| X-29 | |
|---|---|
 A Grumman X-29 in flight | |
| General information | |
| Type | Experimental aircraft |
| National origin | United States |
| Manufacturer | Grumman |
| Status | Retired |
| Primary users | United States Air Force |
| Number built | 2 |
| History | |
| First flight | 14 December 1984 |
TheGrumman X-29 is an Americanexperimental aircraft designed to test aforward-swept wing,canard control surfaces, and other novel aircraft technologies. Funded byNASA, theUnited States Air Force andDARPA, the X-29 was developed byGrumman, and the two built were flown by NASA and the United States Air Force.[1] Theaerodynamic instability of the X-29's airframe required the use of computerizedfly-by-wire control.Composite materials were used to control theaeroelastic divergent twisting experienced by forward-swept wings, and to reduce weight. The aircraft first flew in 1984, and two X-29s were flight tested through 1991.
Two X-29As were built byGrumman after the proposal had been chosen over a competing one involving aGeneral Dynamics F-16 Fighting Falcon. The X-29 design made use of the forward fuselage and nose landing gear from two existingF-5A Freedom Fighter airframes (63-8372 became 82-0003 and 65-10573 became 82-0049).[2] The control surface actuators and main landing gear were from the F-16. The technological advancement that made the X-29 a plausible design was the use ofcarbon-fiber composites. The wings of the X-29, made partially ofgraphite epoxy, were swept forward at more than 33 degrees; forward-swept wings were first trialed 40 years earlier on the experimentalJunkers Ju 287 andOKB-1 EF 131. The Grumman internal designation for the X-29 was "Grumman Model 712" or "G-712".[3]
The X-29 is described as athree surface aircraft, withcanards,forward-swept wings, and aftstrake control surfaces,[4] using three-surface longitudinal control.[5] The canards and wings result in reduced trimdrag and reduced wave drag, while using the strakes for trim in situations where thecenter of gravity is off provides less trim drag than relying on the canard to compensate.[4]
The configuration, combined with acenter of gravity well aft of theaerodynamic center, made the craft inherentlyunstable. Stability was provided by the computerized flight control system making 40 corrections per second. The flight control system was made up of three redundant digital computers backed up by three redundantanalog computers; any of the three could fly it on its own, but the redundancy allowed them to check for errors. Each of the three would "vote" on their measurements, so that if any one was malfunctioning it could be detected. It was estimated that a total failure of the system was as unlikely as a mechanical failure in an airplane with a conventional arrangement.[5] If all of the flight computers failed mid-flight, the aircraft would have disintegrated due to aeroelastic forces before the pilot could keep it stable or even eject.[6]
The high pitch instability of the airframe led to wide predictions of extreme maneuverability. This perception has held up in the years following the end of flight tests. Air Force tests did not support this expectation.[7] For the flight control system to keep the whole system stable, the ability to initiate a maneuver easily needed to be moderated. This was programmed into the flight control system to preserve the ability to stop the pitching rotation and keep the aircraft from departing out of control. As a result, the whole system as flown (with the flight control system in the loop as well) could not be characterized as having any special increased agility. It was concluded that the X-29 could have had increased agility if it had faster control surface actuators and/or larger control surfaces.[7]
In a forward swept wing configuration, the aerodynamic lift produces a twisting force which rotates the wing leading edge upward. This results in a higher angle of attack, which increases lift, twisting the wing further. Thisaeroelastic divergence can quickly lead to structural failure. With conventional metallic construction, a torsionally very stiff wing would be required to resist twisting; stiffening the wing adds weight, which may make the design unfeasible.[8]
The X-29 design made use of theanisotropic elastic coupling between bending and twisting of the carbon fiber composite material to address this aeroelastic effect. Rather than using a very stiff wing, which would carry a weight penalty even with the relatively light-weight composite, the X-29 used a laminate which produced coupling between bending and torsion. As lift increases, bending loads force the wing tips to bend upward. Torsion loads attempt to twist the wing to higher angles of attack, but the coupling resists the loads, twisting the leading edge downward reducing wing angle of attack and lift. With lift reduced, the loads are reduced and divergence is avoided.[8]
The first X-29 took its maiden flight on 14 December 1984 fromEdwards AFB piloted by Grumman's Chief Test Pilot Chuck Sewell.[2] The X-29 was the fourth forward-swept wing jet-powered aircraft design to fly; the other three were the GermanJunkers Ju 287 (1944), the SovietOKB-1 EF 131 (1947) and the West-GermanHFB-320 Hansa Jet (1964).[9] On 13 December 1985, an X-29 became the first forward-swept wing aircraft to fly atsupersonic speed in level flight.
The X-29 began a NASA test program four months after its first flight. The X-29 proved reliable, and by August 1986 was flying research missions of over three hours involving multiple flights. The first X-29 was not equipped with a spin recovery parachute, as flight tests were planned to avoid maneuvers that could result indeparture from controlled flight, such as a spin. In a recorded video interview, NASA test pilot Rogers E. Smith[10] indicated one of the other test pilots performed an unauthorized aileron roll maneuver during a test flight and was subsequently removed from the program.[11] The second X-29 was given such a parachute and was involved in high angle-of-attack testing. X-29 number two was maneuverable up to anangle of attack of about 25 degrees with a maximum angle of 67° reached in a momentary pitch-up maneuver.[12][13]
The two X-29 aircraft flew a total of 242 times from 1984 to 1991.[3][14] The NASA Dryden Flight Research Center reported that the X-29 demonstrated a number of new technologies and techniques, and new uses of existing technologies, including the use of "aeroelastic tailoring to control structural divergence", aircraft control and handling during extreme instability, three-surface longitudinal control, a "double-hinged trailing-edge flaperon at supersonic speeds", effective high angle of attack control, vortex control, and demonstration of military utility.[5]
The first X-29, 82-003, is now on display in the Research and Development Gallery at theNational Museum of the United States Air Force onWright-Patterson Air Force Base nearDayton, Ohio.[15] The other craft is on display at theArmstrong Flight Research Center onEdwards Air Force Base. A full-scale model was on display from 1989 to 2011 at theNational Air and Space Museum's National Mall building in Washington, DC.[16] The full-scale replica was moved to theCradle of Aviation Museum inGarden City, New York in 2011.
Data from Jane's All the World's Aircraft 1988-89,[17] NASA X-Planes,[18] Donald,[3] Winchester[14]
General characteristics
Performance
Avionics
The 1989 flight simulator gameF29 Retaliator was based around the X-29 and imagined a future where it had been developed into a production fighter jet and fitted with various advanced weaponry.[citation needed]
Aircraft of comparable role, configuration, and era
Related lists
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This article incorporatespublic domain material from websites or documents of theNational Aeronautics and Space Administration.
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