Grumman X-29 – The Backward Wing Plane
The Grumman X-29 was a bizarre-looking experimental plane that was one of the first serious attempts to assess the concept of a forward-swept wing. Following in the footsteps of many previous failed attempts, the appearance of lightweight and incredibly strong composite materials in the 1970s finally meant there was a way to stop the wings of such a craft from twisting out of shape.
Despite initial doubts, the X-29’s strange layout would go on to stun researchers with its efficacy, becoming one of aviation history’s most unexpected success stories in the process.
Origins and Development
Before the advent of the Grumman X-29, many researchers had already dabbled with the concept of a forward-swept wing. Foremost was NACA in 1931, which had evaluated the design via wind tunnel tests at Langley Memorial Aeronautical Laboratory, while during the Second World War, the Germans had developed their jet-powered aircraft, the Junkers Ju 287, with forward-swept wings.
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Other examples of early forward-swept winged planes also included the Cornelius XFG-1 Glider, a derivative of the Bell X-1, an experimental variant of the Douglas D-558 Skystreak, a rough design drawn by V.P. Tsibin, and even a version of the American P-51 Mustang that went nowhere.
Most of these experiments were unsuccessful because the technology and material required to prevent the wing from deforming under the pressures of aeroelasticity did not yet exist.
It would take until the early 1960s for the forward swept wing to finally become viable with the assembly of the Hansa Jet HFB-320 by German aeronautical titans Hamburger Flugzuebau, which by 1964 was the only certified civilian jet to use them.
In the 1970s the emergence of composite materials, which were extremely light but stronger than conventional materials, further encouraged engineers to pursue this unusual line of inquiry. In 1977 the Defense Advanced Research Project (DARPA) together with the US Air Force Flight Dynamics Laboratory at Wright-Patterson Air Force Base, Ohio, authorized a program to study this novel wing concept, and to confirm with scientific precision the findings of other studies which had claimed it led to better control and lift qualities during extreme manoeuvres, that it reduced aerodynamic drag, and that it flew more efficiently at cruise speeds.
In December 1981 Grumman was selected as project lead and granted 87 million dollars to produce 2 prototypes. On August 27th 1984 the X-29 was officially unveiled in a rollout ceremony at the Grumman facility in Calverton, New York. Grumman Corporation president George M. Skurla took to the stage that day to proclaim:
“It is a significant milestone in Grumman’s history and further evidence of our continuing commitment to expand the frontiers of manned flight. The X-29 is a means to investigate, in flight, new technologies that could point the way toward future generations of aircraft which are more agile, burn less fuel, and cost less to maintain than any tactical aircraft flying today.”
The X-29 was 16.44 meters long, 4.26 meters high, and had an empty weight of 6,260 kilograms and a maximum weight of 8,074 kilograms. It was powered by a single General Electric F404-GE-400 engine, which gave it a top speed of Mach 1.87, a range of 560 kilometres, and a maximum altitude of 50,000 feet.
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While the rest of the aeroplane was made out of either aluminium or titanium, the forward swept wings, which had a wing span of 8.26 meters, a wing area of 188.80 meters squared, and were mounted very far back on the fuselage, were made of composite materials including graphite and epoxy, which were welded into patterns designed to resist the forces of aeroelasticity that had scuppered all previous attempts.
The X-29’s aeroelastic-tailored wing added even more stabilization by preventing structural divergence from happening within the flight envelope.
Moreover, some measure of artificial stability was added by an electronic triple-redundant digital fly-by-wire flight control system, which made up to 40 commands per second to adjust the control surfaces where necessary, since the wing design was naturally highly unstable.
Forward canards, which were located in front of the wings rather than at the tail, acted as flight control surfaces primarily providing pitch control, which was also achieved by ‘strake flaps’ located on either side of the rudder, an innovation that would become increasingly common on later fighters. In contrast, roll control was determined by ‘flaperons’, a combination of flaps and ailerons, which changed the wing camber.
Another notable feature was that the wing trailing edge actuators that controlled camber were intentionally mounted externally in streamlined fairings due to the thinness of the supercritical airfoil, a component first developed in the 1970s with F-8s.
Flatter on the upper wing surface than its more conventional counterpart, the supercritical airfoil was installed to lessen the power of incoming shockwaves, which would theoretically result in a considerable decrease in drag.
