One technical difficulty is cavitation, where low pressure behind a turning blade causes air bubbles.
一个技术难题是气穴现象，一旦发生气穴现象，位于旋转螺旋桨叶后方的低压会产生气泡。
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But when flying forward the air passing over the blade that is advancing to the front of the helicopter (and so into oncoming air) provides more lift than the one that is retreating to the back.