The vertical tail size is not set to avoid spiralinstability because this mode is not critical.
垂尾的大小不是根据避免螺旋不稳定来确定的。因为这种运动模态不是设计状态。
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Some Africa-watchers fear that parts of the continent may be getting trapped in a downward spiral: more babies mean more competition for resources, more instability—and more babies.