Word for the noise made by a helicopter?

It appears that the different noises produced by helicopers can be pretty complex, and I could not foresee the OP would raise my interest on this topic.

It is sometimes called with the broad and somewhat loose terms “chop chop”, choppy or chopping sound, see for instance:

• Why do helicopter blades make the choppy sound they do when they are spinning around at a constant speed?
• they make the loud chopping noise that’s so familiar (in This Is a Picture of the Sound a Helicopter Makes),
• in Why do helicopters make the pulsing noise when they fly?.

While the “chop” root is loose, I like its similarity with the cut (“to chop”), and “chop chop“, rooted in Cantonese, adopted by English seamen meaning “hurry, hurry”.

More technical terms exist for their different noises, depending on the of enngine, the manoeuvre, the number of blades, the flight “mode”. A more technical SE discussion at The sound of rotating helicopter blades.

[EDIT] Indeed, the “Helicopter sound is rather complex”, as mentioned in Frequency Analysis of Helicopter Sound in the AS332 Super Puma, or in Helicopter blade slap, J. of Sound and Vibration, 1966 (a more accurate technical term proposed by @Sven Yargs) which says that:

Blade slap is the sharp increase in helicopter rotor noise, at the
blade passing frequency, that is characteristic of certain model
helicopters during some régimes of flight.

and

This can be defined as the loud, sharp increase in rotor noise, at the
blade passing frequency, that is so characteristic of particular model
helicopters during certain manoeuvres.

noting that:

Many times, blade slap noise has been mistaken for machine gun fire
and vice versa.

Finally, one can find many other onomatopoeic sounds here, for instance:

• swish/swash/swish/swash
• tocotocotoco
• wuppa wuppa
• whop whop whop
• whumpa-whumpa-whumpa-whumpa
• whup-whup-whup
• thith-thith-thith
• dubdubdubdubdubdub
  -flac-flac-flac
  chakk-chackk-chak-chak,batabatabata