Lockheed YO-3A Quietstar
The YO-3A was designed to provide a very quiet aircraft for low-altitude, night reconnaissance flights over Vietnam. Lockheed based the plane on a Schweizer SGS 2-32 sailplane. The gliders were heavily modified to add a specially muffled engine and a slow-turning three-blade propeller. Flying at 2,000 feet and using night vision equipment the Quietstar tracked North Vietnamese troop movements without the people on the ground ever even hearing the aircraft. After their service in Vietnam YO-3As were used by NASA for studies of aircraft noise, by the Louisiana Department of Wildlife and Fisheries for tracking poachers, and by the Federal Bureau of Investigation.