较大的羽毛可以逐渐增加升力，直到逐渐实现持续的飞行。

Larger feathers could have increased lift incrementally, until sustained flight was gradually achieved.

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这种差异能够产生保持直升机水平飞行的升力。

This difference in lift can make it hard to maintain level flight.

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因此在向前飞行时，旋转叶片可以在每一边产生相同大小的升力来进行机身的平衡。

So in forward flight each rotor can produce an equal amount of lift on each side, thus providing balance.

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