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. 2019 Jan 31;16(150):20180641.
doi: 10.1098/rsif.2018.0641.

Wing morphing allows gulls to modulate static pitch stability during gliding

C Harvey  1 , V B Baliga  1 , P Lavoie  2 , D L Altshuler  1
Affiliations

Wing morphing allows gulls to modulate static pitch stability during gliding

C Harvey et al. J R Soc Interface. .

Abstract

A gliding bird's ability to stabilize its flight path is as critical as its ability to produce sufficient lift. In flight, birds often morph the shape of their wings, but the consequences of avian wing morphing on flight stability are not well understood. Here, we investigate how morphing the gull elbow joint in gliding flight affects their static pitch stability. First, we combined observations of freely gliding gulls and measurements from gull wing cadavers to identify the wing configurations used during gliding flight. These measurements revealed that, as wind speed and gusts increased, gulls flexed their elbows to adopt wing shapes characterized by increased spanwise camber. To determine the static pitch stability characteristics of these wing shapes, we prepared gull wings over the anatomical elbow range and measured the developed pitching moments in a wind tunnel. Wings prepared with extended elbow angles had low spanwise camber and high passive stability, meaning that mild perturbations could be negated without active control. Wings with flexed elbow angles had increased spanwise camber and reduced static pitch stability. Collectively, these results demonstrate that gliding gulls can transition across a broad range of static pitch stability characteristics using the motion of a single joint angle.

Keywords: avian; biomechanics; gliding flight; static pitch stability; wing morphing.

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Conflict of interest statement

We have no competing interests.

Figures

Figure 1.
Figure 1.
Birds can dynamically morph the shape of their wings. (a) Aircraft are designed with fixed wing shapes that satisfy pre-defined stability and performance requirements, thereby restricting their ability to adapt to changing environmental conditions. By contrast, birds can vary their wing configuration during flight. Aircraft figures were adapted from photographs of existing unmanned aerial vehicles with wing spans approximately equivalent to gulls [4,5], and gull figures were adapted from photographs of gliding gulls in the Pacific Northwest. (b) The main forces and moments acting on a gull during steady, gliding flight are summarized as point loads acting on the aerodynamic centre. The pitching moment about the aerodynamic centre is, by definition, independent of the angle of attack. A bird is at equilibrium if all forces and moments about the centre of gravity are balanced. (c) Static pitch stability is the passive or inherent tendency for a glider to return to its equilibrium after perturbation such as a wind gust.
Figure 2.
Figure 2.
Joint angles can be predicted from wing shape. (a) Five peripheral landmarks were identified on photographs of gulls' ventral surface mid-glide (blue points). (b) Wings of gull cadavers were manipulated through the full range of extension and flexion of the elbow and manus joints. Cameras tracked the same five peripheral landmarks (blue points) as well as the position of the humerus, elbow, manus and carpometacarpus (purple points). This allowed us to determine the elbow and manus angles, and the corresponding wing outlines. (c) Wings spanning the full identified elbow angle range were prepared and dried for wind tunnel tests. (d) The range of viable elbow and manus angles was determined for the cadaver wings. (g) A morphospace of cadaver wing shapes was generated, and the in vivo (grey triangles) and prepared (green squares) wing shapes were projected into the space. (e) The first principal component scales with manus angle (f) and the second scales with elbow angle. The relationship between principal component data and the known joint angles of cadavers allowed us to predict the elbow and manus angles used in flight (translucent grey triangles).
Figure 3.
Figure 3.
Elbow extension across the in vivo range decreases aerodynamic efficiency but increases static pitch stability. (a) Wings with high elbow angles show reduced aerodynamic performance. Elbow extension across the in vivo range (grey shading) significantly (b) decreases maximum lift coefficient, (e) increases minimum drag coefficient and (c) decreases aerodynamic efficiency. Turbulence intensity increases aerodynamic efficiency and maximum lift. (d) Passive pitch stability increases as the elbow extends, illustrated by (g) the increasing zero lift pitching moment (intercept of (d)) and (f) the decreasing pitch stability derivative (slope of (d)). The data in (d) are restricted to the pre-stall region, and thus are exclusively linear. Turbulence intensity had a destabilizing effect on both stability parameters. The horizontal dashed line in (g) represents zero. Error bars on (b), (c), (e) and (g) represent the uncertainty due to bias and precision errors. Error bars on (f) represent 95% confidence intervals of the linear model slope prediction.
Figure 4.
Figure 4.
Gulls can actively control elbow angle to adjust spanwise camber, potentially negotiating trade-offs in aerodynamic efficiency and static pitch stability. (a) Across the in vivo range (grey band) the spanwise camber reduces as the elbow angle increases, and encompasses two divergent aerodynamic characteristics. First, wings with the highest elbow angles (138–149°) have the least spanwise camber characterized by (b) reduced aerodynamic efficiency but (c) increased static pitch stability. By contrast, wings with intermediate elbow angles (101–115°) have (b) increased efficiency but (c) reduced static stability and, thus, would require active control. These diverse sets of aerodynamic parameters may allow the same wing to navigate between trade-offs in stability and efficiency. (d) Gulls use lower elbow angles during glides as wind speeds and wind gusts increase, which suggests gulls glide with reduced static pitch stability in unsteady environments.
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References

    1. Anderson JD., Jr 2002. The airplane, a history of its technology. Reston, VA: American Institute of Aeronautics and Astronautics.
    1. Thomas ALR, Taylor GK. 2001. Animal flight dynamics I. Stability in gliding flight. J. Theor. Biol. 212, 399–424. (10.1006/jtbi.2001.2387) - DOI - PubMed
    1. Smith JM. 1952. The importance of the nervous system in the evolution of animal flight. Evolution 6, 127–129. (10.1111/j.1558-5646.1952.tb02818.x) - DOI
    1. DroneDeploy. 2017. Choosing the right mapping drone for your business part I: Multi-rotor vs. fixed wing aircraft. See https://blog.dronedeploy.com/choosing-the-right-mapping-drone-for-your-b....
    1. Flyzone. 2017. Flyzone rapide brushless performance Glider EP Rx-R 60. See https://www.flyzoneplanes.com/airplanes/flza3364/index.html.

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