X-29: America’s SU-47

Continuing in our series on experimental testbed aircraft, today we are taking a look at a popular 1980s concept, a forward swept wing design known as the X-29.

You may remember from our Nazi jet bomber video, the Junkers Ju 287 was arguably the first jet aircraft to utilise forward swept wings. Nazi aeronautical researchers believed that forward swept wings were risky – due to the possibility of wing warping – but if this could be controlled through proper balancing, then it would offer better slow speed performance, and possibly better turning performance.

This was not the first time forward swept wing designs had been tested. In the 1930s, the Polish experimented with forward swept wings, as did Soviet designer Viktor Belyayev, who developed a series of forward swept wing gliders and powered aircraft.

After the war, research continued, mostly based on the Junkers design. Both the Soviets and Americans tried their hand at forward swept wing designs, but limited experience meant a number of issues had to be overcome. By the 1950s breakthroughs were occurring in regard to variable geometry wings, and more industry focus came to bear on the concept.

Hans Wocke – one of the researchers responsible for the JU 287 – developed a forward swept business jet in 1964 – the HFB 320 Hansa Jet. This would be used by the LuftwaffeLuftwaffe: Derived from the German words "Luft" (air) and "Waffe" (weapon), the term literally means "air weapon" and refers to the aerial warfare bra... More through till the 1990s. With the exception of this aircraft, most other forward swept wing designs after the war were limited to the glider world. Gliders suffered from less of the risks that fast jets would, thus forward swept wings provided some of the advantages – such as different low speed flight characteristics – without presenting the dangers one may find on a jet, like warping or vibrations from turbulence.

During the 1970s, DARPADARPA (Defense Advanced Research Projects Agency): A central research and development agency within the United States Department of Defense (DoD) task... More was researching ways to reduce what was known as aeroelastic deformation. They understood that forward swept wings offered unique handling characteristics at low speeds and high angles of attack, but the design carried several serious challenges. For example, aerodynamic divergence would be amplified during any pitch change, which could result in a positive feedback loop. With lift, the control surfaces naturally deflect up or down, but with the wingtips pointing forward, there is a higher chance that lift will be increased exponentially in a positive feedback loop as the control surfaces bend further and further up or down, causing divergence. The other major issue was flutter, or uncontrolled vibrations and oscillations, a situation that could amplify into catastrophic failure of the airframe.

X-29 PROGRAM BEGINS

In 1977 DARPADARPA (Defense Advanced Research Projects Agency): A central research and development agency within the United States Department of Defense (DoD) task... More was looking for a testbed aircraft with forward swept wings at an angle of 33 degrees. Grumman was given the task of building an aircraft – which they referred to as Grumman Model 712 – after a brief competition against a modified F-16 design. In December 1981 they were granted $78 million to produce two working aircraft. Designers would use the F-5A Freedom Fighter as the base airframe, integrating an F-16 landing gear system, and advanced avionics, including a new fly-by-wire system which would be coded to overcome anticipated problems the airframe may encounter. And, of course, the most important part of the development process was the wing design.

Given the expected forces the testbed would experience, the aircraft’s wings were built with carbon fibre reinforced polymer. It would make use of anisotropic coupling. This would be necessary to overcome the divergent forces which would be acting on the wings. The only other option was to produce torsionally-stiff wings which would prohibit movement, but these would be too heavy and severely impede the aircraft. The solution was a carbon fibre laminate, that allowed for the upward bending forces on the wing but prevent the wing from twisting, while maintaining a light-weight construction.

The aircraft would use three control surfaces: aft strakes, wings, and canards. These were designed to minimise any drag from trim correction, while also compensating in an efficient manner for the aircraft’s centre of gravity, which was aft of the aerodynamic centre. This made the aircraft inherently unstable, and this is where the avionics systems would come in. Six computers would be employed: three digital computers, each with an analogue backup. Each individual computer could operate the entire aircraft if necessary, but together their redundancy would allow for data analysis in real time, while highlighting any errors found in any one of the computers. These computers would need to make up to 40 corrections per second. At this stage of development, programmers realised that the design – which was expected to have increased agility – would need to be limited in manoeuvrability so that the flight-control system had a higher chance of keeping the aircraft in controlled flight.

