YO-3A Quiet Star
The YO-3A aircraft was originally a Schweizer SGS-2-32 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. Following work on the X-26B prototype QT-1 and QT-2 (Quiet Thrust) aircraft, Lockheed developed the YO-3A two-seat version with the observer in the front seat and pilot in the rear. The U.S. Army Aviation Systems Command ordered 14 YO-3A in July 1968.
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. A six-blade fixed-pitch propeller was initially installed on the aircraft but was later replaced by a three-blade variable pitch propeller. The result of these modifications was an aircraft so quiet that its noise was drowned out by the background sounds.
Originally built as a miniature, ultra-quiet spy plane, the Lockheed YO-3A was converted into a noise research platform. NASA, in cooperation with the Army, added microphones on the wing tips and tail fin, along with a data-recording system. An air-data boom was installed under the left wing. The sailplane wing, muffled engine, and slow-turning, belt-driven propeller keep the noise extremely low -- enough so as to allow accurate measurement of rotor noise from a nearby helicopter or tiltrotor. The YO-3A was used for acoustics tests with a Bell UH-1H "Huey," two models -- AH-1S and AH-1G -- of Bell Cobras, McDonnell Douglas AH-64 Apache and MD 500 helicopters, and the YUH-61 and YAH-63 prototype military helicopters. The YO-3A was also used to measure sonic booms from an SR-71 "Blackbird."
NASA's Ames Research Center at Mountain Veiw, California, acquired one aircraft 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.
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