HEARN, J.W.D.**; TOBALSKE, B.W.; WARRICK, D.R.; University of Portland; University of Portland; Oregon State University: Contribution of Body Lift to the Aerodynamics of Flap-Bounding

Prior work suggests body lift plays a critical role in making flap-bounding an aerodynamically attractive flight strategy. We used a variety of methods to elucidate the contributions of the body and tail to lift and drag during the bound phase. Digital particle image velocimetry (DPIV) was utilized to generate vector fields to mathematically describe air flow around live birds (Zebra finch, Taeniopygia guttata; n = 4) flying in a variable speed wind tunnel, as well as flow around taxidermically prepared specimens (n = 4) mounted on a force balance. We varied airspeed from 2-12 m/s and body angle from -15 to 50 degrees. To reveal the 3D geometry of the wake we sampled median, parasagittal, and transverse planes. There was good agreement between DPIV and force balance data. For mounted specimens, lift:drag ratios were greater than 1 at body angles from 10 to 30 degrees, and generally peaked near 1.3 at 15 degrees. This is consistent with the mean body angle adopted during flight. Comparing live and mounted birds, lift:drag ratios were similar, but lift and drag coefficients were 3 times greater in mounted specimens versus live birds. When the tail is present, the wake comprises a pair of counter-rotating vortices with downwash present over 75% of the width of the bird�s body. This downwash is also present during upstroke, which indicates that body lift is produced during both flapping and non-flapping phases. There is a significant amount of lift produced by the body even when the tail is removed. Our results indicate that body lift is responsible for 8% of weight support, and functions during all phases of flap-bounding. Existing mathematical theory suggests that the observed body lift would make the flight style attractive at speeds of 6 m/s and faster. NSF IBN-0327380