early X Planes

The Bell Aircraft Corporation X-1-1 in flight. The X-1 series aircraft were air-launched from a modified Boeing B-29 or a B-50 Superfortress bombers. The X-1-1 was painted a bright orange by Bell Aircraft.

At the end of World War II, the United States operated some of the most advanced aircraft in the world, such as the B-29. But the pace of change during the war had been so fast that it became clear to many top scientists and military leaders that unless the United States actively sponsored advanced aeronautics research, it could quickly fall behind. As a result, in 1945 the U.S. Army Air Forces (which became the U.S. Air Force in 1947) and the National Advisory Committee on Aeronautics, or NACA, began the first of a series of experimental aircraft projects, many of which were designed to develop technology for high-speed flight. These soon became known as X-planes. While prototype and experimental aircraft were not new, the X-planes were significant because they were solely intended to develop technology in general, not lead to operational aircraft.

Although not designated an "X vehicle," the D-558-2 was essentially an X-vehicle aircraft in design and function, and contributed much to aeronautics research. The D-558 Phase Two aircraft was quite different from its Phase One predecessor, the Skystreak. German wartime aeronautical research records, reviewed in 1945 by Douglas Aircraft Company personnel, pointed to many advantages gained from incorporating sweptback wing design into future research aircraft.

The X-1 actually had a conventional tail with elevators for pitching the nose up and down. However, at high speeds, a shockwave formed on the tail surfaces near the hinge for the elevators, rendering them useless. But the X-1 also had a system for raising and lowering the entire tail a few degrees to adjust the trim of the airplane in flight (to enable it to fly level). Yeager and the X-1 flight engineers proposed using this system instead of the elevators at high speeds to control the airplane. It worked and this lesson was secretly incorporated into American fighter planes at the time, giving the United States a technological edge over Soviet, French, and British aircraft for several years. Today, all supersonic aircraft use all-moving tail surfaces.

The Bell Aircraft Corporation X-1E airplane being loaded under the mothership, Boeing B-29. The X-planes had originally been lowered into a loading pit and the launch aircraft towed over the pit,  where the rocket plane was hoisted by belly straps into the bomb bay. By the early 1950s a hydraulic lift had been installed on the ramp at the NACA High-Speed Flight Station to elevate the launch aircraft and then lower it over the rocket plane for mating.

The Bell X-1E soon followed these earlier aircraft with its first flight in December 1955. Although it did not achieve speeds or altitudes as high as the X-1A or X-1B, the X-1E proved that an extremely thin wing could be used on supersonic aircraft. This research led to the Lockheed F-104 Starfighter interceptor aircraft. (The X-1C was canceled before completion. The X-1D was destroyed before it could make its first powered flight.)

The X-2 had a more pointed nose than the X-1 and was designed to reach speeds in excess of Mach 3.

In June 1952, the Bell X-2 had its first flight. The X-2 was equipped with a pointier nose and more powerful rocket engine than its predecessors. It was designed to reach speeds in excess of Mach 3 (2,094 miles per hour). At such high speeds, the friction from air brushing against the aircraft heats its skin to high temperatures. The X-2, therefore, had to be made of advanced lightweight heat-resistant steel alloy. The X-2 reached a record altitude of 125,907 feet (38,376 meters). Research on the X-2, including new construction techniques, contributed to the development of advanced materials for high-speed aircraft such as the XB-70 bomber and the SR-71 spyplane.

The twin-turbojet X-3 "Stiletto," the only one built, was designed to test features of an aircraft suitable for sustained flights at supersonic speeds and high altitudes. Its secondary mission was to investigate the use of new materials such as titanium and to explore new construction techniques. It made its first supersonic flight in June 1953.

Even the failure of an X-plane to achieve its goals was useful. The Northrop X-4, which flew from 1948 to 1953, proved that tailless aircraft were unsuitable for high-speed subsonic flight (under Mach 1). Other X-planes were developed to conduct various flight research. Some, such as the X-15, developed soon after the earlier X planes, were very successful whereas others demonstrated that certain technologies were essentially dead-ends.

The Northrop X-4 was developed for the study of flight characteristics of swept wing semi-tailless aircraft at transonic speeds. Northrop built two X-4s. 1 aircraft made its initial flight on December 16, 1948.

The X-planes that did fly were usually equipped with multiple recording instruments, some of which radioed their data to the ground. They often flew numerous flights, each one methodically advancing the flight envelope and providing insight into advanced aerodynamics, engines and materials.

Most X-planes have been developed by either the NACA or the National Aeronautics and Space Administration (NASA) in partnership with the military, usually the U.S. Air Force. Later on, the "X" designation was used in different ways. In one case, the designation was used to mislead people into thinking that a secret spyplane project (the X-16) was actually an experimental aircraft. In other cases, the X designation has been applied to early prototype versions of operational aircraft. But initially, the title "X-plane" indicated that an airplane was built solely to demonstrate and improve aviation technology.