Elsewhere in a bid to save money, the X-29’s undercarriage was taken from the F-16 and possessed anti-skid tires and carbon brakes, while its fuselage and nose-wheel came from a couple of F-5As, including one that had previously served USAF and in the Norwegian Air Force.
Following routine taxi tests, in September 1984 the No.1 X-29 was stripped of its F404 engine, wrapped up in a protective blanket, and loaded onto a transport ship at Bayonne, New York. Sailing through the Panama Canal, it arrived at San Pedro, California where it was next taken to Edwards Air Force Base.
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On December 14th 1984 this first version of the X-29 took to the skies for phase 1 flight testing. Over the course of 242 test runs, evaluators learnt that at moderate angles of attack, because the air moving over the forward wing moved inwards rather than outwards, the wing tips did not stall.
Accident-free flights also revealed the merits of the stabilization measures put in place to counteract the highly unstable forward-swept wing, with pilots consistently reporting good handling characteristics. In fact, one such pilot, Chuck Sewell, enjoyed flying it so much that during the opening round of test flights, he asked ground control permission to enter it into a roll.
Nearly 5 years later on May 23rd 1989, the second version of the X-29 would undertake the first of the 120 flights that made up phase 2, the principal aim being to examine its high angle of attack characteristics as well as the potential military applications a forward-swept wing configuration could offer.
Whereas the No.1 X-29 had only been operated at a 21-degree angle of attack, the No.2 was flown at a vertiginous 67 degrees. To the surprise of all, the manoeuvrability and control it demonstrated exceeded all expectations and was even better than what the computational models had predicted.
At 45 degrees pilots had remarked it still had excellent control characteristics while at 67 degrees it still retains limited controllability, a phenomenon that was attributed to the forward swept wing.
What’s more, effortless control was achieved without the need for the leading edge flaps on the wings to provide additional lift nor were the moveable vanes situated on the engine’s exhaust nozzle to change the thrust direction necessary for such conditions to arise.
Although the X-29 had not reduced aerodynamic drag as previous studies had attested, it did show the benefits of several novel devices such as the aeroelastic tail and the close-coupled canard for longitudinal control, while proving possible that with the right design, high angle of attack control could be achieved. It was this last breakthrough that would persuade policymakers to employ it in one last experiment.
Vortex Flow Control
In 1992 the No.2 Grumman X-29 was drafted in by the Air Force to take part in a program that would explore how vortex flow control, overriding normal flight control systems, could be implemented at a high angle of attack to maintain control.
Consequently, the X-29 underwent a modification process in which it was installed with two high-pressure nitrogen tanks linked to two small nozzle jets situated in the forward upper portion of the nose, the idea being to pump air into the vortices that flew off the nose during high angles of attack.
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Evaluators were eager to test the practicalities of a system extensively tested at the Air Force’s Wright Laboratory previously, which had seemingly illustrated that an injection of air into the vortices changed the direction of the vortex flow and created the forces necessary to change the direction of the nose.
Between May and August 1992 over 60 flights, this tweaked Grumman X-29 helped assessors discover the advantages of vortex flow control (VFC), which at higher angles of attack, when the rudder loses its effectiveness, unexpectedly allowed the craft to move left and right with relative ease.
But VFC also had its drawbacks, and was unable to impose control when side winds were present nor did it reduce consistent oscillation.
Presently, this second X-29 is on display at Dryden Flight Research Center while the first is exhibited at the Air Force Museum.
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- Crew: 1
- Capacity: 4,000 lb (1,814 kg) payload
- Length: 53 ft 11.25 in (16.4402 m) including nose probe
- Wingspan: 27 ft 2.5 in (8.293 m)
- Height: 14 ft 3.5 in (4.356 m)
- Empty weight: 13,800 lb (6,260 kg)
- Max takeoff weight: 17,800 lb (8,074 kg)
- Powerplant: 1 × General Electric F404-GE-400 afterburning turbofan engine, 16,000 lbf (71 kN) with afterburner
- Maximum speed: 956 kn (1,100 mph, 1,771 km/h) at 33,000 ft (10,058 m)
- Range: 350 nmi (400 mi, 650 km)
- Service ceiling: 55,000 ft (17,000 m)