GLIDING BIRDS - REDUCTION OF INDUCED DRAG BY WING TIP SLOTS BETWEEN THE PRIMARY FEATHERS

1. The feathers at the wing tips of most birds that soar over land sep arate both horizontally and vertically in flight to form slotted tips. The individual feathers in the slotted tips resemble the winglets use d on the wing tips of some aircraft to reduce induced drag. 2. A wing that produces lift leaves a pair of vortex sheets in its wake. Wing th eory shows that winglets can reduce the kinetic energy left in the vor tex sheets, and hence the induced drag, by spreading vorticity both ho rizontally and vertically. 3. This paper describes the aerodynamic for ces on a wing made of a base wing and different wing tips. The feather ed wing tip was slotted and was made of four primary feathers from a H arris' hawk (Parabuteo unicinctus). The Clark Y tip was unslotted and was made of balsa wood shaped to a Clark Y aerofoil. The balsa feather tip was slotted and was made of three balsa wood wings shaped like fe athers. 4. The base wing in a wind tunnel at an air speed of 12.6 m s- 1 generated upwash angles as high as 15-degrees at the end of the wing when the angle of attack of the wing was 10.5-degrees. The feathered tip responded to upwash by increasing its lift to drag ratio (L/D) by 107%, from 4.9 to 10.1, as the angle of attack of the base wing increa sed from 4-degrees to 14-degrees. The L/D values of the balsa feather tip and the Clark Y tip increased by 49% and 5%, respectively, for the same change in angle of attack. 5. With the angle of attack of the ba se wing fixed at 13-degrees, changing the angle of attack of the wing tip changed the drag of the base wing. The drag of the base wing incre ased by 25% as the angle of attack of the Clark Y tip increased from 0 -degrees to 15-degrees. The base wing drag decreased by 6% for the sam e change in the angle of attack of the feathered tip. 6. The total dra g of the wing with the feathered tip was 12% less than that of a hypot hetical wing with the same lift and span, but with tip feathers that d id not respond to upwash at the end of the base wing. This value is co nsistent with wing theory predictions on drag reduction from winglets. 7. Wings with the tip and the base wing locked together had lift and drag that increased with increasing base wing angle of attack, as expe cted for conventional wings. Span factors were calculated from these d ata - a large span factor indicates that a wing has low induced drag f or a given lift and wing span. The wing with the Clark Y tip had a spa n factor that decreased from 1 to 0.75 as the angle of attack of the b ase wing increased. Over the same range of angle of attack. the span f actor of the wing with the feathered tip remained constant at 0.87. As the angle of attack of this wing increased, aerodynamic forces spread the feathers vertically to form slots. With fully formed slots, the w ing had a higher span factor than the wing with the unslotted Clark Y tip. 8. Flow visualization with helium-filled bubbles showed that the addition of two winglets to the tip of a model wing spread vorticity b oth horizontally and vertically in the wake of the tip. 9. These obser vations taken together provide strong evidence that the tip slots of s oaring birds reduce induced drag in the sense that the separated tip f eathers act as winglets and increase the span factor of the wings.