This Silent Plane Flew Over Vietnam’s Treetops Undetected

Photo Credit: NASA

When thinking of stealth aircraft, designs like the Lockheed SR-71 Blackbird, Northrop B-2 Spirit, and the Lockheed U-2 spy plane come to mind, but the U.S. actually developed a relatively mundane-looking stealth aircraft: the Lockheed YO-3 Quiet Star. The YO-3 wasn’t hard to detect because it was extremely fast, had radar-absorbing paint, or a shape specifically tuned to disperse detection systems, no, its party trick was complete silence.

For aircraft, ‘stealth’ is simply the ability to avoid detection via any means necessary. Aircraft that use extreme measures are usually the most well-known, like the SR-71 Blackbird and its 2,200 mph top speed, or the B-2 Spirit’s complex shape and cutting edge suite of electronics that significantly reduce its radar cross-section and help avoid detection. However, there are other less “cool” ways to avoid detection.

NASA's converted YO-3A observation plane, now used for acoustics research, touches down at Edwards Air Force Base following a pilot checkout flight.
Photo Credit: NASA

During the Vietnam War, the U.S. wanted a small spotter aircraft that could fly above the enemy-held jungle at just over 1,000 feet without being detected. At the time, the Navy and Air Force did not possess an aircraft that could do this without alerting North Vietnamese and Viet Cong forces long before their arrival. For this job, speed, complex shapes, or expensive paint would not suffice, so Lockheed, the selected contractor to design this new aircraft, would go back to the basics of stealth.

Skunk Works’ silent plane

Lockheed was given the contract by the Department of Defense in 1968, or more specifically, Lockheed’s Advanced Development Programs, better known as Skunk Works. Skunk Works had a record of producing exceptional aircraft, including the aforementioned SR-71 Black Bird and the U-2.

Lockheed YO-3 at the Museum of Flight, Seattle
Lockheed YO-3A at the Museum of Flight, Seattle. (Photo Credit: Articseahorse / Wikimedia Commons CC BY-SA 4.0)

Thankfully, Lockheed had already dabbled with silent aircraft just a few years before, when they placed a quiet engine in a glider. The project went so far as to ship the aircraft over to Vietnam for testing, which it completed successfully. When the DoD issued the requirement for the new low-level observation plane Lockheed’s recent experience would prove to be helpful.

Lockheed used a Schweizer glider as a starting point. They swapped much of the original materials for lightweight metals and fiberglass, which was rarely found on aircraft at the time. Lockheed wanted the aircraft to be as light as possible to reduce fuel consumption and increase its duration in the air.

They enlarged the cockpit and added a bigger canopy for great visibility. An extra crew member (observer/spotter) was added in front of the pilot.

Naturally, the engine was the hardest part to make “stealthy,” as even relatively small aircraft internal combustion engines create a lot of noise. And, unlike car engines, aircraft motors need to run at nearly maximum power for extended periods of time, meaning they are often much larger in displacement to reduce stress and improve reliability. These larger engines make much more noise.

Even the propeller chopping through the air makes noise.

These were problems facing the Skunk Works engineers.

Lockheed YO-3 at Pima Air & Space Museum.
Lockheed YO-3 at Pima Air & Space Museum. (Photo Credit: aeroprints.com via Wikimedia Commons, CC BY-SA 3.0)

As a glider with further weight reduction, the aircraft didn’t need a huge engine, so a small horizontally opposed 6.0 liter 6 cylinder engine was used. This was connected to a slow-turning propeller via a belt-and-pulley system.

To contain the sound produced by the engine, fiberglass was packed around it and its exhaust would flow through a muffler and ducts towards the aircraft’s rear. This virtually eliminated all sound coming from the aircraft.

The Lockheed YO-3 Quiet Star

Work was completed in 1969, and Lockheed named it the YO-3 Quiet Star. A number of them were soon taken to Vietnam for their baptism of fire.

NASA pilot Ed Lewis rear briefs NASA test pilot Dick Ewers on the flight instruments of NASA's YO-3A acoustics research aircraft prior to a checkout flight.
Photo Credit: NASA

The Lockheed YO-3s performed brilliantly and even exceeded the original requirements. Operating at night, they were so quiet that those below simply could not hear them, and if they ever did figure out they were being watched, the YO-3 would be long gone.

Before any mission, the Lockheed YO-3 would take off and fly around the airbase. Those on the ground would listen out for any abnormal noises, or just any noise at all, and signal the aircraft to come down if they did. Any rattles would be fixed and repairs would be quickly carried out, and then the aircraft would go on its way.

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The YO-3 was originally required to be silent from an altitude of over 1,000 feet, but crews realized they were so quiet that they could drop down to treetop level without detection.

Over its entire service, no YO-3 had ever received enemy fire. Unfortunately, they arrived in Vietnam too late and made a negligible impact. After the war ended, some of them ended up in the hands of NASA and the FBI.