Helicopters are noisy beasts; it's just in their nature. But for one mission at the tail end of the Vietnam War, a combination of ARPA funds and hard work by the aircraft division of Hughes Tool led to the development of a chopper known as the Quiet One. The Hughes 500P wasn't totally silent, but it was about as surreptitious as a helicopter could be, and in December 1972, it flew in a CIA mission to wiretap a key communications line for North Vietnamese forces. Helicopter historian James R. Chiles (The God Machine) tells the full story of the machine, the men, and the mission.
Read it in Air & Space magazine: "Air America's Black Helicopter"