Experimental aircraft intended to test new configurations or unexplored aerodynamics.
In the United States, the federal government, under the direction of the National Advisory Committee for Aeronautics (NACA), its successor, the National Aeronautics and Space Administration (NASA), the U.S. Air Force, or the U.S. Navy, has sponsored a number of dedicated research aircraft, starting with the X-1 in 1944. As of 2001, the most recent X plane in progress was the X-43. Many of these aircraft were initially top secret. Additionally, new fighter and bomber prototypes are often initially given an “X” designation (for example, the XP-59 and XB-70) in the United States as well as in other countries.
The X-1 program was initiated in response to difficulties World War II fighter aircraft were experiencing as they approached the speed of sound in dives. These included especially Lockheed’s P-38 Lightning fighter and Republic’s P-47 Thunderbolt fighter planes. Even at flight speeds of about 75 percent of the speed of sound (that is, at a Mach number of 0.75), regions around the fuselage and around the thickest part of wing were experiencing supersonic flow and causing shock waves to form there. The consequences were a pitch-down trim which tended to increase the airspeed even more, separated flow behind the shock waves which caused control surface buffet and ineffectiveness, and a great increase in drag. Recovery, if possible, often relied on a reduction in Mach number produced by an increase in the speed of sound as the air temperature increased at lower altitudes. A specialized research aircraft seemed to be the only available approach because it was not known at that time how to build a wind tunnel with a supersonic test section.
Since its rocket engine had fuel for only 2.5 minutes, the only option was to attach the X-1 to the bomb bay of a B-29 bomber and release it at the maximum altitude and speed, and then glide to a landing on Muroc Dry Lake in California (now Edwards Air Force Base). The X-1 made its first powered flight in August, 1947, and in October, with test pilot Chuck Yeager, made the world’s first supersonic flight. In the following year, Yeager reached Mach 1.45. He also flew the second series X-1A; in 1953 he achieved Mach 2.44 in it but nearly lost the plane and himself when it tumbled out of control due to what was later recognized as inertia coupling. (Roll or inertia coupling occurs when a rolling motion creates a pitch moment because the aerodynamic axis is not aligned with the inertial axis. It caused the X-1 to tumble out of control on one flight and later caused the loss of an X-2. A solution to inertia coupling was found with the X-3 in time to solve the similar problem with the F-100 Super Sabre.
Starting from its first powered flight in 1955, the X-2 used a two-chamber rocket engine and, in 1956, set a new world speed record of 1,900 miles per hour and a new altitude record of 126,200 feet.
Throughout the 1950’s and 1960’s, one of the main focuses of X plane research was the exploration of supersonic flight. The Douglas X-3 Stilleto was designed to explore the transonic speed range, where mixed subsonic and supersonic flow exits around a plane in the range from about Mach 0.8 to about Mach 1.2, using two afterburner-equipped jet engines. (An afterburner uses raw fuel in the exhaust, after the compressor and combustor and turbine that powers the compressor.) Its first flight was in 1952.
The Northrop X-4 Bantam, a small piloted research aircraft, was a test of the semitailless configuration (that is, it had no horizontal tail surface). Its first flight was in 1948. A porpoising instability was found to limit flight speeds to less than Mach 1. Beginning in 1951, the Bell X-5 tested in-flight variable sweep of the wing, from 20 degrees to 60 degrees. (The later Navy F-14 Tomcat uses variable wing sweep.) For the Convair X-6, two standard ten-engine B-36 bombers were to be modified to carry a nuclear-powered turbojet, which was expected to yield almost indefinite range. However, only the modified aircraft which tested the shield of the nuclear reactor ever flew.
Six Lockheed X-7’s made 130 flights from 1951 to 1960, and many speed (Mach 4.3) and altitude (106,000 ft) records were set. The knowledge obtained was applied to the Lockheed F-104 Starfighter aircraft. The Aerojet General X-8 Aerobee was an uncrewed aircraft, recovered via parachute, designed to measure properties of the air above the atmosphere. With flights that began in 1947, it eventually reached Mach 5.96 and an altitude of 360,000 feet above Earth’s surface.
Many X planes were designed to perfect their missile capabilities. Beginning in 1950, the uncrewed Bell X-9 Shrike provided valuable information relevant to the guidance and control of an air-launched air-to-ground missile. The uncrewed North American X-10, with a first flight in 1953, provided needed information for the design of supersonic cruise missiles. A canard surface was shown to be useful. The uncrewed Convair X-11demonstrated the successful use of vectored thrust to control the trajectory of rocket-powered ballistic missiles. First flight was in 1948. The X-12, a follow-on to the X-11, became the prototype for the B-65 Atlas intercontinental ballistic missile (ICBM). First flight was in 1957.
