system
Given the following post, respond as if you're commenting on Reddit:
human
If helicopter propellors spin smoothly, why do they make percussive "buh buh buh" sounds instead of a more constant hum?
gpt
"Rotor flap" is the correct answer, but it's more complicated than that.

There are two main controls in a helicopter - cyclic pitch and collective pitch.

Cyclic pitch alters the blade's angle of attack depending on where the blade is in rotation. This is controlled by the joystick, and it's what controls tilt/roll in the helicopter - how it moves side to side or front to back.

Collective pitch alters the pitch over the entire rotor disk, and it's what controls the altitude and velocity. This is controlled by the collective pitch stick usually to the left of the pilot. Pull up on that stick and the entire rotor changes pitch, increasing lift, causing the helicopter to go up.

Combine inputs to both sticks by pushing forward on the cyclic joystick, and pulling up on the collective - and with the right mix of inputs the helicopter pitches forward and accelerates forward. The rotor disk tilts and "pulls" the helicopter forward kind of like an airplane propeller, but at the same time it's providing the lift to keep the helicopter up in the air.

When a helicopter rotor isn't under load, and the cyclic and collective blade pitches are neutral - the rotor simply hums and doesn't make that distinctive chopping sound.

The sound also depends on the rotor type and blade count. High performance helicopters often have four, five or more blades in the rotor, spin at higher RPMs and unless under severe load - they produce less of that distinctive chopping sound because they need less pitch to do the same lift/work.

Propellers on airplanes make the same sound but at higher frequencies since they have higher RPM, but smaller rotors.