In the last dark nights of the Vietnam War, however, a secret government organization did use a helicopter for a single, sneaky mission. But it was no ordinary aircraft. The helicopter, a limited-edition model from the Aircraft Division of Hughes Tool Company, was modified to be stealthy. It was called the Quiet One—also known as the Hughes 500P, the "P" standing for Penetrator.
Just how quiet was the Quiet One? "It was absolutely amazing just how quiet those copters were," recalls Don Stephens, who managed the Quiet One's secret base in Laos for the CIA. "I'd stand on the [landing pad] and try to figure out the first time I could hear it and which direction it was coming from. I couldn't place it until it was one or two hundred yards away." Says Rod Taylor, who served as project engineer for Hughes, "There is no helicopter today that is as quiet."
The Quiet One grew out of the Hughes 500 helicopter, known to aviators in Vietnam as the OH-6A "Loach," after LOH, an abbreviation for "light observation helicopter." ...
... Hughes identified one of the worst of the noisemakers: the tail rotor. By doubling the number of blades to four, Hughes was able to cut the speed of the rotor in half, which reduced the helicopter's noise.
... The slapping noise that some helicopters produce, which can be heard two miles away or more, is caused by "blade vortex interaction," in which the tip of each whirling rotor blade makes tiny tornadoes that are then struck by oncoming blades. The Quiet One's modifications included an extra main rotor blade, changes to the tips on the main blades, and engine adjustments that allowed the pilot to slow the main rotor speed, making the blades quieter (see "How To Hush a Helicopter," p. 68). The helicopter also had extra fuel tanks in the rear passenger compartment, an alcohol-water injection system to boost the Allison engine's power output for short periods, an engine exhaust muffler, lead-vinyl pads to deaden skin noise, and even a baffle to block noise slipping out the air intake.The mission:
The extensive alterations did not blank out all noise, Taylor says. Rather, they damped the kinds of noise that people associate with a helicopter. "Noise is very subjective," he says. "You can reduce the overall noise signature and an observer will still say, 'I can hear it as well as before.' It's related to the human ability to discriminate different sounds. You don't hear the lawnmower next door, but a model airplane is easily heard. It has a higher frequency and seems irritating."
The Quiet One's single, secret mission, conducted on December 5 and 6, 1972, fell outside Air America's normal operations. The company's public face—what spies might call its "legend"—was that of a plucky charter airline delivering food and supplies to civilians in Laos, and flying occasional combat evacuation missions in Laos and South Vietnam. While it did substantially more than that, and at considerable peril (217 of its employees died in Laos), Air America crews did not make it a practice to fly deep into North Vietnam.
The mission was intended to fill an information gap that had been galling Henry Kissinger, secretary of state under President Richard Nixon. Negotiations to end the 11-year war had begun in March 1972 but stalled in part because South Vietnamese leaders feared that North Vietnam would invade not long after U.S. troops left. A five-month Air Force and Navy bombing campaign called Operation Linebacker had brought the North Vietnamese to the negotiating table in Paris that October, but even that campaign could not force a deal. Kissinger wanted the CIA to find out whether the North Vietnamese were following the peace terms or just using them as a smokescreen for attack plans.
. . . From its intelligence work a year earlier, the CIA knew about a weak point in the North Vietnamese wall of security: a telephone line used by the country's military commanders, located near the industrial city of Vinh. A patrolled bicycle path ran alongside the string of telephone poles, but at one spot, about 15 miles southwest of Vinh and just east of the Cau River, the phone line went straight up a bluff, over a ridge, and down the other side. The terrain was too steep for bikes, so the path followed the river, which flowed around the bluff, rejoining the telephone poles on the bluff's far side (see hand-drawn map, p. 67). This would be the best place to drop off commandos to place a wiretap.
Because the Vinh tap would be sending its intercepts out of North Vietnam, across Laos, and into Thailand, it would need a solar-powered relay station that could catch and transmit the signal, broadcasting from high ground. The station would be within earshot of enemy patrols, so both the tap and relay would have to be dropped in by helicopter—a very quiet one.Read the whole article.