Thomas A L, Taylor G K
Department of Zoology, Oxford University, UK.
J Theor Biol. 2001 Oct 7;212(3):399-424. doi: 10.1006/jtbi.2001.2387.
Stability is as essential to flying as lift itself, but previous discussions of how flying animals maintain stability have been limited in both number and scope. By developing the pitching moment equations for gliding animals and by discussing potential sources of roll and yaw stability, we consider the various sources of static stability used by gliding animals. We find that gliding animals differ markedly from aircraft in how they maintain stability. In particular, the pendulum stability provided when the centre of gravity lies below the wings is a much more important source of stability in flying animals than in most conventional aircraft. Drag-based stability also appears to be important for many gliding animals, whereas in aircraft, drag is usually kept to a minimum. One unexpected consequence of these differences is that the golden measure of static pitching stability in aircraft--the static margin--can only strictly be applied to flying animals if the equilibrium angle of attack is specified. We also derive several rules of thumb by which stable fliers can be identified. Stable fliers are expected to exhibit one or more of the following features: (1) Wings that are swept forward in slow flight. (2) Wings that are twisted down at the tips when swept back (wash-out) and twisted up at the tips when swept forwards (wash-in). (3) Additional lifting surfaces (canard, hindwings or a tail) inclined nose-up to the main wing if they lie forward of it, and nose-down if they lie behind it (longitudinal dihedral). Each of these predictions is directional--the opposite is expected to apply in unstable animals. In addition, animals with reduced stability are expected to display direct flight patterns in turbulent conditions, in contrast to the erratic flight patterns predicted for stable animals, in which large restoring forces are generated. Using these predictions, we find that flying animals possess a far higher degree of inherent stability than has generally been recognized. This conclusion is reinforced by measurements of the relative positions of the centres of gravity and lift in birds, which suggest that the wings alone may be sufficient to provide longitudinal static stability. Birds may therefore resemble tailless aircraft more closely than conventional aircraft with a tailplane.
稳定性对于飞行而言与升力本身同样至关重要，但以往关于飞行生物如何保持稳定性的讨论在数量和范围上都很有限。通过推导滑翔生物的俯仰力矩方程，并讨论滚转和偏航稳定性的潜在来源，我们考量了滑翔生物所使用的各种静态稳定性来源。我们发现，滑翔生物在保持稳定性的方式上与飞机有着显著差异。特别是，当重心位于翅膀下方时所提供的摆锤稳定性，在飞行生物中是比大多数传统飞机更为重要的稳定性来源。基于阻力的稳定性对于许多滑翔生物似乎也很重要，而在飞机中，阻力通常会被保持在最低限度。这些差异带来的一个意外结果是，飞机静态俯仰稳定性的黄金指标——静稳定裕度——只有在指定平衡攻角的情况下才能严格应用于飞行生物。我们还推导出了几条经验法则，借此可以识别稳定的飞行者。预计稳定的飞行者会展现出以下一个或多个特征：（1）在慢速飞行时翅膀向前掠。（2）后掠时翅膀尖端向下扭转（下反角），前掠时翅膀尖端向上扭转（上反角）。（3）如果额外的升力面（鸭翼、后翅或尾巴）位于主翼前方，则向上倾斜，位于主翼后方则向下倾斜（纵向二面角）。这些预测中的每一个都是有方向性的——预计不稳定的生物会呈现相反的情况。此外，与稳定生物在湍流条件下预计会产生大的恢复力而呈现的不稳定飞行模式相反，稳定性降低的生物预计会展现直接的飞行模式。利用这些预测，我们发现飞行生物拥有比普遍认知更高程度的固有稳定性。对鸟类重心和升力相对位置的测量强化了这一结论，这表明仅翅膀可能就足以提供纵向静态稳定性。因此，鸟类可能比带有尾翼的传统飞机更类似于无尾飞机。
