DRAG REDUCTION BY WING TIP SLOTS IN A GLIDING HARRIS HAWK, PARABUTEO-UNICINCTUS

The anterior-most primary feathers of many birds that soar over land b end upwards and separate vertically to form slotted wing tips during f light, The slots are thought to reduce aerodynamic drag, although drag reduction has never been demonstrated in living birds, Wing theory ex plains how the feathers that form the tip slots can reduce induced dra g by spreading vorticity horizontally along the wing and by acting as winglets, which are used on aircraft to make wings non-planar and to s pread vorticity vertically. This study uses the induced drag factor to measure the induced drag of a wing relative to that of a standard pla nar wing with the same span, lift and speed, An induced drag factor of less than 1 indicates that the wing is non-planar. The minimum drag o f a Harris' hawk gliding freely in a wind tunnel was measured before a nd after removing the slots by clipping the tip feathers, The unclippe d hawk had 70-90 % of the drag of the clipped hawk at speeds between 7 .3 and 15.0 m s(-1). At a wing span of 0.8 m, the unclipped hawk had a mean induced drag factor of 0.56, compared with the value of 1.10 ass umed for the clipped hawk. A Monte Carlo simulation of error propagati on and a sensitivity analysis to possible errors in measured and assum ed values showed that the true mean value of the induced drag factor f or the unclipped hawk was unlikely to be more than 0.93. These results for a living bird support the conclusions from a previous study of a feathered tip on a model wing in a wind tunnel: the feathers that form the slotted tips reduce induced drag by acting as winglets that make the wings non-planar and spread vorticity both horizontally and vertic ally.