FORT RUCKER, Ala. -- Many aircraft in the Army Aviation inventory are some of the deadliest airships in the world, but not all aircraft were meant to pack a punch.

The Lockheed QT-2PC Quiet Thruster aircraft weren't known for their airspeed, firepower or high-flying capabilities, but more so for their ability to go unnoticed, according to Bob Barlow, U.S. Army Aviation Museum volunteer.

The QT-2PC's roots reach back to the late 1950s in the U.S. Navy's X-26 Frigate program, which was a program designed to help train test pilots on the phenomenon of inertia coupling.

"Aircraft were getting faster, and as they got faster the wings got smaller because they weren't needed as much to provide lift," said Barlow. "As the wings got shorter, you started having more inertia in the fuselage than you did in the wing, so if you put the aircraft in a roll, sometimes it would become unstable because the inertia would shift forward and back as a process of precession, and the aircraft would tumble."

The X-26 was based on the civilian sailplane, the Schweitzer SGS 2-32, which had an impressive wingspan of nearly 60 feet, and provided a much slower control response than high-speed jets being tested at the time, which allowed the pilots to train to sense the onset of inertia coupling.

Fast forward to 1967, Lockheed became interested in creating an aircraft for light observation and surveillance that was quiet, and came up with the idea of putting an engine on a sailplane.

"Sailplanes have very efficient wings, so it doesn't need much power or thrust to stay in the air, so the less engine you've got, the slower your engine turns and the less noise you have," said Barlow.

Lockheed felt they could combine sailplane aerodynamics with a slow-turning propeller to get the results they were looking for, and the Navy provided them with the two X-26s. After acquiring the aircraft, an engine wasn't placed in the nose or wings of the aircraft, but rather mounted as close to the center of gravity as possible located behind the pilot and observer.

A crankshaft ran from the center of the plane on the exterior of the craft above the pilots to a large propeller at the front of the aircraft.

Lockheed wanted the aircraft to be as quiet as possible, so the engine they mounted only put out about 100 horsepower, but even then the plane didn't require that much power, said Barlow. In order to slow down the propeller, a reduction gear was installed to reduce the RPM of the propeller, providing a quieter ride. Additionally, the propeller had a substantial pitch to give it the most authority with the least RPM.

"During testing, they found out that at an altitude of about 1,000 feet and at a speed of between 70-80 knots, (the aircraft) was practically silent," he said. "Judging by the fact that the test program was successful, it was decided that the aircraft would deploy to Vietnam for service testing."

Before being sent off for service testing, further modifications were made by adding avionics, mission equipment, observer windows in the lower fuselage and a deep blue/grey paint job for night operations.

In January of 1968, the two QT-2PC's were sent to Southeast Asia for combat evaluation. Shortly after, the Tet Offensive broke out, which allowed the aircraft to prove their worth.

"The QT-2PCs with their crews, which were multiservice, started flying over the hot areas, night after night, and a lot of times they were launching while under artillery fire," said the museum volunteer. "They continued to fly night after night for several weeks and the enemy never took notice -- they flew virtually with impunity."

Through these missions, the crews were able to gather a wealth of information, including spotting the fleets of sampans coming from Viet Cong bases and supply centers in Cambodia. This discovery confirmed that the target for their offensive was Saigon, and now friendly forces could now plan to deal with the threat, said Barlow.

Following the successful evaluation of the aircraft, they were returned to the U.S. Naval Test Pilot's School in 1969, he said. There were only two ever made, designated Tail No. 1 and 2, and Tail No. 1 currently sits in the U.S. Army Aviation Museum's collection in storage. Tail No. 2 was decommissioned and sold, and de-converted back to its original sailplane configuration and still flies to this day, Barlow said.