Helicopter, Hush Thyself

We've seen many articles the last two days about the stealthy helicopter allegedly used by the 160th SOAR in the Bin Laden raid, said to have been wrecked against a wall in a mishap called settling with power, and left behind as an intentionally demolished wreck with a mostly intact tailboom. Remarkable to me is the fact that the tail rotor blades are essentially undamaged ... normally in a tail-strike incident these are turned into small fragments. So the tail rotor must have been at a full stop before it encountered any hard object. 

Taking it at face value, two qualities are claimed in the Abbottabad stories: low radar cross section, and low noise.

How do such noisy machines hush themselves? I wrote about the original low-noise helo in this article for Air&Space. It was the 500P, the "Quiet One," a highly modified variant of the OH-6A Cayuse. It was financed by the CIA and built by the aircraft division of Hughes Tool. 

 

The 500P went into action to place a wiretap on an enemy telephone line near Vinh, North Vietnam, in December 1972. It flew out of a secret base called PS-44 near Pakse, Laos. The operation was successful, and I interviewed the participants.

The story begins in 1968, when Hughes Tool Aircraft Division sold two piston-powered Model 269 helicopters to an affluent Los Angeles suburb for police patrols. Citizens soon called to complain about the noise, and the city told Hughes to either make them quieter or take them back. An emerging market for police patrols was at stake. Engineers at Hughes identified the tail rotor as one of the biggest noisemakers. 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.

Coincidentally, the Advanced Research Projects Agency was hunting for contractors who could cut noise from military helicopters of all sizes. After hearing about Hughes' work on the police helicopters, ARPA offered the company $200,000 in 1968 to work similar magic on a Hughes OH-6A light helicopter. Hughes Tool made a short movie about the modifications, which included a new set of gears to slow the tail rotor, and showed it to ARPA.  ARPA approved money for an all-out quietizing effort, Phase II, and gave the project the code name Mainstreet. Even before work was fully under way, the CIA ordered two (later registered as N351X and N352X) for use in the field. Test flights began at Culver City, California, in 1971, followed by a brisk training program for the U.S. instructor-pilots who would later train mission pilots.

Flights of the Quiet One included low-level work at the secret Air Force base Area 51 in Nevada and touchdowns on peaks in California to familiarize pilots with close-quarters maneuvering and landing in darkness. Pilots needed at least eight hours to get comfortable with steering by sole reference to the comparatively narrow view of the forward-looking infrared (FLIR) camera, which was mounted just above the skids.

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. 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 extensive alterations were not to eliminate all noise -- an impossibility -- but to damp the frequencies of noise that people associate with a helicopter. 

How quiet was it? I'm told that because of the quieting gear, the helicopter couldn't be heard from the porch of the PS-44 main building unless it was flying overhead. Even then, at night, it sounded like a far-off airplane. The helicopter had its own hangar so Soviet spyplanes and satellites could not get a look at the peculiar profile produced by the extra main rotor blade, a tail rotor with blades in an odd scissored configuration, and big muffler on the rear fuselage. More technical information is available in this unclassified report. 

Did the remarkable 500P continue to fly by night? Officially, the Vinh wiretap was the first and only wartime mission of the original black helicopter. Its remarkable qualities were offset by the reduced flight performance.