Hughes 500P
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.
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