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

Glancing through the results in Google Books for "deafened by the" + "of the helicopter", it seems the most common term is roar, roaring. I doubt there's anything more specific in common use ¹.

It's probably an overestimate, but Google Books thinks it has an estimated 8280 hits for...

roar of the helicopter

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¹ I don't think it's specific to helicopters, but chuntering + helicopter gets 12,100 Google hits.

Helicopters are said to whir. The whirring helicopters. Due to their blades. Any machine with blades is said to whir.

Merriam Webster:

Examples of whir in a sentence

Origin of whir Middle English (Scots) quirren, probably of Scandinavian origin; akin to Dan hvirre to whirl, whir

First Known Use: 15th century Related to whir

noun

Simple Definition of whir : the sound made by something that is spinning very fast

Full Definition of whir : a continuous fluttering or vibratory sound made by something in rapid motion

I'd suggest, thumping sound

The thump of a helicopter drew nearer.

Rebel Heart

The thump-thump-thump of the helicopter got louder and louder until they could feel it vibrating through them.

Rivertime

One onomatopoeic word for the sound of whirling helicopter rotors is "chuf" or "chuff" (often repeated in sets of two or three syllables). Here are some examples from a Google Books search. From Aviation Week, volume 60 (1954) [combined snippets:

Sikorsky, of course, is the recognized "father" of the helicopter industry in the United States — an industry that has grown from his original VS-300 (first flown in September, 1939) to a field of 17 manufacturers turning out everything from tiny single-place ships to giant tandem-rotor craft accommodating 40 troops.

...

Then, on May 6, 1941, Sikorsky took the VS-300 aloft and stayed airborne for one hour, 32 minutes and 30 seconds, a new world record.

Since then the chuf-chuf-chuf of the rotary wing has become familiar in Korea, in air mail and express schedules, in utility uses such as power line inspection and mapping flights and in passenger operations as well.

From Aircraft, volume 20, (1958) [snippet view]:

The little "chuff-chuff" Fairey Ultra Light Helicopter, now in Royal Navy colours, was flown on and off its transporter truck while being driven along the runway to simulate a ship's platform.

From The Aeronautical Journal, volume 78 (1974) [snippet view]:

It is noticed that there is a significant content of low frequency discrete frequency sound in the helicopter recording which is absent in the road traffic noise (Fig. 2). These low frequency discrete frequency components are heard as the chuff-chuff of the helicopter. If these tones do not decrease in amplitude with increase in frequency then the helicopter will make the banging sound which is often called blade slap.

From Ivan Smith, Come Break a Spear (1980) [combined snippets]:

The clattering chuff-chuff of heli-borne troops filled the air. Black against the bright, silver sky the machines rose up and over the hill and dipped swiftly towards the valley.

From Hob Brown, Inner Tube: A Novel (1985):

Chuff-chuff-chuff: the soundtrack for embassy evacuations. A bulbous black helicopter passes over our heads, carrying, with equal probability, soldiers or hunters or survey geologists.

From Reader's Digest Treasury of Humorous Writing (1988) [snippet view]:

Down on the first floor, the lieutenant paused in his supervising of the handing out of riot guns to listen to the unmistakable chuff-chuff of a nearby helicopter. "My God!" he whispered. "They must be supplied by Fidel Castro!"

From Larry Kammholz, Moc Hoa (mŏck Wauh): A Vietnam Medical-Military Adventure (1990) [combined snippets]:

Then there was the sudden excitement of the sound of the half-dozen choppers accelerating their engines and rotors, churning up the dust. The whine of the engines and the chuf, chuf of the whirling blades then increased in pitch and intensity and rapidly heightened to a final whizzing takeoff.

From Rob Loughran, High Steaks (2003):

But his voice floundered in the chuff-chuff, chuff-chuff of a US Navy helicopter that bulleted down the valley about forty feet off the ground.

From James Doss, Stone Butterfly(2007):

The chuff-chuff of an approaching helicopter awakens the feline dreamer.

From T.M. Bilderback, Mama Told Me Not To Come - A Justice Security Novel (2010):

The first helicopter was almost even with the barn now, moving fast over them. Felix heard a muffled chuff, chuff, chuff from it.

From Matthew Ewald, Human Nature (2015):

The sounds from outside intensified, he cocked his head as he heard a soft whisper, the chuff chuff whirring of a helicopter, a news helicopter, he...no, they had been here to make a statement.

