They once cost millions, and had limited uses, until researchers worked out how to make small aircraft hover using multiple co-ordinated rotors.

曾经它们的成本为是百万，且使用有限，直到研究人员研究出了如何使用多协调转子使小型飞机悬停。

These aren't just fancy helicopters - they're  much quieter than your average chopper, multiple rotors make them more manoeuvrable  and safer and because they're electrified, they're cleaner and cheaper to run too.

这些不仅仅是花哨的直升机——它们比普通的直升机安静得多，多个旋翼使它们更灵活、更安全，而且因为它们是电气化的，所以它们更清洁，运行起来也更便宜。