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Lockheed’s Combined Sailplane & Slow-Turning Propeller

6/12/2006 • Aviation History, Gear

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.

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.

The DARPA contract called for a field trial, dubbed Operation Prize Crew, to be conducted in Vietnam. After the initial flight tests, Lockheed shipped the QT-2s to Bien Hoa, a special operations air base northeast of Saigon. The Navy furnished the aircrew; the Army provided ground services and also a couple of small starlight scopes–electronic image intensifiers intended as night-weapons sights–for use as hand-held viewers.

The aircraft arrived in January 1968. On the 31st, the Communists took advantage of Tet, the Vietnamese lunar new year holiday, to launch a massive surprise offensive against installations in cities and towns throughout the South. Every asset was needed in the often desperate fighting, and the quiet airplane test director repeatedly had to reject requests for daylight reconnaissance missions in order to maintain the secrecy of the concept. For the first several weeks, the QT-2s took off every night, sometimes during rocket or mortar bombardment of the airfield. To every-one’s immense relief, the guerrillas, completely absorbed in their activities, took no notice of the quiet airplanes only 1,000 feet above their heads. As for targets and intelligence, the QT-2 observers reported more than could possibly be dealt with. Their greatest coup was a sight that at first made the aircrews rub their eyes in disbelief: fleets of supply sampans making their way along the Mekong Delta’s network of channels and canals from VC sanctuaries upriver in Cambodia. The nightly sightings confirmed other intelligence that indicated the Tet Offensive was aimed at Saigon.

While Prize Crew was testing and refining its operation, Lockheed bought another Schweizer sailplane and another Continental engine and made some badly needed aeronautical improvements–including a 7-square-feet larger empennage (tail surfaces), stronger wing spars, thicker skin, and conventional landing gear with brakes and a steerable tail wheel to replace the sailplane’s tandem wheels. Following the corporate penchant for celestial names, the company called its own quiet airplane the Q-Star. Nine different propeller designs were tried out before Lockheed settled on a six-bladed, fixed-pitch version. The company tested a 185-hp Wankel rotary engine, using an automobile radiator for the liquid cooling–the first-ever aeronautical application of that engine type. For maintenance and support reasons, however, conventional Continental engines were chosen for the production aircraft.

The propeller and engine tests presented no great difficulties, but another experiment almost ended the Q-Star’s brief life. The Department of Defense had secretly developed a family of unattended sensors–acoustic, seismic, magnetic and human effluence (people sniffers)–for its Ho Chi Minh Trail interdiction campaign, and for political reasons DARPA asked Lockheed to consider them for the quiet airplane’s payload. Those sensors, which were intended to be airdropped and radio monitored, could detect activity only within a few tens of feet. If the quiet airplane were to deploy them and still stay high enough not to be heard, it would have to tow them in an aerial pod.

Lockheed designed an aerial pod and agreed to use the Q-Star for a trial because the QT-2s were still in Vietnam. A somewhat skeptical company pilot took up the plane, fitted with a power reel, more than 1,000 feet of cable and a dummy pod, and reeled out his payload 1,000 feet above the Sacramento Delta. When the pilot made a 180-degree turn, the pod didn’t–and nearly yanked the frail craft out of the sky. The terrified pilot dropped the pod in order to regain flying speed and flatly refused to consider taking it up again. To the pilot’s–and Lockheed’s–great relief, DARPA quietly dropped that idea.

Freed of the restrictions on structural modifications, Lockheed reconfigured the Schweizer airframe into a surprisingly smart-looking production aircraft. With low-mounted wings, nose-mounted engine (which eliminated the ungainly propeller-shaft extension over the pilot’s head), and conventional retractable landing gear (wheels folding inward into wing roots widened for the purpose), the YO-3A, as it was designated by the government, resembled a small, prop-driven fighter plane. Other improvements included a trailing-edge extension over the inner half of the 57-foot wingspan, adding 25 square feet of lifting surface to the original 180 square feet, and a large, one-piece canopy-windshield for better visibility. The pilot sat in the rear cockpit, with the observer in front.

The payload carried by the YO-3A consisted of the night-vision system–a horizon-stabilized image-intensifier unit with a wide-angle objective lens mounted in a turret in the bottom of the fuselage. The viewing scope was located in the observer’s cockpit. Since no suitable night-observation system was available off the shelf, a Lockheed engineer designed one for subcontract fabrication.

All of those changes boosted the YO-3A’s empty weight to 3,129 pounds (almost twice the QT-2’s), so Lockheed once again went to a larger engine: the 210-hp air-cooled, 6-cylinder Continental IO-360D. That power plant gave a top speed of 138 mph and a cruising speed of 110–or as slow as 70 mph for maximum noise reduction. Endurance was six hours of flight time. The company used a constant-speed three-bladed propeller because it was found to be nearly as quiet at slow cruise as the six-bladed unit, but much more efficient at higher speeds.

A final question was whether the plane could be armed. For a while, gravity-dropping a load of flechettes seemed feasible, but the laws of physics and probability combined to decree that the YO-3A would go to war without a weapon.

Lockheed anticipated an initial multiservice production contract for at least 50 aircraft when DARPA pronounced Prize Crew a success. However, time had already begun to run out for American involvement in Vietnam. Tet had been the turning point. Lockheed received its initial–and final–production contract in late 1968, a $2 million order for 14 aircraft for the U.S. Army.

The YO-3As passed their acceptance tests in the fall of 1969, and the Army immediately shipped 13 of them to the 1st Army Security Agency (ASA) Company in Vietnam. (The Army Aviation Agency at Fort Rucker, Ala., got the remaining airplane for advanced testing.) Based at Long Binh, a few miles northeast of Saigon, the YO-3As proved valuable for night reconnaissance of enemy troop movements during the next two years. But on April 30, 1972, as part of its phased withdrawal from Vietnam, the Army deactivated the 1st ASA Company and returned the aircraft to the United States.

Earlier, DARPA had returned the QT-2s (redesignated the X-26A/Bs) to the Naval Test Pilot School at Patuxent River, Md. The Navy cannibalized one to provide spares for the other, and eventually sent the surviving aircraft to the Army Aviation Museum at Fort Rucker. Lockheed donated the Q-Star to the privately owned Flying Lady Museum at Morgan Hill, Calif. Of the 14 YO-3As built in 1969, only six could still be accounted for 11 years later. Four had gone to law enforcement agencies (a market that Lockheed tried without success to exploit): two each to the FBI field office in Oxnard, Calif., and the Louisiana Department of Wildlife and Fisheries. A fifth was registered to a private owner in Connecticut. In 1980, the sixth plane was serving as an airborne microphone platform in a helicopter rotor-blade research program at the NASA Ames Research Center at Moffett Field in Sunnyvale, Calif.–just across the field from its builder, the Lockheed Missiles & Space Company.

This article was written by Ronald R. Gilliam and originally published in the July 1996 issue of Aviation History.

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