USHERWOOD, J.R.; HEDRICK, T.L.; BIEWENER, A.A.; Harvard University: Aerodynamics of flapping bird wings: insights from direct pressure measurements.

Vertebrates power flight with flapping wings of complex and changing geometries. The aerodynamics of such dynamic aerofoils remains obscure. While a fixed-wing view of aerodynamics is likely to be appropriate for gliding and soaring flight, flapping flight involves accelerations, rotations, wing-wing interactions, and dynamic changes in both planform and sectional wing shape, with potential for strongly 3-dimensional flows. Thus, analogies based on either fixed-wing or propeller aerodynamics are potentially misleading for much of animal flight. We describe techniques allowing direct, dynamic pressure measurements along flapping wings, with results for differential pressures across the wings of Canada geese in take-off flight, and initial results for absolute dorsal and ventral pressure measurements for ringed turtle-doves flying in a wind-tunnel. Pressure differentials at five sites along goose wings reach over four times the wing loading during downstroke, and reverse at the wingtip during upstroke to a dorsal-ventral pressure close to wing loading. The results show: maintenance of ventral-to-dorsal pressure at the most proximal site throughout the wingstroke cycle; reversal of the pressure sense at the most distal site; and double pressure peaks at the distal sites during the downstroke. Maintenance of pressure differential at wing base, and reversal of pressure sense at wingtip, suggest the view of slow flight as a ‘vortex-ring gait’ may conceal some interesting details. The double pressure peaks during downstroke may be related to management of the kinetic energy of flapping wings.