|A Grumman X-29 in flight|
|Role||Experimental Research Aircraft|
|First flight||14 December 1984|
The Grumman X-29 was an experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. The aerodynamic instability of the airframe required the use of computerized fly-by-wire control. Composite materials were used to control the aeroelastic divergent twisting experienced by forward-swept wings, also reducing the weight. Developed by Grumman, the X-29 first flew in 1984; two X-29s were flight tested over the next decade.
Design and development
Two X-29As were built by Grumman from two existing Northrop F-5A Freedom Fighter airframes (63-8372 became 82-0003 and 65-10573 became 82-0049) after the proposal had been chosen over a competing one involving a General Dynamics F-16 Fighting Falcon. The X-29 design made use of the forward fuselage and nose landing gear from the F-5As with the control surface actuators and main landing gear from the F-16. The technological advancement that made the X-29 a plausible design was the use of carbon-fiber composites. The wings of the X-29, made partially of graphite epoxy, were swept forward at more than 33 degrees. The Grumman internal designation for the X-29 was "Grumman Model 712" or "G-712".
Three-surface design and inherent instability
The X-29 is described as a three surface aircraft, with canards, forward-swept wings, and aft strake control surfaces, using three-surface longitudinal control. The canards and wings result in reduced trim drag and reduced wave drag, while using the strakes for trim in situations where the center of gravity is off provides less trim drag than relying on the canard to compensate.
The configuration, combined with a center of gravity well aft of the aerodynamic center, made the craft inherently unstable. 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 redundant analog 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.
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. 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.
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. This aeroelastic 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.
The X-29 design made use of the anisotropic 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.
The first X-29 took its maiden flight on 14 December 1984 from Edwards AFB piloted by Grumman's Chief Test Pilot Chuck Sewell. The X-29 was the third forward-swept wing jet-powered aircraft design to fly; the other two were the Nazi Germany Junkers Ju 287 (1944 and post-war by the USSR) and the HFB-320 Hansa Jet (1964). On 13 December 1985, a X-29 became the first forward-swept wing aircraft to fly at supersonic 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 in departure from controlled flight, such as a spin. The second X-29 was given such a parachute and was involved in a high angle-of-attack testing. X-29 number two was maneuverable up to an angle of attack of about 25 degrees with a maximum angle of 67° reached in a momentary pitch-up maneuver.
The two X-29 aircraft flew a total of 242 times from 1984 to 1991. 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.
Aircraft on display
The first X-29 is now on display in the R&D/Flight Test Hangar at the National Museum of the United States Air Force on Wright-Patterson Air Force Base near Dayton, Ohio. The other craft is on display at the Armstrong Flight Research Center on Edwards Air Force Base. A full scale model was on display from 1989 to 2011 at the National Air and Space Museum's National Mall building in Washington, DC.
- Crew: one pilot
- Payload: 4,000 lb (1,810 kg)
- Length: 48 ft 1 in (14.7 m)
- Wingspan: 27 ft 2 in (8.29 m)
- Height: 14 ft 9 in (4.26 m)
- Wing area: 188.8 ft² (17.54 m²)
- Empty weight: 13,800 lb (6,260 kg)
- Max. takeoff weight: 17,800 lb (8,070 kg)
- Powerplant: 1 × General Electric F404 turbofan, 16,000 lbf (71.2 kN)
- Maximum speed: Mach 1.8 (1,100 mph, 1,770 km/h at 33,000 ft (10,000 m))
- Range: 350 mi (560 km)
- Service ceiling: 55,000 ft (16,800 m)
- Related development
- Aircraft of comparable role, configuration and era
- Related lists
- Gehrs-Pahl, Andreas, ed. "The X-Planes: From X-1 to X-34." ais.org, 1995. Retrieved: 1 September 2009.
- Donald 1997, p. 483.
- Roskam 1985, pp. 85–87.
- "X-29 fact sheet." NASA Dryden Flight Research Center. Retrieved: 1 September 2009.
- Flying Qualities Evaluation of the X-29A Research Aircraft , AFFTC TR-89-08, May 1989.
- Pamadi, Bandu N. "Performance, Stability, Dynamics, and Control of Airplanes. 2nd Ed., Section 1.11." AIAA, 2004.
- Green 1970, pp. 493–496.
- AFFTC-TR-91-15, X-29 High Angle-of-Attack Flying Qualities. USAF, July 1991.
- Winchester 2005, p. 261.
- Winchester 2005, p. 262.
- "Exhibits on view." National Air and Space Museum. Retrieved: 14 October 2011.
- Jenkins, Dennis R. et al. "SP-2003-4531: American X-Vehicles, An Inventory—X-1 to X-50." NASA, June 2003. Retrieved: 1 September 2009.
- "X-29 3-view." NASA/Dryden Flight Research Center. Retrieved: 14 July 2010.
- Donald, David ed. "Grumman X-29A". The Complete Encyclopedia of World Aircraft. New York: Barnes & Noble Books, 1997. ISBN 0-7607-0592-5.
- Green, William. Warplanes of the Third Reich. New York: Doubleday & Company Inc., 1970. ISBN 0-385-05782-2.
- Roskam, Jan. Airplane Design II: Preliminary Configuration Design and Integration of the Propulsion System. Ottawa, Kansas: Roskam Aviation and Engineering Corporation, 1985. ISBN 978-1-88488-543-3.
- Thruelsen, Richard. The Grumman Story. New York: Praeger Publishers, Inc., 1976. ISBN 0-275-54260-2.
- Treadwell, Terry. Ironworks: Grumman's Fighting Aeroplanes. Shrewsbury, UK: Airlife Publishers, 1990.ISBN 1-85310-070-6.
- Warwick, Graham. "Forward-sweep Technology." Flight International, 16 June 1984, pp. 1563–1568.
- Winchester, Jim. "Grumman X-29". X-Planes and Prototypes. London: Amber Books Ltd., 2005. ISBN 1-904687-40-7.
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