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This was one of the stealthiest aircraft ever flown by the US military

Imagine an airplane so quiet that it's virtually impossible to hear it coming and going from the ground. This may seem like science fiction to most, but for the US Army's YO-3 'Quiet Star' scout aircraft, it was an incredible and unparalleled reality — still unmatched today.


In the late 1960s, the Army put forward a requirement for a small observation aircraft that could fly just above 1,000 feet without being detected by North Vietnamese and Viet Cong troops. Navy, Air Force and Marine reconnaissance aircraft were too noisy and easily detectable, allowing for NVA commanders to hide their soldiers well in advance of surveillance flights, rendering such missions useless.

To solve this problem, in 1968 the Department of Defense contracted Lockheed's storied Skunk Works black projects division to build an aircraft suitable for the job. Skunk Works had, by this time, already developed the U-2 Dragon Lady and SR-71 Blackbird spy planes for the Air Force and CIA, so designing something substantially smaller, slower and cheaper would be a considerably easy task, well within their capabilities.

According to Rene Francillon in his book, "Lockheed Aircraft Since 1913," the aerospace company had already attempted to build something similar two years earlier using a Schweizer glider fitted with a 'silenced' powerplant for quiet flights. Known by the codename PRIZE CREW, this glider was sent to Vietnam for operational testing and was determined successful enough that the concept was worth exploring further.

When the 1968 request appeared, Lockheed was already well-prepared.

A Schweizer SGS 2-32, the glider which the YO-3 was based on (Photo Wikimedia Commons)

To meet the Army's needs, Lockheed took another Schweizer glider and modified it heavily, using fiberglass — a fairly novel technology on aircraft at the time — and lightweight metals to reduce weight and increase endurance. The cockpit was redesigned to hold a pilot and an observer/spotter in a tandem configuration under a large bubble canopy for enhanced visibility.

Propeller aircraft aren't normally known for being very quiet or inconspicuous. The noise of their piston engines and the propeller blades beating the air around it into submission can be heard from a fair distance off. However, Lockheed's best and brightest made it work.

By connecting a small 6-cylinder engine to the propeller using a belt and pulley system, and by adding fiberglass shielding to the engine compartment, the aircraft became nearly noiseless, even with its engines on at full power. Exhaust from the engine would be ducted and funneled to the rear of the plane using a special muffler, further reducing any potential for sound generation.

A Quiet Star during a test flight in the United States. Nine were deployed to Vietnam (Photo US Army)

Lockheed finished developing this new stealth aircraft in 1969, dubbing it the YO-3 Quiet Star. By 1970, nine Quiet Stars were sent on their maiden combat deployment to Vietnam, beginning a 14-month rotation to the country in support of American troops on the ground.

Before a typical observation mission, a YO-3 would be fueled up and launched, then flown around the air base it had recently taken off from so that personnel on the ground could listen for any sounds out of the ordinary — note that "ordinary" for the Quiet Star was almost absolute silence.

If any rattles were heard, the aircraft would land immediately, be patched up with duct tape or glue, and be sent out on its mission.

Though the Quiet Star was designed to fly safely at 1,200 feet and above, it was so undetectable that its pilots were able to take it down to treetop level with NVA or VC troops being none the wiser. The effectiveness of night missions was enhanced through the use of a low-light optical system designed by Xerox, the same company known for building copying machines.

A Quiet Star as seen from a chase aircraft over the US (Photo Wikimedia Commons)

No YO-3 ever took a shot from the bad guys during its deployment to Vietnam, simply because the Communists weren't able to detect it. With its spindly wings and dark paint scheme, the YO-3 couldn't be distinguished easily from the darkness of the night, and by the time enemy troops realized something had passed overhead, it was already gone.

Sadly, the Quiet Star arrived in Vietnam far too late to make much of a difference at all. It was pulled out of the country and relegated to testing roles with NASA, though a few of the 11 units produced by Lockheed were acquired by the FBI and the Louisiana Department of Fish and Game.

The FBI used its Quiet Stars to locate kidnappers, while Louisiana game wardens used theirs to catch poachers.