Department of Mechanical Engineering, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, Republic of China.
J R Soc Interface. 2012 Jul 7;9(72):1674-84. doi: 10.1098/rsif.2011.0737. Epub 2012 Jan 18.
We demonstrate experimentally that a passerine exploits tail spreading to intercept the downward flow induced by its wings to facilitate the recovery of its posture. The periodic spreading of its tail by the White-eye bird exhibits a phase correlation with both wingstroke motion and body oscillation during hovering flight. During a downstroke, a White-eye's body undergoes a remarkable pitch-down motion, with the tail undergoing an upward swing. This pitch-down motion becomes appropriately suppressed at the end of the downstroke; the bird's body posture then recovers gradually to its original status. Employing digital particle-image velocimetry, we show that the strong downward flow induced by downstroking the wings serves as an external jet flow impinging upon the tail, providing a depressing force on the tail to counteract the pitch-down motion of the bird's body. Spreading of the tail enhances a rapid recovery of the body posture because increased forces are experienced. The maximum force experienced by a spread tail is approximately 2.6 times that of a non-spread tail.
我们通过实验证明，雀形目动物利用尾部展开来截获翅膀产生的向下流，以促进其姿势的恢复。白眼鸟的尾巴周期性展开，与悬停飞行时的翅膀挥动运动和身体摆动具有相位相关性。在向下挥动翅膀时，白眼鸟的身体经历了显著的下俯运动，尾巴向上摆动。这种下俯运动在下挥动作结束时被适当抑制；然后，鸟的身体姿势逐渐恢复到原来的状态。我们采用数字粒子图像测速法表明，翅膀向下挥动产生的强烈向下流充当外部射流冲击尾部，对尾部施加压力以抵消鸟体的下俯运动。尾部的展开增强了身体姿势的快速恢复，因为这样可以产生更大的力。展开的尾巴所经历的最大力大约是未展开尾巴的 2.6 倍。
