ON THE AERODYNAMICS OF BIRDS TAILS
ON THE AERODYNAMICS OF BIRDS TAILS
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DOI：
10.1098/rstb.1993.0079
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发表时间：
1993-06-29
期刊：
影响因子：
--
通讯作者：
THOMAS, ALR
中科院分区：
文献类型：
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作者：
THOMAS, ALR
利用细长升力面理论可以预测鸟尾的气动力特性及其产生的力。该模型的结果表明，与传统机翼不同，传统机翼产生的升力与其面积成正比，尾翼产生的升力与其最大连续翼展的平方成正比。只要尾翼的宽度沿着流动方向展开，升力就不受尾翼形状变化的影响。在尾翼最大宽度点之后，流动由前部分的尾流控制。因此，该点后面的任何区域只产生阻力，而不会产生升力。升力中心位于最大宽度点前面的尾翼部分的区域中心。因此，尾翼顶点附近的力臂是常规机翼前缘附近力臂的两倍多。尾翼的阻力是与升力成比例的诱导阻力和与表面积成比例的剖面阻力的组合。诱导阻力可以通过下垂外尾羽产生前缘吸力而减半。这可以用于控制，特别是在机翼和尾翼都产生最大升力的慢速飞行中。细长升力面模型在迎角小于15 °时非常精确。在大迎角时，前缘形成的涡流可以稳定流过尾翼的气流，从而通过分离涡机制产生增加的升力。前缘相对于自由姿态的方位不对称（滚转、偏航或由平面形状不对称引起）在流场中被放大，并导致大的滚转和偏航力，这些力可用于转弯机动的控制。细长升力面模型可用于研究尾翼形状和尾翼展开变化对尾翼气动性能的影响。具有三角形平面形状的叉形尾翼在展开到略大于120度时给出最佳的空气动力学性能，并且就空气动力学效率而言，对于控制俯仰和偏航的手段而言，这可能接近于通用的最佳值。然而，当尾巴没有广泛传播时，自然选择可能会起到优化尾巴性能的作用。尾翼通常只在机动或低速时展开得较宽，选择尾翼展开角度时可以提高尾翼的效率--例如，为了使鸟的总升阻比最大化--在任何展开角度下的最佳形状都是使尾翼具有直后缘的形状。 这将总是给出一个稍微分叉的平面形状，但深度将取决于选择作用时尾巴的伸展程度，而这取决于自然选择下的优化标准。 叉形尾翼比其他尾翼类型对迎角和展度的变化更敏感。 叉形尾翼比其他尾翼形态更容易受到损坏，并且在损坏后遭受更大的性能损失。 叉形尾翼的固有稳定性也比任何其他类型的尾翼都要小。 对于在杂乱环境中飞行或不经常飞行的鸟类，空气动力学性能可能不是重要的优化标准。 在这种条件下，自然选择可能会偏爱其他形状的尾巴，根据模型可以预测经过性选择的细长尾巴的空气动力学成本。这些预测可以用来区分各种模型的细长尾巴的演变。细长的渐变尾巴和针尾可能是通过费舍尔或残疾机制进化而来的。长叉尾的进化最初可能受到自然选择的青睐，在性选择的叉尾中看到的羽毛伸长模式是由费舍尔假说（Fisher 1930）预测的，但不是由任何其他的性选择理论。
The aerodynamic properties of a bird's tail, and the forces produced by it, can be predicted by using slender lifting surface theory. The results of the model show that unlike conventional wings, which generate lift proportional to their area, the lift generated by the tail is proportional to the square of its maximum continuous span. Lift is unaffected by substantial variations in tail shape provided that the tail initially expands in width along the direction of flow. Behind the point of maximum width of the tail the flow is dominated by the wake of the forward section. Any area behind this point therefore causes only drag, not lift. The centre of lift is at the centre of area of the part of the tail in front of the point of maximum width. The moment arm of the tail, about its apex, is therefore more than twice the moment arm of a conventional wing about its leading edge. The drag of the tail is a combination of induced drag proportional to lift, and profile drag proportional to surface area. Induced drag can be halved by drooping the outer tail feathers to generate leading edge suction. This may be used for control, particularly in slow flight when both wings and tail are generating maximum lift.The slender lifting surface model is very accurate at angles of attack below about 15-degrees. At higher angles of attack vortex formation at the leading edge can stabilize the flow over the tail and thereby generate increased lift by a detached vortex mechanism. Asymmetry in the orientation of the leading edges with relation to the freestream (either in roll, yaw or caused by asymmetry in the planform) is amplified in the flow field and leads to large rolling and yawing forces that could be used for control in turning manoeuvres. The slender lifting surface model can be used to examine the effect of variations in tail shape and tail spread on the aerodynamic performance of the tail. A forked tail that has a triangular planform when spread to just over 120-degrees gives the best aerodynamic performance and this may be close to a universal optimum, in terms of aerodynamic efficiency, for a means to control pitch and yaw. However, natural selection may act to optimise the performance of the tail when it is not widely spread. The tail is normally only widely spread during manoeuvres, or at low speeds, selection may act to improve the efficiency of the tail when it is spread to only a relatively narrow angle - for example to maximize the bird's overall lift to drag ratio - the optimum shape at any angle of spread is that which gives a straight trailing edge to the tail. This will always give a slightly forked planform, but depth will depend on how widely the tail is spread when selection acts, and this depends on the criteria for optimization under natural selection. A forked tail is more sensitive to changes in angle of attack and angle of spread, than other tail types. Forked tails are more susceptible to damage than other tail morphologies, and suffer a greater loss of performance following damage. Forked tails also confer less inherent stability than any other type of tail. Aerodynamic performance may not be an important optimization criterion for birds that fly in a cluttered environment, or do not fly very much. Natural selection, under these conditions, may favour tails of other shapes.The aerodynamic costs of sexually selected elongated tails can be predicted from the model. These predictions can be used to distinguish between the various models for the evolution of elongated tails. Elongated graduated tails and pintails could have evolved either through a Fisherian or Handicap mechanism. The evolution of long forked tails can be initially favoured by natural selection, the pattern of feather elongation seen in sexually selected forked tails is predicted by the Fisher hypothesis (Fisher 1930) but not by any of the other theories of sexual selection.