Characteristics of the alula in relation to wing and body size in the Laridae and Sternidae

An aerodynamic structure ubiquitous in Aves is the alula; a small collection of feathers muscularized near the wrist joint. New research into the aerodynamics of this structure suggests that its primary function is to induce leading-edge vortex (LEV) flow over bird's outer hand-wing to enhance wing lift when manuevering at slow speeds. Here, we explore scaling trends of the alula's spanwise position and its connection to this function. Specifically, we test the hypothesis that the relative distance of the alula from the wing tip is that which maximizes LEV-lift when the wing is spread and operated in a deep-stall flight condition. To test this, we perform experiments on model wings in a wind tunnel to approximate this distance and compare our results to positional measurements of the alula on spread-wing specimens. We found the position of the alula on non-aquatic birds selected for alula optimization to be located at or near the lift-maximizing position predicted by wind tunnel experiments. These findings shed new light on avian wing anatomy and the role of unconventional aerodynamics in shaping it. A bird's alula consists of a small cohort of feathers, approximately one-eighth the length of the bird's wing, that stem from the bird's primary digit, or thumb. It is an evolutionary adaptation observed in fossils of primitive birds 1-3 , and exists on all modern birds (minus hummingbirds) 4. The function of the alula is widely considered to be aerodynamic, although some research has indicated a possible sensory role 5. During landing, birds tilt their wings to high angles to slow descent 4,6 and protract their alula upwards from the plane of the wing 6 to prevent wing stall and the subsequent loss of wing lift 7-14 (see Fig. 1). This function enables birds to perform steeper descents with greater changes in body orientation when landing 10. Thus, additional knowledge of these flight feathers can advance our understanding of avian flight. Aerodynamics of the alula. Despite consensus among researchers regarding the importance of the alula in avian flight, the aerodynamic mechanisms underlying its function remain debated. The gap formed between the deflected alula and the top surface of the wing (see Fig. 1) has led to early comparisons of it to flow control devices on aircraft such as leading-edge slots/slats 7-9. These devices prevent wing stall by ensuring the flow remains smoothly attached to the wing. Subsequent research depicting separated flow over real and model swift wings in steady flight 15,16 and Passerines in slow-flapping flight 17 suggests that the alula likely prevents wing stall through the maintenance of separated-edge flows rather than preventing flow separation from occurring in the first place. These observations have prompted a revaluation of the aerodynamics of the alula for which two updated interpretations of its function have been proposed 4,6. First, that the alula generates a small vortex which separates the attached-flow system on the inner, thick-profiled, arm-wing section and the separat...

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“…Scaling trends in flight feathering. Measurements of the alular feathers are sparse in the literature 7,10,11 .…”

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confidence: 99%

Scaling trends of bird’s alular feathers in connection to leading-edge vortex flow over hand-wing

Linehan

Mohseni

2020

Sci Rep

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“…Each final wing shape was attached to a gull-inspired body. We selected a NACA0020 aerofoil based on an estimated body length (41 cm) and body height that was estimated using an allometric relationship with a body mass of 0.91 kg [34,35]. The shoulder joint was positioned at the quarter-chord of the body.…”

Section: Determining the Wing-body Configurationsmentioning

confidence: 99%

Gull-inspired joint-driven wing morphing allows adaptive longitudinal flight control

Harvey

Baliga

Goates

et al. 2021

J. R. Soc. Interface.

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Birds dynamically adapt to disparate flight behaviours and unpredictable environments by actively manipulating their skeletal joints to change their wing shape. This in-flight adaptability has inspired many unmanned aerial vehicle (UAV) wings, which predominately morph within a single geometric plane. By contrast, avian joint-driven wing morphing produces a diverse set of non-planar wing shapes. Here, we investigated if joint-driven wing morphing is desirable for UAVs by quantifying the longitudinal aerodynamic characteristics of gull-inspired wing-body configurations. We used a numerical lifting-line algorithm (MachUpX) to determine the aerodynamic loads across the range of motion of the elbow and wrist, which was validated with wind tunnel tests using three-dimensional printed wing-body models. We found that joint-driven wing morphing effectively controls lift, pitching moment and static margin, but other mechanisms are required to trim. Within the range of wing extension capability, specific paths of joint motion (trajectories) permit distinct longitudinal flight control strategies. We identified two unique trajectories that decoupled stability from lift and pitching moment generation. Further, extension along the trajectory inherent to the musculoskeletal linkage system produced the largest changes to the investigated aerodynamic properties. Collectively, our results show that gull-inspired joint-driven wing morphing allows adaptive longitudinal flight control and could promote multifunctional UAV designs.

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“…Through a summary and analysis of the current research, we can find that some pretty good insights have been gained from studies on the alula. Firstly, studies have found that the alula is mostly used by birds during takeoff and landing, playing the role of delaying flow separation and increasing lift, similar to high-lift devices in fixed-wing aircraft [1,2]. Secondly, there have been two main predictions regarding the aerodynamic mechanisms underlying the lift enhancement of the alula, which are the leading-edge slot effect [4,5] and the vortex generator effect [6].…”

Section: Aerodynamic Mechanism Of the Alulamentioning

confidence: 99%

“…Also their results showed that the ratio of the length of the alula to the wing span decreases with the increasing aspect ratio of the wing. Lee and Choi [2] investigated the relationship between the size and shape of the alula and the features of the wing in the Laridae and Sternidae. They found that for Laridae, the aspect ratio of the alula was smaller in the species that have relatively longer wings, but the pattern was opposite in the Sternidae, the species with high loading potential and long wings exhibit long alula.…”

Section: Morphological Measurements and Statistics Of The Alulamentioning

confidence: 99%

“…However, with the increase in AoA, there is a risk of stalling, which is of great danger to flight. At this time, birds often deflect their alula, playing a role similar to high-lift devices in fixed-wing aircraft [2]. This paper includes two major sections: one evaluates the current research on the function of the alula, and the other predicts studies for future research on the alula.…”

Section: Introductionmentioning

confidence: 99%

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The Progress of Aerodynamic Mechanisms Based on Avian Leading-Edge Alula and Future Study Recommendations

et al. 2021

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Birds in nature have many unique devices to help them acquire excellent flight abilities under various complex flight conditions. One of the unique devices is the leading-edge alula, located at the junction of the arm wing and the hand wing of most birds. It often spreads out during takeoff and landing, probably playing a similar role to high-lift devices in fixed-wing aircraft. This paper analyzed and reviewed the results of current research on leading-edge alula, finding some important factors, such as the complex flapping motions, flexibility, and the plane and section shape of the wing, that have been ignored in current research to a certain extent. These would greatly affect the conclusions obtained. Hence, for a deeper understanding of the aerodynamic mechanisms and functions of the alula, some new study predictions for future research are presented. In addition, the feasible models and methods for further research based on these predictions are discussed and proposed. For example, the higher-accuracy LES or hybrid LES/RANS method and the combinations of these methods with wind-tunnel experiments using PIV technology are recommended.

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Characteristics of the alula in relation to wing and body size in the Laridae and Sternidae

Cited by 10 publications

References 17 publications

Scaling trends of bird’s alular feathers in connection to leading-edge vortex flow over hand-wing

Scaling trends of bird’s alular feathers in connection to leading-edge vortex flow over hand-wing

Gull-inspired joint-driven wing morphing allows adaptive longitudinal flight control

The Progress of Aerodynamic Mechanisms Based on Avian Leading-Edge Alula and Future Study Recommendations

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