Awesome article on the use of a heavily modified OH-6 in Vietnam to tap wire communications in North Vietnam.
http://www.airspacemag.com/military-aviation/the_quiet_one.html?c=y&page=1
http://www.airspacemag.com/military-aviation/the_quiet_one.html?c=y&page=1
BLACK HELICOPTERS ARE A FAVORITE FANTASY when conspiracy theorists and movie directors conjure a government gone bad, but in fact, the last vehicle a secret organization would choose for a stealthy mission is a helicopter. A helicopter is a one-man band, its turbine exhaust blaring a piercing whine, the fuselage skin's vibration rumbling like a drum, the tail rotor rasping like a buzzsaw.
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.