English: NASA's YO-3A parked on the Dryden ramp. The YO-3A aircraft was originally a Schweizer SGS-2-23 sailplane. During the late 1960s Lockheed modified over a dozen of these sailplanes to create ultra-quiet observation aircraft for use over South Vietnam during the conflict there. This particular YO-3A flew combat missions and was later sold to an airframe and powerplant mechanics school. NASA's Ames Research Center at Mountain Veiw, California, acquired the aircraft from the school in 1978. It restored the YO-3A to flight status and fitted it with wing- and tail-mounted microphones as an accoustic research aircraft. Ames operated it at Edwards Air Force Base for noise measurements of helicopters and tilt rotor aircraft. One set of tests in December 1995 obtained free-flight noise data on the XV-15 tilt rotor. NASA also used the YO-3A for sonic boom measurements of a NASA SR-71 assigned to the Dryden Flight Research Center. NASA transferred the YO-3A to Dryden in December 1997, and as of April 2001 it was in flyable storage there. The designation YO-3A indicates that this aircraft was a pre-production (Y) observation (O) aircraft. Even though the YO-3A saw operational use, the Y designation was never removed. Its 210-horsepower Continental V-6 was modified to reduce noise. The engine was connected to a propeller through a belt-driven reduction system. This reduced the propeller's rotation speed. The propeller blades themselves were made of birch plywood and were wider than standard propellers. The result of these modifications was an aircraft so quiet that its noise was drowned out by the background sounds.