Leading-edge flow reattachment and the lateral static stability of low-aspect-ratio rectangular wings

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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“…Wind tunnel. The wind tunnel set-up has been described in multiple publications by our group 19,47,48 .…”

Section: Methodsmentioning

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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“…It emerges that as the sideslip angle increases, the windward tip vortex core grows in size and strength, displacing and leading-edge separation further toward the leeward wing. 49 Such a flow phenomenon is similar to the flow structure over the suction surface of a free-to-roll LAR rectangular flat-plate at moderate to high angles of attack. 43 Figure 15 presents a surface tufts visualization for a rolling rectangular flat-plate wing with AR = 2 at a large roll angle, when α = 17°, β = 0°, and Re = 1.14 × 10 5 .…”

Section: Self-induced Roll Oscillations Of Non-slender Wingsmentioning

confidence: 73%

“…More recently, research has been conducted investigating the sideslip effect on the aerodynamic characteristics with particular focus on the roll stabilities of LAR wings at Re ¼ 5 Â 10 4 -1 Â 10 5 . 6,[47][48][49] It has been found that the roll moment becomes the most pronounced for the high sideslip case and increases linearly with angle of attack until it reaches roll stall. This roll moment is created by spanwise-asymmetric distributed tip vortices that induce uneven loads between the windward/leeward sides of the wing, as shown in Figure 14.…”

Section: Self-induced Roll Oscillations Of Non-slender Wingsmentioning

confidence: 99%

Review of self-induced roll oscillations and its attenuation for low-aspect-ratio wings

2019

Proceedings of the Institution of Mechanical Engineers, Part G:

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Micro aerial vehicles are currently receiving growing interest because of their broad applications in many fields. In their flight tests, the onset of unwanted large amplitude roll oscillations was reported, which resulted in difficulties with flight control, and this has become one of the major challenges of current micro aerial vehicles design. In this review type of article, the low Reynolds number flow characteristics of a low-aspect-ratio wing are reviewed, and the self-induced roll oscillations are discussed with special attention being payed to the interaction between the three-dimensional flow structure and wing in reciprocatively rolling motion. The roll attenuation methods are introduced via flow control approaches, which can suppress the roll oscillations effectively by manipulating the leading-edge flow separation and tip vortices of the low-aspect-ratio wings.

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“…As a result of flying at low Reynolds numbers in gusty environments [12], MAVs are often subject to large sideslip angles as well. In sideslip, the aerodynamic structures on the suction side of low-aspect-ratio wings [13,14] become asymmetric [15][16][17]. Figure 1 shows isovorticity contours from stereoscopic digital particle image velocimetry (S-DPIV) measurements by DeVoria and Mohseni [16] of the flow about an rectangular wing at three different sideslip angles.…”

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confidence: 99%

“…Unfortunately, low-aspect-ratio wings confound many of the assumptions these theories are founded upon. This is due to the highly three-dimensional nature of the wing aerodynamics [6,16,17], as seen in Fig. 1, and because of the LESR, which represents bound circulation on the wing itself [19].…”

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confidence: 99%

Roll Control of Low-Aspect-Ratio Wings Using Articulated Winglet Control Surfaces

O'Donnell

Mohseni

2019

Journal of Aircraft

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The use of tip mounted winglets with independently variable cant angles was investigated as a means of roll control on wings with an aspect ratio of one. Wind-tunnel testing was performed in which a six-axis force balance was used to measure the total aerodynamic load on wings with winglet control surfaces. Stereoscopic digital particle image velocimetry of the near-wake plane was used to show how the topology of the tip vortices changed with winglet deflection. Shifts in the location of the right tip vortex core are considered to be responsible for roll moment generation because they indicate changes in the symmetry of suction-side flow structures. All winglet deflections were observed to shift the right tip vortex core inboard, and thereby shorten the effective span of the wing. The effect of a winglet deflection may be approximated as a change in the wing aspect ratio and a lateral shift in the wing aerodynamic center. Prandtl's lifting line theory provides a closed-form estimate for the reduction in lift caused by a winglet deflection. A geometrical argument was made to account for the induced roll moment. The right tip vortex core also shifts vertically, following the deflected wing tip. Vertical shifts in the right tip vortex result in an angle between the wing span line and a line connecting the two tip vortices. A positive angle is defined as the right tip vortex higher over the wing than the left, and it is accompanied by a positive roll moment. While in sideslip, the wing with no winglet deflection experiences a considerable roll moment as a result of a vertical and lateral shift in the two tip vortices. The articulated winglets are observed to partially mitigate these effects when the upstream winglet is actuated, and thus show promise as a direct means of disturbance rejection.

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Leading-edge flow reattachment and the lateral static stability of low-aspect-ratio rectangular wings

Cited by 16 publications

References 31 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

Review of self-induced roll oscillations and its attenuation for low-aspect-ratio wings

Roll Control of Low-Aspect-Ratio Wings Using Articulated Winglet Control Surfaces

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