我的这个悬挂式滑翔机和凯利的发明在原理上是差不多的，所以尽管我知道它能够飞起来，但怎么控制它对我来说仍然是个问题。
My student hang glider was almost as low concept as Cayley's, and though I knew it could fly, control clearly remains an issue.
300年后，一位英国工程师乔治·凯利在经过了无数次失败的实验后，终于得出了一个重要的结论：飞行需要抬升力、推进力和控制力。
It took more than 300 years and many more failed experiments until Sir George Cayley, a British engineer, determined that flight required lift, propulsion, and control.
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