Why do helicopters make a chopping noise instead of a smooth spinning noise?
In: Engineering
The rotor isn’t spinning at a very fast RPM, compared to many other spinning fans, but the blades are long, so they’ve got a long circumference to cover in one revolution. When going at full speed, a typical helicopter blade’s tips are moving at a significant portion – perhaps around half – of the speed of sound!
Maybe you’ve heard of the Doppler effect. It’s what changes the pitch – and also, importantly, the energy – of a sound, depending on whether its source is moving towards or away from you. At extreme speeds, the Doppler effect becomes extreme too. (In fact as the source approaches the speed of sound, the Doppler shift theoretically approaches infinity – and that’s one way of explaining what a sonic boom is.)
So for a certain part of the rotor’s spin, one blade tip is approaching you very fast, and the other’s whipping away from you very fast, and then a quarter-turn later, they’re both travelling in a perpendicular direction to you. These are two situations of very different loudness. And each of these situations happens, alternatingly, about 15 times per second.
edit: The actual mode of sound production is a bit different, because it has an additional spinny bit on the end, but a [bullroarer](https://www.youtube.com/watch?v=2ODGE2f7gLQ) is another good example of how Doppler shifting works on an object moving in a circle.
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