When the alula was present, the duration of total descending flight was shorter (GLMM, F 1,20 = 9.76, P = 0.005 Fig. During the second wingbeat, the speed was greatly reduced and the deflection of secondary coverts was visible which indicates flow separation over the suction surface of the wing 2. They performed steep and fast descents during the first wingbeat and leveled their bodies during or at the end of the second wingbeat.
%20wing%20(500%20x%20661).jpg "bird wing alula")
From a 170 cm-high perch, the birds landed on the ground after three to four wingbeats.
Bird wing alula free#
In order to describe how the alula affects the flight of birds, we analysed the kinematics of free descending flights of four juvenile captive Eurasian magpies ( Pica pica). In the present study, we conducted field observations and force and velocity measurements to evaluate the hypotheses regarding the three-dimensional effects of the alula in avian flight. However, no experiments have been conducted to evaluate these hypotheses. Recently, two possible three-dimensional effects of the alula were hypothesized 5, 6: (i) the induction of leading edge vortex over the hand-wing (distal part of the wing) and (ii) the prevention of the spanwise thickening of the boundary layer (which otherwise would encourage flow separation) by inhibiting the spanwise flow with small-scale vortices created by the alula (i.e., similar to the wing fence and vortilon used in backward-swept wing aircraft). Also, the alula is a three-dimensional structure, so its function may not be similar to the stall-delaying effect of two-dimensional leading edge slat 11. However, not all the previous studies affirmed the stall-delaying effect of the alula and the results of those studies were not conclusive and failed to provide a generalized view on the function of alula in actual flights of birds. Particularly, the classic paper by Nachtigall and Kempf 7 rigorously conducted force measurements on the dried wings of three avian species and described how birds used the alula in landing. Among the previous studies that have attempted to reveal the aerodynamic mechanism of alula functioning as a leading-edge slat 7, 8, 9, 10, some found the lift enhancing effect of the alula 7, 9, 10 (in one study, one out of four specimens had measurable lift enhancement 10). Thus, it has been widely accepted that the alula aids slow flights or flights at high attack angles such as landing 1, 5, 7. It has long been assumed that the alula functions in a similar manner to that of the extended leading-edge slat in an aircraft, which increases the lift force at high angles of attack with delaying the stall 6. Its presence is universal in extant flying birds and can also be found in the fossils of several early ancestors of birds 2, 3, 4. The alula is a small structure that is composed of a digit bone and two to six feathers 1.
Bird wing alula generator#
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.
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. Force measurements revealed that the alula increases the lift and often delays the stall. With the alula, the birds performed steeper descending flights with greater changes in body orientation. Here, we investigated the function of alula on the aerodynamic performance of avian wings based on data from flight tasks and wind-tunnel experiments. 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. It is assumed to function similarly to a leading-edge slat that increases lift and delays stall. 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.
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