The Lockheed YO-3 Quiet Star is an American single-engined, propeller-driven aircraft that was developed for battlefield observation during the Vietnam War.
Designed to be as quiet as possible, it was intended to observe troop movements in near silence during the hours of darkness.
The YO-3A was designed to a United States Army specification of 1968, which called for an observation aircraft that would be acoustically undetectable from the ground when flying at an altitude of 1,500 feet (457 m) at night.
Lockheed Missiles and Space Company located in Sunnyvale; California was contracted to produce two prototype aircraft.
In 1966, the company built two QT-2 “Quiet Thrusters”, using modified Schweizer SGS 2-32 gliders.
The prototype QT-2s were then modified to the QT-2PC “PRIZE CREW” configuration.
The QT-2PC had a silenced engine and a slow-turning propeller for quiet operation.
Following operational trials with the QT-2PC in Vietnam, a production aircraft, designated the YO-3A was ordered.
This aircraft’s design was also based on the Schweizer SGS 2-32 glider.
Like the QT-2PC, the YO-3A has a large wingspan and a larger canopy area for observation.
Two crew members (a pilot and an observer) are seated in tandem.
The observer is located at the front of the cockpit.
The YO-3A is an all-metal low-wing monoplane of semi-monocoque construction.
The control surfaces of the YO-3A including the ailerons and rudder are fabric-covered.
The engine cover, canopy, engine exhaust shroud, wing-root fairings, and wheel-well fairings were constructed of fiberglass.
The YO-3A has retractable tailwheel-type landing gear.
The YO-3A was powered by an air-cooled, six-cylinder, horizontally opposed, fuel-injected, Continental Model No. IO-360D engine.
The engine is coupled to a slow-turning propeller through a belt pulley-drive system.
The propeller reduction ratio is 3.33:1.
Originally equipped with a six-bladed ground-adjustable-pitch propeller, this was replaced in March 1971 with a three-bladed laminated constant-speed wooden propeller designed by Ole Fahlin.
The engine cowling and firewall were lined with fiberglass material to dampen and contain engine noise.
The YO-3A is equipped with an Asymmetrical Exhaust System.
A crossover exhaust pipe is used to remove exhaust from the left bank of engine cylinders to the right side of the engine compartment.
This crossover joins the right bank exhaust pipe and exits along the lower right side of the engine compartment.
The exhaust gases are then moved through an acoustical fairing into a dissipating and resonating muffler continuing to the aft end of the fuselage.
Nine of the eleven YO-3As produced operated in South Vietnam, at night, from 1970 to 1971 (Late June 1970 to September 1971) and, although three were destroyed in crashes, were never damaged by enemy fire or shot down.
The YO-3A was very successful in spotting movement by the Viet Cong and North Vietnam Army (NVA) operating in South Vietnam.
QT-1 Quiet Thruster
Proposed single seat powered glider based on the Schweizer SGS 2-32, not built.
Two modified Schweizer X-26 two-seat sailplanes for evaluation, later modified with sensor packages as the QT-2PC.
QT-2PC PRIZE CREW
Two QT-2s with combat sensor packages for evaluation in Vietnam theatre, one reduced to spares and the other returned to the United States Navy as the Schweizer X-26B.
Modified Schweizer SGS 2-32 for engine/propeller development.
Production aircraft for the United States Army, 11 built.