early X Planes
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
The first aircraft produced by the joint
team was the XS-1. The "S" stood for supersonic and was dropped early in
the program. The X-1 was the first crewed vehicle to break the sound
barrier. It was built by Bell Aircraft Company. Its fuselage was modeled
on a 50-caliber bullet because that was the one shape that aerodynamics
experts knew did not tumble at supersonic speeds. It had straight, very
thin wings. It was powered by a rocket engine and dropped from the belly
of a B-29 bomber. Its first flight was in January 1946. On October 14,
1947, the X-1, piloted by Captain Charles (Chuck) Yeager reached a speed
of 700 miles per hour (1,127 kilometers per hour) while at 45,000 feet
(13,716 meters), breaking the sound barrier. The X-1 proved that an
aircraft could be controlled at speeds faster than the speed of sound,
Mach 1. It led to several aerodynamic advances that were quickly
incorporated into U.S. fighter aircraft designs.
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.
After the success of the X-1 program,
the Air Force and NACA teamed up again to develop the second generation
X-1, which was intended to fly at twice the speed of sound, or Mach 2.
Four aircraft were planned. The X-1A had its first flight on July 24,
1951. It and its sister craft the X-1B established new speed records,
eventually reaching a speed of Mach 2.44 (1,650 miles per hour) (2,655
kilometers per hour) and an altitude of 90,440 feet (27,566 meters).
Bell Aircraft Corporation X-1E airplane being loaded under the mothership,
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
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.)
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 Douglas X-3, which first flew in
1952, was not as successful as its predecessors. Unlike the earlier
aircraft, it was not rocket-powered or dropped from the belly of a bomber,
but instead took off from the ground like a conventional aircraft with jet
engines. It had a short, thin wing that did not generate much lift except
at high speeds. This meant that it did not lift off from the runway until
it was traveling very fast, which caused its tires to overheat. As a
result, several tire companies developed high temperature materials for
twin-turbojet X-3 "Stiletto," the only one built, was designed to test
of an aircraft suitable for sustained flights at supersonic speeds and
Its secondary mission was to investigate the use of new materials such as
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.
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