The Ryan X-13 Vertijet was a tail-sitting, delta-winged, jet-powered vertical takeoff and landing (VTOL) aircraft. It made its first full transition from vertical takeoff to fully horizontal flight and back in 1957. The complexity of the system reduced payload, but jet-powered VTOL was shown to be possible.
The Bell X-14 was a VTOL aircraft that took off and landed in a horizontal attitude by diverting the jet thrust downward and then reducing the deflection in stages to transition to horizontal flight. Starting with preliminary tests in 1954, it ended up making research flights for nearly twenty-five years.
The North American X-15 was perhaps the most valuable high performance research aircraft of the X planes. The rocket-powered X-15 used a bullet-shaped fuselage with dorsal and ventral fins along with an all-moving slab-type horizontal tail. First flying in 1959, it eventually reached Mach 6.33 (considered to be hypersonic flight since it was greater than Mach 5) and 354,200 feet above Earth’s surface. The full pressure suit worn by the pilots was the basis for later astronaut pressure suits.
The Bell X-16 was intended to be a long-range, subsonic reconnaissance aircraft flying at or above 70,000 feet. The “X” designation was a cover-up. The project was canceled before the first flight could take place.
Beginning in 1955, twenty-six Lockheed X-17 multistage rockets were used to test the reentry requirements of long-range missiles. It showed that a blunt nose-cone shape reduced aerodynamic heating upon reentry because it forced the shock wave to form well in front of the nose.
Bell Aerospace Textron X-22 explored VTOL using four turboprop engines utilizing ducted fans (that is, there was a channel or duct around each of the four propellers). It was able to demonstrate sustained hover at over 8,000 feet. First flight was in 1966. A complex transmission system insured that an engine failure would only result in a reduction in total power without any tendency to roll.
Making its first flight in 1966, the uncrewed Martin Marietta X-23A proved the feasibility of a hypersonic lifting body for an orbiting, reentering space vehicle, playing a key role in the design of the space shuttle. (A lifting body is a fuselage that is broadened enough to generate a sufficiently large lift-to-drag ratio that it can be landed safely after a steep glide through the atmosphere.)
The single Martin Marietta X-24A was a Mach 2, rocket-powered, air-launched aircraft that was designed to explore the low-speed characteristics of a maneuverable lifting body; its first flight was in 1969. The X-24B was a modified X-24A with a new nose and a different tail configuration and made its first flight in 1973.
The X-26 aircraft, based on a Schweizer sailplane, were intended to perform ultraquiet reconnaissance over enemy territory. The X-26A was made by Schweizer and the X-26B by Lockheed. A low radar cross section was also part of the design. One aircraft was used in the Vietnam War. Two aircraft were used by the Navy to train pilots in inertia, or roll, coupling.
Forward wing sweep had been recognized for many years as conveying the same benefits as rearward wing sweep, as well as potentially better low-speed and maneuverability characteristics because the stall begins at the root and progresses to the wingtips, leaving the ailerons effective as long as possible. The problem with forward sweep is aeroelastic divergence: When the forward swept wing is loaded, it will try to bend so as to increase the angle of attack, which causes more loading, and this can quickly lead to structural failure. Advanced composite materials in which stiffness and strength can be tailored for the directions in which it is needed were used to overcome this problem in the Grumman X-29A.
The Rockwell/MBB X-31 was a multinational enhanced fighter maneuverability test aircraft that used vanes on the exhaust to direct the thrust over a wide range of angles. It was able to demonstrate a 180-degree turn from a deep stall at an angle of attack of 70 degrees after a dynamic entry (the Herbst maneuver). Its first flight was in 1990. The first of two X-31’s was lost on its two hundred ninety-second flight in 1995 when the airspeed probe iced up, but the pilot ejected safely.
Boeing X-32 was designed as a joint strike fighter (JSF) concept demonstrator aircraft, rather than as a pure research or experimental vehicle. It was in competition with the Lockheed Martin X-35 to become a successor (as the F-24) to the early models of the F-16 for the United States Air Force, replace early models of the F-18 for the Navy, and (in its short takeoff and vertical land, or STOVL, version) replace the Harrier and the F-18 for the U.S. Marine Corps and the British Royal Navy and Air Force. Its first flight was in September, 2000. The STOVL version first flew in March, 2001. The first flight of Lockheed Martin’s non-V/STOL version was in October, 2000.
Lockheed Martin X-33, a liquid hydrogen/oxygen-powered space launcher using a new aerospike engine, was designed to test a particular single-stage-to-orbit configuration as part of a future reusable launch vehicle (RLV). Payload was projected at 25,000 pounds or more, and hypersonic speeds of greater than Mach 15 were anticipated. Its lifting-body design was expected to yield touchdowns at 190 miles per hour (about 60 miles per hour slower than the space shuttle). Lockheed Martin’s contract with NASA expired on March 31, 2001. Assembly continued, but the future of the vehicle was placed in doubt.
