The Grumman X-29 was an experimental aircraft that explored a number of new technologies; the most immediately obvious being the forward-swept wings and canard control surface. The inherent aerodynamic instability of this arrangement required the use of computerized fly-by-wire control, and advanced composite materials were needed to make the wing sufficiently rigid without being unacceptably heavy. The X-29 first flew in 1984 and two X-29s were flight tested over the next decade.
Design and development
The X-29 in flight
Two X-29As were built by Grumman Aerospace Corporation 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 Grumman internal designation for the X-29 was "Grumman Model 712" or "G-712". The X-29 first flew in 1984 from Edwards AFB with Grumman's Chief Test Pilot Chuck Sewell at the controls.
On December 13, 1985 one of the X-29s became the first forward swept wing aircraft to fly in supersonic, level flight. The earlier Junkers Ju 287 had been the first jet aircraft with forward-swept wings. The NASA test program of the two X-29s, continued from 1984 to 1991.
The X-29A demonstrated excellent control and maneuvering qualities at an angle of attack up to 45°. However, the wing configuration made the craft inherently unstable and it could fly only with the constant corrections (up to 40 per second) provided by the computerized flight control system. The 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 first craft built is now on display in the National Museum of the United States Air Force at Wright-Patterson Air Force Base near Dayton, Ohio. The other craft is on display at the Dryden Flight Research Center on Edwards Air Force Base. A full scale model is on display at the National Air and Space Museum's National Mall building in Washington, DC.
 Aeroelastic considerations
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 non-isotropic nature 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.
 Specifications (X-29)
Data from NASA X-Planes NASA/Dryden, Donald, Winchester
* 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)
* Range: 350 mi (560 km)
* Service ceiling: 55,000 ft (16,800 m)