Few aircraft are as storied as the experimental series known as the X-Planes. These flying laboratories date to the mid-1940s and took us ever higher, further and faster.
The first, the Bell X-1, was developed to explore transonic flight after fighter pilots began experiencing control problems as they approached the speed of sound in dives. Since then, a long list of X-Planes – and other test aircraft lacking the official ‘X’ moniker – have explored the unknown edge of aerodynamics and aviation. From the early days of supersonic flight and speed records to the possibilities of unmanned combat aircraft, X-Planes have, as their pilots say, pushed the edge of the envelope.
And so to mark the 62nd anniversary of Chuck Yeager's historic supersonic flight in the Bell X-1, here are 10 X-Planes that have led to some of the most innovative and useful aircraft designs.
Above: Bell X-1. Originally called the XS-1, the Bell X-1 is the grandfather of X-Planes. The X-1 resembles a .50-caliber bullet because that was one of the few supersonic objects known at the time. The rocket-powered plane broke the sound barrier for the first time on October 14, 1947, during its 50th flight with then Captain Charles E. “Chuck” Yeager at the controls.
One of the more important discoveries made during the X-1 flights was the need for an all flying tail or stabilator. In earlier powered flights the X-1 did not respond to some control inputs and it was discovered a shockwave was effectively creating an aerodynamic shadow around much of the elevator, the part of the horizontal tail responsible for pitching the airplane up and down. Engineers made minor changes to the tail so the entire horizontal surface could be more easily controlled by the pilot instead of just the trailing edge. This has been standard on all supersonic aircraft ever since.
Bell X-5 – One of the things learned in the early X-Planes was the need for swept wings at higher speeds. A swept wing can reduce the drag on an airplane as it approaches the speed of sound. But it also creates less lift at lower speeds, which can lead to problems with maneuvering and landing.
The solution was a wing that could move during flight. The German Messerschmitt P.1101 had wings that could be adjusted on the ground, but they never flew. In 1951 the Bell X-5 became the first successful jet aircraft to fly with a variable sweep wing capability. The wing sweep angle could be changed from 20 degrees to 60 degrees. This innovation was later used on the F-111 and most famously on the F-14 Tomcat of Top Gun fame.
North American X-15 – One of the most legendary of the X-Planes, the X-15 set speed records that still stand today. Designed to explore the realm of hypersonic flight as well as very high altitudes, the X-15 first flew in 1959.
By the last flight, its 199th, in 1968, the X-15 had flown faster and higher than any aircraft. In 1963 Joe Walker flew the X-15 into space achieving an altitude of 354,200 feet, a record only recently broken by SpaceShipOne in 2004. In 1967 Pete Knight flew the X-15 to a top speed of Mach 6.7 (~4,534mph), a record for winged vehicles yet to be broken.
In addition to numerous material (Inconel and titanium) and piloting (energy management techniques) developments, the X-15 was the first aircraft to use reaction controls to maneuver in space. These small thrusters allowed the pilot to position the aircraft at high altitudes where the normal aerodynamic controls were useless. Reaction controls are widely used on vehicles such as the space shuttle and most spacecraft today.
Hiller X-18 – Not all X-Planes were about going higher and faster. The Hiller X-18 was developed to explore the realm of vertical take off and landing (VTOL). First flown in 1959, the X-18 made just twenty flights during a short career that ended in 1961. The program came to a premature close after a propeller malfunction led to an inverted spin. The pilots did recover, but authorities terminated the program amid ongoing safety concerns.
The X-18 (and subsequent X-19 and X-22 programs) led to much of what is known today about VTOL airplanes. The Marine Corps’ V-22 Osprey is an example of an aircraft using the interesting configuration of this type of aircraft.
Boeing X-20 – The X-20 Dynasoar is an example of an X-Plane that contributed mightily to our knowledge of flight without ever leaving the ground. The X-20 was designed to be a piloted orbital reentry vehicle, but it was canceled before its first flight.
