Night in Vietnam mostly belonged to the Viet Cong. Despite the South Vietnamese army’s well-known abhorrence of night operations, the Saigon government insisted on maintaining outposts–little triangular, mud-walled, brick-towered forts built by the French–in areas dominated by the Viet Cong (VC). In 1962, these unsupported outposts were frequently overrun during VC night assaults. The United States had 222 aircraft in Vietnam by the end of that year, including 149 helicopters. Many of the helicopters were armed gunships, but they proved to be of little help in night operations because their noise always warned the guerrillas.

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In frustration, the U.S. Department of Defense turned to its scientific and technical arm, the Defense Advanced Research Projects Agency. DARPA handed the problem to the Lockheed Missiles & Space Company in Sunnyvale, Calif., in 1966. The company was already working on counterguerrilla problems and could call on aeronautical expertise within its parent corporation.

Lockheed Aircraft Company decided that what was needed was audio stealth–a quiet airplane. The number one source of noise, the engine, would have to be small, well muffled, and slow running, with a large, multibladed propeller. The airframe would consequently have to be light–and should be acoustically ‘clean–with long wings for plenty of low-speed lift. To be successful, it would have to be an aircraft that, once over the target area, almost did not need an engine. A powered sailplane was the solution.

Lockheed selected the Schweizer model SGS 2-32, a large (two-place, 57-foot wingspan), well-tested design. For power, a Continental 57-hp 4-cylinder engine was selected, fitted with a reduction gear to keep propeller rpm low. A large automobile exhaust muffler was deemed sufficient to reduce engine noise to a reasonable level at low rpm–at least if the aircraft stayed 1,000 feet above the ground. To minimize costly structural changes, Lockheed located the engine at the plane’s center of gravity, just aft of the cockpit, and designed a long propeller-shaft extension to run over the top of the cockpit to a pylon mounted on the nose. The initial design, dubbed QT-1 (for Quiet Thruster, first model), would never have won a beauty contest and was never built, but it was the progenitor of three quiet-airplane designs that were.

While the concept design satisfied DARPA, that agency had to solicit participation by the armed services in order to proceed. The Air Force declined active participation, sensing a possible competitor to its reduced-noise Pave Eagle (the Beech Aircraft QU-22) for night reconnaissance of the Ho Chi Minh Trail infiltration routes. The Army was more enthusiastic, but insisted on a two-man crew (pilot and observer). The Navy also was supportive, envisioning Marine Corps use, and agreed to loan DARPA two SGS 2-32s they had originally ordered for test-pilot training.

Lockheed converted the two sailplanes after the Army delivered them to California. The QT-1 design was followed except for the engine, which had to be larger because of the added aircrew weight. The 4-cylinder, 100-hp air-cooled Continental O-200-A was installed and an oversized automotive muffler fitted. That power plant provided a 75-mph cruising speed and 115-mph top speed. Finally, a part-time maker of wooden propellers–one of the few in the world and coincidentally already on the Lockheed payroll–carved a four-bladed propeller. Designated QT-2s, the converted aircraft were the first quiet airplanes actually built.

The two QT-2s underwent flight tests at a secret base in the Mojave Desert in August 1967. The consensus was that they flew well. At the Navy’s riverine warfare training station in the Sacramento River delta, the larger engine was just audible–if one listened for it. Neither the Navy nor DARPA knew how the Mekong Delta’s ambient night noise level–a steady hum of insects and perhaps a dull roar of frogs–compared with that of the Sacramento Delta. They would soon find out.

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