s^-∈1 and body angle from -15 to 50. The wake of bounding birds and mounted specimens consisted of a pair of counterrotating vortices shed into the wake from the tail, with induced downwash in the sagittal plane and upwash in parasagittal planes lateral to the bird. This wake structure was present even when the tail was entirely removed. We observed good agreement between force measures derived from PIV and force transducers over the range of body angles typically used by zebra finch during forward flight. Body lift:drag (L:D) ratios averaged 1.4 in live birds and varied between 1 and 1.5 in specimens at body angles from 10 to 30. Peak (L:D) ratio was the same in live birds and specimens (1.5) and was exhibited in specimens at body angles of 15 or 20, consistent with the lower end of body angles utilized during bounds. Increasing flight velocity in live birds caused a decrease in C_L and C_D from maximum values of 1.19 and 0.95 during flight at 6 m s^-∈1 to minimum values of 0.70 and 0.54 during flight at 10 m s^-∈1. Consistent with delta-wing theory as applied to birds with a graduated-tail shape, trimming the tail to 0 and 50% of normal length reduced L:D ratios and extending tail length to 150% of normal increased L:D ratio. As downward induced velocity is present in the sagittal plane during upstroke of flapping flight, we hypothesize that body lift is produced during flapping phases. Future efforts to model the mechanics of intermittent flight should take into account that flap-bounding birds may support up to 20% of their weight even with their wings fully flexed.