title: The function of the alula in avian flight.
abstract: The alula is a small structure located at the joint between the hand-wing and arm-wing of birds and is known to be used in slow flight with high angles of attack such as landing. It is assumed to function similarly to a leading-edge slat that increases lift and delays stall. However, in spite of its universal presence in flying birds and the wide acceptance of stall delay as its main function, how the alula delays the stall and aids the flight of birds remains unclear. Here, we investigated the function of alula on the aerodynamic performance of avian wings based on data from flight tasks and wind-tunnel experiments. With the alula, the birds performed steeper descending flights with greater changes in body orientation. Force measurements revealed that the alula increases the lift and often delays the stall. Digital particle image velocimetry showed that these effects are caused by the streamwise vortex, formed at the tip of the alula, that induces strong downwash and suppresses the flow separation over the wing surface. This is the first experimental evidence that the alula functions as a vortex generator that increases the lift force and enhances manoeuvrability in flights at high angles of attack.
authors.affiliation: 1] Institute of Advanced Machines and Design, Seoul National University, Seoul, Korea [2] School of Biological Sciences, Seoul National University, Seoul, Korea.
authors.forename: Sang-im
authors.initials: SI
authors.lastname: Lee
authors.affiliation: Department of Mechanical &Aerospace Engineering, Seoul National University, Seoul, Korea.
authors.forename: Jooha
authors.initials: J
authors.lastname: Kim
authors.affiliation: 1] Institute of Advanced Machines and Design, Seoul National University, Seoul, Korea [2] Department of Mechanical &Aerospace Engineering, Seoul National University, Seoul, Korea.
authors.forename: Hyungmin
authors.initials: H
authors.lastname: Park
authors.affiliation: 1] School of Biological Sciences, Seoul National University, Seoul, Korea [2] Museum of Zoology, Polish Academy of Sciences, Warsaw, Poland.
authors.forename: Piotr G
authors.initials: PG
authors.lastname: Jabłoński
authors.affiliation: 1] Institute of Advanced Machines and Design, Seoul National University, Seoul, Korea [2] Department of Mechanical &Aerospace Engineering, Seoul National University, Seoul, Korea.
authors.forename: Haecheon
authors.initials: H
authors.lastname: Choi
mesh_terms: D000818:Animals; D001717:Birds / Q000033:anatomy & histology* / Q000502:physiology*; D005426:Flight, Animal; D008297:Male; D014921:Wings, Animal / Q000033:anatomy & histology*