Why do helicopters make a chopping noise instead of a smooth spinning noise?
In: Engineering
The tips of each rotor blade produce vortices that wrap around the tip. The noise, or “blade slap”, is caused when the vortices are struck by the next advancing blade. Most of the noise a helicopter makes is from the tail rotor as it spins much faster than the main rotor.
Source: am chopping noise maker.
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.
Helicopter rotors operate a low RPM, usually around 600. That’s much much slower than things like fans, and not fast enough to produce a continuous noise:
Each blade create a pressure wave as it passes by, and the time between one wave and the next is long enough that they stay distinct.
Because you’re hearing the sound of each blade as it goes by. Listen to different helicopters with different numbers of blades. The UH-1 Huey has 2 blades and makes a very distinctive whip whup sound but a 4 bladed copter has more of a buzz to it.
Each blade goes whomp. For you to hear a continuous noise, you would need the rotor speed times number of blades to be over 50Hz, or for a dual blade rotor go over 1500rpm.
This would unfortunately make blade go over the speed of sound.
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