The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies.
The X-29 was developed by Grumman, and the two built were flown by NASA and the United States Air Force.
The aerodynamic instability of the X-29’s airframe required the use of computerized fly-by-wire control.
Composite materials were used to control the aero-elastic 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 by Grumman 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 two existing F-5A Freedom Fighter airframes (63-8372 became 82-0003 and 65-10573 became 82-0049).
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 of carbon-fibre composites.
The wings of the X-29, made partially of graphite epoxy, were swept forward at more than 33 degrees; forward-swept wings were first trialled 40 years earlier on the experimental Junkers Ju 287 and OKB-1 EF 131.
The Grumman internal designation for the X-29 was “Grumman Model 712” or “G-712”.
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 centre of gravity is off provides less trim drag than relying on the canard to compensate.
The configuration, combined with a centre of gravity well aft of the aerodynamic centre, 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 analogue 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 anyone 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 manoeuvrability.
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 manoeuvre 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.
4,000 lb (1,814 kg) payload
53 ft 11.25 in (16.4402 m) including nose probe
48 ft 1 in (15 m) fuselage only
27 ft 2.5 in (8.293 m)
14 ft 3.5 in (4.356 m)
188.84 sq ft (17.544 m2)
Grumman K MOD 2 (6.2%)
Grumman K MOD 2 (4.9%)
13,800 lb (6,260 kg)
Max take-off weight
17,800 lb (8,074 kg)
3,978 lb (1,804 kg) in two fuselage bladder tanks and two strake integral tanks
1 × General Electric F404-GE-400 afterburning turbofan engine,
16,000 lbf (71 kN) with afterburner
956 kn (1,100 mph, 1,771 km/h) at 33,000 ft (10,058 m)