Grumman G-712 / X-29
Grumman Aerospace, Rockwell International and General Dynamics, proposed to DARPA that a forward-swept wing demonstrator should be built. The demonstrator would be used to: verify that composites could provide the wing stiffness required without severe weight penalties; verify the aerodynamic advantages of a forward-swept wing indicated in wind tunnel testing; and expand the forward-swept wing technology base.
DARPA responded favourably and there followed three years of intensive competitive design studies, culminating in Grumman Aerospace being selected to build two demonstrators in 1981 under an $80 million contract, based on their Project Design G-712. This was officially designated X-29A by the U.S. Air Force in mid-1981.
The X-29A was a single-seat jet aircraft fitted with a wing mounted at the rear of the fuselage, swept forward at 35 degrees, and having shoulder-mounted canards just behind the cockpit.
To save development time and money, Grumman used as many parts as practical from existing aircraft. The forward fuselage and nose landing gear are from a Northrop F-5A, and the main landing gear and control surface actuators are from a General Dynamics F-16A. Off-the-shelf equipment includes the emergency power unit and the flight control servo actuators, also from an F-16A. The aircraft is powered by a single General Electric F404 afterburning turbofan, developing 16,000 lb thrust, and is the engine type used in the McDonnell Douglas F/A-18 Hornet.
The tapered variable-incidence foreplane was selected after intensive wind-tunnel testing and is used to provide pitch trim and control movements, as this type of surface provides lift for trim. It also acts as a slat to help the heavily loaded in-board section of the wing.
The X-29A is a highly unstable aircraft, the c.g. being no less than 35% aft of the aerodynamic centre of the wing/canard surfaces. Initially the planned instability was intended to be only 20%, and accordingly the canard had an area equal to 15% of the wing. Wind tunnel tests, however, showed that this would not give the degree of control required during transonic manoeuvres, and the canard size was increased to 20% of the wing, giving the current degree of instability.
The heart of the X-29A is its distinctive forward-swept wing. It is a very thin wing; the thickness-to-chord ratio being less than 5%. The wing area is 188 sq. ft. and the angle of forward sweep 30 deg.
The inboard end of the leading-edge is swept aft, to alleviate some of the root stall problems associated with forward sweep. To preserve the structural integrity of the vital lower wing skin, the main landing gear retracts forward, into the fuselage ahead of the wing. The trailing-edge of each wing root extends aft to form a large body strake ending in a controllable flap. The strakes add area behind the c.g. and hence improve directional stability.
It is on the outer, forward-swept portion of the wing that the unique directional properties of carbon-fibre laminates construction are used to overcome the adverse wing twist, or “divergence", without the prohibitive weight penalty of a conventional aluminium alloy structure. A total of 752 plies is used, with 156 layers at the thickest section of the skin. To resist the natural tendency of the forward-swept wing to twist, the layers are “rotated" some 10 deg. forward of the wing's structural axis.
The laminated wing skins, at an angle to the bending axis, shear forward under compression and backward under tension. The effect of shearing under load on the wing torsion box is to generate a nose-down torque which counters the natural tendency of the wing to twist leading edge up. The carbon-fibre skins are attached to a sub-structure of conventional aluminium alloy construc-tion, the front spar being of electron-beam welded titanium to cater for the high loads on the front part of the wing due to the forward sweep.
Full-span “variable camber" flaperons are fitted to the wing trailing edge, these being used symmetrically for pitch control and asymmetrically for roll. The flaperons are in three sections, being hinged at two chord-wise locations, so that they may be used to change the camber of the wing. The primary hinge is at 75% chord and the secondary hinge is at 90% of the chord. The flap sections are geared so that for every 1 deg. of flap deflection the aft section deflects an additional 1 deg. The flap increases manoeuvrability and reduces drag across the entire speed range. Programmed by the flight control computer, the flap alters the wing shape in flight as a function of changing conditions. The result is a constantly optimum wing shape.
The X-29A flew for the first time on 14th December 1984, from NASA's Dryden Flight Research Facility at Edwards Air Force Base, Cali-fornia, with Grumman chief test pilot Chuck Sewell at the controls. For this first flight, which lasted 57 minutes, the landing gear and the variable-camber trailing-edge devices were kept down. The gear and flaps were retracted during the second flight on 4th February 1985. Two further flights were made on 25th February and 1st March. Two 360-deg. rolls were made during the third flight. After just these four flights the demonstrator was turned over to NASA in March 1985 for further flight testing.
In the initial phase of testing low-altitude, high-speed manoeuvres, the X-29 demonstrated high-g turns tighter than anything achieved by a conventional fighter and displayed awesome potential for combat aircraft. For an extended test phase, the second aircraft was fitted with a vortex flow control system to test the possibility of using high-pressure nitrogen injected directly into the vortices coming off the nose to help maintain control at high AoA. With this, pilots were able to achieve good control response to an AoA of 67 degrees.
The first aircraft flew on 14 December 1984 but was grounded on 6 December 1988 after its 242nd flight. The second X-29A, flown for the first time on 23 May 1989, completed its flight test programme in October 1991. Between them the two aircraft completed 374 flights (more than any other X-craft) and demonstrated angles of attack up to 67 degrees (the target was 80). They also flew at Mach numbers up to 1.52 and reached altitudes up to 12200m. Both aircraft were now in store at the Ames-Dryden Flight Research Facility of NASA at Edwards AFB, California.
The X-29 tests ended in 1992 after 436 flights.
Engine: 1 x General Electric F404-GE-400 after-burning turbofan, 69847 N / 16,000-lb. s.t.
Wing span: 27 ft. 2.5 in. / 8.3 m
Length: 16.4 m / 53 ft 10 in
Height: 14 ft. 3.5 in. / 4.4 m
Wing area: 17.5 sq.m / 188.37 sq ft
Foreplane area: 35.96 sq.ft.
Empty wt: 13,800 lb. / 6260.0 kg
MTOW: 17,800 lb. / 8074.0 kg
Max. payload weight : 12819.9 lb / 5814.0 kg
Max speed: Mach 1.6 approx.
Ceiling: 15300 m / 50200 ft