Loading noise (Helicopter noise reduction)

Loading noise is an aerodynamic adverse effect due to the acceleration of the force distribution on the air around the rotor blade due to the blade passing through it, and is directed primarily below the rotor. In general, loading noise can include numerous types of blade loading: some special sources of loading noise are identified separately.

Changes in blade-section motion relative to the observer as the steadily loaded propeller rotates, generally referred to as "loading" noise. This source tends to dominate at low blade speed.[citation needed] –Wiki

If you hear it "chopping" it's predominantly loading noise.

Helicoptor Noise

1. blade slap CED

   • (Aeronautics) the regular noise beat generated by the rotor blades of a helicopter
   • the regular noise beat generated by the rotor blades of a helicopter (Collins Dictionary)

This is a demonstration of my new sound-effect-gauge for FSX-Helicopters. It simulates the blade slap in several flight conditions YouTube video link

Rotorcraft Aeromechanics

Impulsive noise or blade slap is a sharp cracking, popping, or slapping sound occurring at the blade passage frequency....

More references...

There are numerous characteristics of helicopter sound that cause it to be objectionable to many within earshot. To begin with, noises with a beat or pulsating qualities tend to be more annoying. The main-rotor and anti-torque systems in turbine helicopters tend to dominate the acoustical signature.

The distinctive aerodynamic rotor blade slap noise generated by helicopter rotors is difficult to disguise. Blade slap (technically “Blade Vortex Interaction,” or simply BVI) noise occurs during descent for landing and results from interaction of a main-rotor blade with previously shed tip vortices. These interactions generate a complex unsteady pressure field that propagates below the rotor as high impulsive noise. High-Speed Impulsive (HSI) noise is caused by transonic flow shock formation on the advancing main-rotor blade, primarily near the blade’s tip. This noise tends to propagate forward of the helicopter.

_{Aviation week.com}

From SE Physics

In some cases (turns at high speed, descent) the blades may also be hitting the wake vortex shed by the previous blade resulting in sharp increase in the puslating sound called "blade slapping".

From Straight Dope The Helicopter Noise

What you are talking about though is the noise you hear primarily in low speed descent (when the helicopter is coming in to land). This is when the helicopter makes its distinctive "whop whop" noise. This results from the tip vortices produced by preceding blades passing through the disk plane and interacting with later blades. The term "blade slap" is archaic and it is now referred to as BVI, or Vlade-Vortex Interaction.

I come up with heli noise and just find some evidences as follows:

A loud heli noise filled the air. The B.S.A.A. heli appeared behind the ruins and shoot to the B.O.W.. Rika looked up and ran to Piers. Source

Seems fair to me since he's AFK and its irritating having a really loud heli noise above you when you're in spawn waiting for an MHQ to deploy. Source

Different helicopter types make different noises. The twin-bladed Huey of the Vietnam era was a whop - whop sound. The twin rotor Chinook (Vietnam and later, still in service today) is more of a wokka - wokka. That's what its crews call it. Earlier helicopters were sometimes described as 'clattering'. Modern ones can make a 'thrumming' sound when far away, becoming more of 'beating' sound close to. I've often seen the word 'low' added in front of all the above -the low beating of the helicopter...

My son's British childminder, when we were living in France, called the sound a helicopter makes:

F f f f

I suppose the vowel sound connecting the consonants is more or less a schwa. It's not like saying the letter F. The F sound is definitely the beginning of a distinct syllable. I'm not sure how to notate it properly.

I'm not sure if she learned it in the UK or in France.

A scuttering sound

You can hear examples at this link:

https://www.soundsnap.com/tags/scuttering

‘A scuttering sound’ is a repeated intermittent sound, similar to the sound made when an insect or animal moves or ‘scutters’ along the ground.

It is also often used for intermittent engine sound: ‘the engine scutters into life’.

https://www.soundsnap.com/tags/scuttering

On onomatopoeic possibilities...

A search for "thwack-thwack" got me 385000 webpage links from Google today, with a search for "thwack-thwack" AND "helicopter" getting 335000 from the same company. So while it isn't exclusive to helicopters, we can at least say that the indications are that around 85%-90% of the hundreds of thousands of uses of "thwack-thwack" on the web denote the noise that said vehicles make.

That's better than "choo-choo" (surely a train sound!) as against "choo-choo" AND "train" (5.6 million, 3.9 million, 70%).

For helicopters, an onomatopoeic term may exist that has better numbers than "thwack-thwack" but if so I didn't find it. ("Blade slap" is highly specific, but only the second word is onomatopoeic.)