飞机在1 500英尺的高空保持水平飞行。
在发射的最初阶段会产生一个阻力，使得炮弹可以保持水平飞行，很快阻力就消失了，自然力使得它向下运动。
Well there is this initial impedance from explosion, sure the cannon ball can keep going horizontally but as soon that impedance goes it store its natural motion which is downward.
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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