Reynolds Kate V, Thomas Adrian L R, Taylor Graham K
Department of Zoology, University of Oxford, Oxford OX1 3PS, UK.
Department of Zoology, University of Oxford, Oxford OX1 3PS, UK
J R Soc Interface. 2014 Dec 6;11(101):20140645. doi: 10.1098/rsif.2014.0645.
Turbulent atmospheric conditions represent a challenge to stable flight in soaring birds, which are often seen to drop their wings in a transient motion that we call a tuck. Here, we investigate the mechanics, occurrence and causation of wing tucking in a captive steppe eagle Aquila nipalensis, using ground-based video and onboard inertial instrumentation. Statistical analysis of 2594 tucks, identified automatically from 45 flights, reveals that wing tucks occur more frequently under conditions of higher atmospheric turbulence. Furthermore, wing tucks are usually preceded by transient increases in airspeed, load factor and pitch rate, consistent with the bird encountering a headwind gust. The tuck itself immediately follows a rapid drop in angle of attack, caused by a downdraft or nose-down pitch motion, which produces a rapid drop in load factor. Positive aerodynamic loading acts to elevate the wings, and the resulting aerodynamic moment must therefore be balanced in soaring by an opposing musculoskeletal moment. Wing tucking presumably occurs when the reduction in the aerodynamic moment caused by a drop in load factor is not met by an equivalent reduction in the applied musculoskeletal moment. We conclude that wing tucks represent a gust response precipitated by a transient drop in aerodynamic loading.
不稳定的大气条件对翱翔鸟类的稳定飞行构成挑战，人们经常看到它们以一种我们称之为“收拢”的短暂动作收起翅膀。在这里，我们使用地面视频和机载惯性仪器，研究圈养的草原雕（Aquila nipalensis）翅膀收拢的力学原理、发生情况和原因。从45次飞行中自动识别出2594次收拢动作，并进行统计分析，结果表明，在大气湍流较强的情况下，翅膀收拢更为频繁。此外，翅膀收拢之前通常是空速、载荷系数和俯仰率的短暂增加，这与鸟类遇到逆风阵风的情况一致。收拢动作本身紧接着攻角迅速下降，这是由下沉气流或低头俯仰运动引起的，导致载荷系数迅速下降。正向空气动力载荷会使翅膀上扬，因此在翱翔时，由此产生的气动力矩必须由相反的肌肉骨骼力矩来平衡。当载荷系数下降导致的气动力矩减少未能被相应减少的肌肉骨骼力矩所抵消时，翅膀可能就会收拢。我们得出结论，翅膀收拢是由空气动力载荷的短暂下降引发的阵风反应。