The uncrewed Orbital Sciences X-34 was intended to provide design information for next-generation spacecraft. A powered version was expected to reach Mach 8. By 2001, three had been built.
McDonnell Douglas/Boeing X-36, a 28-percent-scale prototype jet, was designed to test the agility of future fighter aircraft that lack the traditional tail surfaces. A forward canard, split ailerons, and thrust-vectoring were used for directional control. Designed to be unstable in both pitch and yaw, it used a fly-by-wire (computer-controlled) control system. Thirty-one flights were made in 1997.
The Orbital Sciences X-37 was expected to validate new propulsion, thermal protection materials, and lightweight structures that would eventually lead to less expensive access to space. The Boeing X-40A was an 85-percent-scale version of the X-37. Its first flight was on August 11, 1998.
Three lifting-body, uncrewed Scaled Composites X-38’s, intended to provide a rescue vehicle for astronauts, had been built. The first flight of an X-38 was in 1998. A parafoil deployed to slow the final descent to Earth.
Micro Craft X-43, an uncrewed scramjet-powered hypersonic research vehicle, also known as Hyper-X, was intended to reach Mach 10, becoming the first air-breathing (jet-powered) aircraft to achieve hypersonic speeds (above Mach 5). The first flight attempt in 2001 ended in the loss of the first X-43 because of a launch vehicle failure.
The two Boeing X-45’s that had been built by 2001 were intended to acquire data leading to future uncrewed combat air vehicles (UCAVs), building on the success of uninhabited aerial vehicles (UAVs) for reconnaissance.
A number of X planes proved impractical for a number of reasons—they were too expensive, they were good ideas that could not be translated into realistic designs, or they simply fell afoul of congressional funding agendas. For instance, the Hiller X-18, a test bed for a vertical/short takeoff and landing (V/STOL) large cargo transport, made twenty flights, starting in 1959, before a failed prop control caused an unrecoverable spin. The whole wing, along with the turboprop engines, tilted from horizontal to vertical.
Curtiss-Wright X-19 was a tandem-winged aircraft with fully tilting turboprops at each of its four wingtips. However, on its only flight, the aircraft and its crew were lost when a propeller separated from the aircraft. The project was terminated in 1965.
Boeing X-20 Dyna-Soar (for “dynamic soaring”) was intended to be a prototype of a piloted, maneuverable hypersonic research aircraft that would go into orbit and then glide in for landing, much as the space shuttle does now. However, the success of the Mercury and Gemini suborbital and orbital missions caused this project to be terminated in 1963.
Designed to explore the feasibility of producing extensive laminar flow on a large aircraft by using suction through tiny holes in the wing surface, the Northrup X-21 demonstrated the ability to obtain close to 75 percent laminar flow over the wings, with a potential increase of perhaps 50 percent in its range. However, maintenance costs (keeping the holes free of water and dust and insects) and production costs made it appear to be too expensive to be practical. The X-21’s first flight was in 1963.
Three versions of the Benson X-25 autogyro were built and tested. It was intended as a stowable emergency flight vehicle for pilots who were forced to abandon their aircraft behind enemy lines. (An autogyro, or autogiro, obtains lift from rotating blades rather than from a wing, and the rotor blades can be folded to minimize the space required.) The concept proved to be feasible but not practical. The first flight of the X-25A was in 1968.
The Lockheed X-27 Lancer was intended as a low-cost, advanced lightweight fighter that would appeal to countries using the company’s F-104 Starfighter, but it was never funded.
The Pereira X-28A Osprey I was a homebuilt design, a low-cost, single-place seaplane that the government thought could be usefully employed in Southeast Asia for civil police patrols. It was successfully tested in 1971, but no production contract resulted.
The X-30 Spaceplane, announced by President Ronald Reagan in 1986 as the “Orient Express,” was intended to be a crewed, single-stage-to-orbit vehicle that could take off and land on conventional runways. It was expected to use turbojets at low speeds, ramjets at the lower supersonic speeds, scramjets (supersonic combustion ramjets) above Mach 4, and rocket engines while in orbit. The program was canceled in 1995 with no planes built.
Miller, Jay. The X-Planes: X-1 to X-45. 3d ed. St. Croix, Minn.: Specialty Press, 2001. The only up-to-date, complete, and detailed reference to the U.S. research X planes. Pace, Steve. X-Fighters: USAF Experimental and Prototype Fighters, XP-59 to YF-23. Osceloa, Wis.: Motorbooks International, 1991. Some thirty-seven different airplanes are described and illustrated, starting with the first U.S. jet fighter.
Air Force, U.S.
National Advisory Committee for Aeronautics
National Aeronautics and Space Administration
Uninhabited aerial vehicles