Lockheed combined a sailplane and slow-turning propeller in an effort to steal the night back from the Viet Cong.

YO-3A

Lockheed combined a sailplane and slow-turning propeller in an effort to steal the night back from the Viet Cong. 

By Ronald R. Gilliam 

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. 

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.