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more at http://scitech.quickfound.net/ "The most famous project conducted by the NACA sound lab group at the Langley Research Center's Full Scale Tunnel was with the Stinson L-5. This propeller-driven military aircraft was modified in an effort to demonstrate the ability to reduce noise. Modifications included reducing the tip speed of the propeller blades and muffler design changes. The propeller, engine, and muffler modifications reduced noise pressure levels 90%. The modified airplane was demonstrated as a "quiet airplane" at Langley in 1947. Reports, documents, and photographs are included with the information on the Aircraft Noise Reduction Facility information at http://crgis.ndc.nasa.gov/historic/1208 . Langley Film #170. For more on the Full Scale Tunnel, and other research conducted there, see http://crgis.ndc.nasa.gov/historic/643 . " This film is silent, so the airplane is very, very quiet. The actual overall noise reduction was from 90 decibels to 66 decibels (the decibel scale is logarithmic, so this was a 90% reduction in the sound pressure level). Public domain film from NASA, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied. http://crgis.ndc.nasa.gov/crgis/images/7/75/1947-05-23_Quiet_Airplane_Demonstrated.pdf http://creativecommons.org/licenses/by-sa/3.0/ http://en.wikipedia.org/wiki/Stinson_L-5_Sentinel The Stinson L-5 Sentinel was a World War II era liaison aircraft used by all branches of the U.S. military and by the British Royal Air Force. Along with the Stinson L-1 Vigilant, the L-5 was the only other American liaison aircraft of World War II that was purpose-built for military use and had no civilian counterpart. All other military liaison airplanes adopted during World War II were lightly modified "off-the-shelf" civilian models... Development The origins of the L-5, affectionately known as the "Flying Jeep", can be traced to the pre-war civilian Stinson HW-75. The 75 horsepower civilian high-wing design was built by the Stinson Aircraft Company at Wayne, Michigan and first flew in 1939. The HW-75 featured two seats up front side-by-side, and a third "jumpseat" in the rear in which a small passenger could sit sideways. The design was easy to fly, and it was difficult to stall or spin. Shortly after the introduction of the HW-75, Stinson became a subsidiary of the Vultee Aircraft corporation. Under Vultee management, the HW-75 was equipped with an 80-horsepower four-cylinder engine for the 1940 model year and the HW-75 became known as the Model 105 "Voyager", touting its 105 mph cruise speed. Fitted with a four-cylinder 90hp Franklin engine for the 1941 model year, the type became known as the Model 10A. In the post-war era, the fuselage of the Model 10A was enlarged accommodate four seats, and the four-cylinder powerplant was replaced with a Franklin 150hp six-cylinder engine. This conversion became the Stinson Model 108 Voyager and the only civilian aircraft commercially produced by Stinson after WWII... In March 1943, with the creation of the liaison category of light observation aircraft, the designation was changed to L-5. The primary purpose as a liaison aircraft was courier and communication work, artillery spotting and casualty evacuation. The fuselage of later models was redesigned so the aircraft could also be used as an air ambulance, or for cargo work. With a wider and deeper rear fuselage section and a large rear door that folded downward, a litter patient or 250 pounds of cargo could be quickly loaded aboard. The L-5 series was manufactured between December 1942 and September 1945, during which time 3,590 of the unarmed two-seaters were built for the United States armed forces, making it the second most widely used light observation aircraft of the war behind the Piper L-4 Cub. Construction The fuselage was constructed using chrome-moly steel tubing covered with doped cotton fabric and the wings and empennage were constructed of spruce spars and plywood ribs and skins, also covered with fabric. The use of aluminum, which was in critically short supply and more urgently needed for other aircraft, was limited to the engine cowling, tail cone, framework for the ailerons, rudder and elevator and the landing gear fairings. The L-5 was powered by a six-cylinder 190 horsepower Lycoming O-435 engine.
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