Several pilots were chosen in 1960 to fly the aircraft, including a little-known NASA test pilot by the name of Neil Armstrong, and flights were scheduled for later in the decade. But by the end of 1963, with the Gemini program moving ahead, the X-20 was killed. Still, the X-20 is considered a valuable X program. Engineers developing the space shuttle used much of the reentry heating data gleaned by X-20 materials team.
Martin Marietta X-24 – The X-24 family of planes followed up on earlier research into the concept of lifting body aircraft. Instead of using the typical wing for lift, these aircraft used the shape of the fuselage to create most of the lift.
Similar to the Northrop M2-F2 (perhaps most famous as the airplane crashing in the opening sequence of the Six Million Dollar Man television show), the X-24A and X-24B were considered the most advanced of the lifting body aircraft flown during the 1960s and 1970s. The aircraft demonstrated the viability of the lifting body design as an unpowered space reentry vehicle. Much of the research was used in the development of the space shuttle.
Grumman X-29 – This is an example of an X-Plane designed to test several ideas on one airplane, and one of the more interesting designs to fly over the deserts of Edwards Air Force Base.
Those forward-swept wings are the most obvious wild idea, but the X-29 also served as a test bed for advanced composite materials, a forward ‘canard’ wing and fly-by-wire control systems to keep the extremely maneuverable airplane flying. While the forward swept wing didn’t go anywhere, the X-29 program provided valuable insight into aerodynamics for extreme maneuverability as well as the computer control systems used on the latest fighter aircraft.
Rockwell/Messerschmitt X-31 – Continuing the maneuverability trend, the X-31 was designed to investigate the use of thrust vectoring in fighter jets. High speed aircraft don’t do well at low speeds and are rendered useless at speeds near or below their stall speed when the wing is no longer providing lift.
In order to look into the possibility of close-in air combat when extremely tight turning at slow speeds may be useful, engineers came up with the idea of directing the jet exhaust to steer the aircraft. Using three large paddle-like devices at the end of the exhaust nozzle, the X-31 successfully demonstrated the ability to direct the thrust and maneuver the aircraft at extremely high angles of attack and in post-stall maneuvers.
While limited to pitch changes only, thrust vectoring is used on the latest F-22 fighter for increased agility in close combat situations.
X-43 – Designed to take speed to a whole new level, the X-43 has flown faster than any air breathing aircraft to date. The unmanned scale model, flew to a speed of 9.8 Mach (~7,546mph) in 2004.
The X-43 uses a scramjet engine, which unlike a typical rocket motor, uses the oxygen from the atmosphere rather than a liquid oxygen supply carried by the vehicle. Instead the hydrogen fuel is combined with atmospheric oxygen in a combustion chamber at hypersonic speeds. The entire design of the aircraft aids in this process making the X-43 a flying motor of sorts.
Still in development as part of NASA’s Hyper-X program, future scramjet vehicles could provide a more efficient way of delivering payloads to space. And there are always the dreams of scramjets being used to transport people at 10 times the speed of sound, making that flight to Tokyo shorter than the in-flight movie.
Boeing X-45 – One of the latest X-Planes investigating future fighter jet designs, the X-45 represents the latest trend in combat aircraft, leaving the pilot on the ground. First flown in 2002, the X-45 demonstrated the capabilities of an unmanned combat air vehicle. Unlike the better known unmanned aerial vehicles such as the Predator being used over Iraq and Afghanistan, the X-45 demonstrated the possibility of semi-autonomous aircraft to operate in a combat environment.
On one demonstration flight, a single ground-based operator controlled two aircraft. Once a target on the ground was noticed by the aircraft, the on-board systems were able to determine which of the two airplanes was best able to attack. That aircraft then changed its course and after being given permission by the ground operator, was allowed to attack the ground target.
The X-45 may signal a shift in the long history of X-Planes to aircraft designed to push a whole new set of boundaries now that the safety of the pilot is removed from the research.
Photos: NASA and U.S. Air Force.
