The Alula and Its Aerodynamic Effect on Avian Flight

Abstract

The alula, a small thumb-like appendage on a bird wing, is often credited with increasing lift and decreasing the risk of stall during bird flight. Using field based studies; researchers have observed that the alula lifts away from the wing at critical moments in flight, such as take-off and landing. However, to date, there has been no conclusive experimental evidence to support the idea that use of the alula affects lift. To determine the effect of the alula on avian flight, we used a wind tunnel to study the wings of four ducks: the Wood Duck (Aix sponsa), the Redhead Duck (Aythya americana), the Black Scoter (Melanitta americana), and the Lesser Scaup (Aythya affinis). We used a combination of lift/drag measurements and Particle Image Velocimetry (PIV) to test the wings at velocities from 10–16 m/s and angles of attack from −20 to 25 degrees. The alula was observed to naturally lift as the stall angle was approached. Of the four wings, the Black Scoter demonstrated the largest maximum lift coefficient (1.4), followed by the Wood Duck (1.3), the Lesser Scaup (1.2) and lastly, the Redhead Duck (0.9). All four wings had minimum drag coefficients near 0.1. The Lesser Scaup was the only wing which had a measurable change in lift (10%) attributable to alula deployment. PIV results for the flow field around the Lesser Scaup wing showed higher velocities on the top side of the wing when the alula was deflected.