Some of the research was achieved by using a 22% scale model of the aircraft at NASANASA (National Aeronautics and Space Administration): The independent agency of the United States federal government responsible for the civilian spac... More Langley. This radio-controlled model was used to research the capability of the airframe, and helped them develop early flight laws which would be instrumental during the program

X-29 INTRODUCED

The result was the Grumman X-29. Two would be rolled out for testing. The first flight would occur at Edwards Air Force base on December 14^th, 1984. It would be the third jet aircraft – after the JU287 and Hansa – to take flight with forward swept wings. The program would encompass NASANASA (National Aeronautics and Space Administration): The independent agency of the United States federal government responsible for the civilian spac... More, the US Air Force, DARPADARPA (Defense Advanced Research Projects Agency): A central research and development agency within the United States Department of Defense (DoD) task... More, and Grumman, all take part in the research process.

The two X-29s would be designed for different roles. The first would be tasked with general research without departing from controlled flight, the second would be equipped with a drag chute and tasked with research into higher angles of attack.

After its first flight in 1984, NASANASA (National Aeronautics and Space Administration): The independent agency of the United States federal government responsible for the civilian spac... More Ames-Dryden took the first aircraft for a four-month test program, entering ‘Phase 1’ of the project. They would explore the basics of forward swept wing flight, documenting outcomes from various tests. What they confirmed was that air flowed inwards – not outwards – along the wings, resulting in better low speed handling and stall characteristics. This stage also demonstrated the structural integrity of the wings, and that the flight profile could be safely constrained thanks to the onboard computer system.

Research was also conducted around the super-critical airfoils on the wings, which are designed to reduce drag and the onset of shockwaves on the upper part of the wing. During phase 1, aircraft number 1 performed well in tight turns, general flight, and showed better than average performance at transonic speeds. A total of 242 flights would be flown during this initial phase.

Phase 2 began with the first flight of aircraft number 2 on May 23^rd, 1989. A primary focus now, was on high angles of attack, an increasingly popular topic of research at the time. During tests, the airframe demonstrated impressive results; both aircraft could easily achieve controlled flight at angles of attacks up to 21 degrees. The second aircraft would be allowed to go beyond that, achieving an AOA of 45 degrees and retain excellent control response. It would be pushed all the way to a maximum of 67 degrees AOA, and even then, the pilot reported it still had some controllability.

These impressive AOA capabilities were put down to the combined use of forward swept wings, strakes, and canards. Unlike the similar F-16 VISTA testbed, which required thrust vectoring veins around the engine, the X29 required none of this, and likely would have achieved even higher performance if such a system was integrated. Phase 2 would end after a total of 120 flights.

The final action for the aircraft would occur in May 1992. The Air Force wanted to test a vortex control system. The second X29 would be used. Several modifications were added, including two small nitrogen jets near the forward upper region of the nose. These were attached with control valves to tanks. The idea was to fire these jets into the vortices flowing off the nose when the aircraft entered high AOA’s. This was called ‘vortex flow control’ or VFC. It was found that this system provided increased yaw forces, allowing the aircraft to shift left or right when the rudders were ineffective. However, it was also found that such a system didn’t have much effect when sideslip was present, nor did it prevent oscillation from rocking the aircraft. By August 1992, the program would end, with a total of 60 flights completed in the VFC phase.

OUTCOME

The program highlighted the fact that forward swept wing designs could be used effectively, albeit with the aid of onboard computers. The inherently unstable design could be controlled effectively, while still allowing for good performance. Aeroelastic tailoring proved that the problems of forward swept wing tips could be overcome.

After the program, both aircraft were retired, and both now sit as exhibit pieces.

No doubt the project ignited a renewed interest in forward swept wing designs. In 1997, SukhoiSukhoi – A prominent Russian aerospace and defense corporation specializing in the design and manufacture of military aircraft, particularly fighter a... More would show off a forward swept wing prototype at the Paris Air ShowThe Paris Air Show is the world’s oldest and one of the most influential aerospace exhibitions, held every two years at Le Bourget Airport near Paris,... More. This aircraft – the SU-47 – would never enter service but would receive more publicity than the X-29. In 2015, another working prototype took flight. This aircraft – the KB SAT SR-10 – is a proposed training aircraft for the Russian Air Force. It has been said that the design was rejected in favour of the YAK-130 – an advanced trainer with inflight simulator abilities replicating other aircraft. But regardless of its fate, the fact that this prototype was even proposed is proof of continued interest in the forward swept wing design.

The X-29 program was an interesting one, and certainly it was a unique looking airframe. Whether or not forward swept wings have a future in the world of aeronautics is anyone’s guess. For some, at least, it continues to be a viable design concept.