X Planes: Experimental aircraft bring tomorrow’s technology to the present day

by Stuart Ibberson, editor
When people hear the term “X-Planes” they often think of cutting edge aircraft used to test technology.

Some of the most famous X-Planes that come to mind are the Bell X-1 in which Chuck Yeager broke the sound barrier, the X-15 in which multiple pilots earned their astronaut wings, and the X-35 that later became the F-35 Lightning II fighter, in service with the U.S. Air Force, the U.S. Navy, the U.S. Marine Corps, and many allies around the world.

However, not all X-Planes are aircraft (some are rockets and missiles), and not all aircraft that have the ‘X’ in their name are officially designated as X-Planes — the X-91, X-92 and XB-70 come to mind.

The Grumman X-29. (NASA photograph)

Beginning in 1946, two XS-1 experimental research aircraft (later redesignated X-1s) conducted pioneering tests at Muroc Army Air Field (now Edwards Air Force Base) in California, to obtain flight data on conditions in the transonic speed range. These early tests resulted in the first piloted flight faster than Mach 1.0, the speed of sound, on Oct. 14, 1947.

The XS-1 was the first high-speed aircraft built purely for aviation research purposes. The model was never intended for production. The XS-1 was designed largely in accordance with specifications provided by the National Advisory Committee for Aeronautics (NACA) [now National Aeronautics and Space Administration], paid for by the Army Air Forces, and built by Bell Aircraft Inc. The XS-1 #2 (serial number 46-063) was flight tested by NACA to provide design data for later production high-performance aircraft.

The research techniques used in the X-1 program became the pattern for all subsequent X-craft projects. The NACA X-1 procedures and personnel also helped lay the foundation of America’s space program in the 1960s. The X-1 project defined and solidified the post-war cooperative union between U.S. military needs, industrial capabilities, and research facilities. The flight data collected by NACA in the X-1 tests then provided a basis for American aviation supremacy in the latter half of the 20th century, which continues to this day.

The X-3 in flight. (NASA photograph)

X-Planes have since accomplished many aviation “firsts” including breaking speed and altitude barriers, varying wing sweep in flight, implementing exotic alloys and propulsion innovations, and many more.

As a result of the X-1’s initial supersonic flight, the National Aviation Association voted its 1948 Collier Trophy to be shared by the three main participants in the program. Those honored at the White House by President Harry S. Truman included Lawrence “Larry” Bell for Bell Aircraft, Capt. Charles E. “Chuck” Yeager for piloting the flights, and John Stack of NACA for the NACA contributions.

The U.S. X-Plane Program has evolved from being the first rocket-powered airplane to break the sound barrier, to testing over 30 different major research designs — although not all were developed into flying prototypes.

As the program progressed, other non-rocket-powered experimental aircraft were built and tested. These aircraft included: a range of vertical takeoff and horizontal landing vehicles; smaller, propeller-driven reconnaissance vehicles; and a series of unmanned missile testbeds of both single and multistage designs. Although the program grew to include conventional propeller-driven aircraft, all designs had in common the aspect of being highly valuable research tools for advancement of aerodynamics and astronautics.

The Bell X-5. (Air Force photograph)

Accomplishments of the X-Plane family have been many. The program included: the first aircraft to break the sound barrier; the first aircraft to use a variable-sweep-wing in flight; the first to fly at altitudes in excess of 100,000, 200,000 and 300,000 feet; the first to use exotic alloy metals for primary structure; the first to test gimbaled jet and rocket engines; the first to use jet-thrust for launch and landing; the first to fly three, four, five, and six times the speed of sound; the first to test boundary-layer-airflow control theories over an entire wing at transonic speeds; the first to successfully complete a 180-degree turn using a post-stall maneuver; and the first missile to reach an intercontinental flight range.

The majority of testing for the X-Plane family has occurred at Edwards Air Force Base (formerly known as Muroc Army Airfield). Hosts within Edwards include the Air Force Test Center and Armstrong Flight Research Center (formerly known as the Air Force Flight Test Center and Dryden Flight Research Center, respectively.)

Other sites which have served as X-Plane testing sites include: Langley Research Center, Va.; Ames Research Center, Calif.; various U.S. government-owned ships; White Sands Missile Range, N.M.; Wright-Patterson Air Force Base, Ohio; Cape Canaveral Air Station, Fla.; Pinecastle Air Force Base, Fla.; Buffalo, N.Y.; and the National Aviation Facilities Experimental Center in Atlantic City, N.J. However, Edwards has seen more X-Plane programs and test flights than any other similar facility in the United States.

The Ryan X-13 Vertijet being tested at Edwards Air Force Base, Calif., circa 1957. (Courtesy photograph)

As with every research program testing prototype equipment, the X-Plane Program has not been without technical glitches and equipment failures. Since the beginning of the program’s manned flight operations in 1946, approximately 15 major accidents and four pilot fatalities have been associated with manned vehicle tests.

So who determines whether an X-Plane is an “X-Plane?” The U.S. Department of Defense has specific guidelines and detailed protocols that identify all aircraft, helicopters, rockets, missiles, spacecraft and other aerial vehicles in military use. These guidelines are what give us F for fighter, C for cargo, A for attack, and X for experimental, among other designations.

Aerotech News and Review is going to take a look at some of the better known X-Planes, some of the less well-known aircraft, a few of the current programs — as well as the X-Plane that wasn’t really an X-Plane.

The X-18 showing its elaborate engine configuration. (Air Force photograph)


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