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.