Alula on a bird’s wing: Tiny but mighty!

Abstract

The alula, often called as a bird’s “thumb”, is a tiny structure that is composed of two to six feathers on a bird’s wing. This small structure can be found not only in extant flying birds such as falcon, kestrel, magpie, and mallard, but also in the fossils of several early ancestors of birds. From countless observations, it has been known that the bird folds its alula in most situations, but moves it slightly upwards and forward in slow and steep flight such as landing. Despite its small size, the alula is assumed to have significant fluid-dynamic effects of increasing lift and delaying stall. However, in spite of its universal presence in flying birds and the wide acceptance of stall delay as its main function, it still remains unclear how the alula delays the stall and aids the flight of birds. In this presentation, the mechanism of aerodynamic performance enhancement by alula will be discussed based on the wind-tunnel experiments with a real bird’s wing, and the three-dimensional effects of the alula will be compared to the stall-delaying effect of two-dimensional leading edge slat. In addition, the potential application of this mechanism in engineering will be suggested with some examples.