Straight Up Difficult It's difficult to build a working four-rotor helicopter that spans 100 feet and only weighs 80 pounds. It's even harder when your engine is a 0.7-horsepower person. But two teams of young engineers hope to do just that.
![A Human-Powered Helicopter: Straight Up Difficult](https://cdn-ak-scissors.b.st-hatena.com/image/square/9ebdf54463314fe82f6559a1cc1f61780efe355e/height=288;version=1;width=512/https%3A%2F%2Fmedia.npr.org%2Fassets%2Fimg%2F2012%2F10%2F12%2Fhph_poster_wide-8282a71a8356b0478fb729277de8b46ed9f547ef.jpg%3Fs%3D1400%26c%3D100%26f%3Djpeg)
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