Why do choppers (helicopters) "chop"?

[Droning on and on...]

We're discussing drone prop noise in several threads. Got me thinking about the pulsing, rythmic chopping noise we hear from full-sized helicopters. Why?

I've been an RC helo hobbyist for a long time, even my biggest helicopters never "chopped", even with blades 2ft or more in length. Just a smooth hum.

 

[Rchawks]

I've got a Corona from back in the day. Pretty scary if you a newbie. I guess anything with a airfoil that slices the air makes a sound. Airplane wings, Helicopter blades, bullwhips, man those can be very loud. I think probably because of the size of the blades on a full scale helicopter the pitch is different because of it's size and rpm. The small drones have a different pitch than the larger one because of size and rpm, my guess anyway.

 

[AlanL]

Helicopter sounds

 

[justhangingaround]

And it's the coolest sound,
Blade vortex interaction air vortex between rotating blades and air vortex they create at tips which causes chopping sound
Blade slap chopping sound created as blade passes through vortex left from other blade and rotor speed significantly affects pitch and intensity of chopping sound my 2 cents worth
I got old Robbie Mosquito nitro,44inch wooden blades and never heard the chop but it nearly chop my head off , oh good old days lol(dismantled for safety)

 

[hank970]

The rotor tips may be moving through the air faster than the speed of sound, thus creating continuous sonic booms. Of course, the speed of the rotor tips relative to static air is greater on one side of the craft than the other if the craft is moving.

 

[RickDan]

It is the air movement created by the blades interacting with the tail rotor.