In the 1980s, experiments were begun with a radical new aircraft design–the forward-swept wing. This odd-looking configuration promised a number of aerodynamic benefits. However, these benefits came with some severe limits as well.
One of the primary advantages of forward-swept wings is greatly improved maneuverability, which is always a desired characteristic in a military fighter jet. With conventional swept-wing jets, the air flows along the wing from the fuselage towards the wingtips, and this makes the wingtips stall before the wing root. This can disrupt the airflow over the control surfaces and cause a loss of control, especially during a steep climb at a high angle of attack. In a forward-swept wing, however, the airflow runs from the wingtip inwards towards the fuselage, allowing the wingtip to remain stable and shifting the instability towards the wing root. This improves the fighter’s agility and helps keep it under control during extreme maneuvers. The shift in airflow also distributes the lifting force more evenly along the length of the wing, which improves performance.
However, the forward-swept design also places a much larger strain on the wings, far more than the conventional materials of aluminum or titanium can handle, and this makes it very difficult to build a forward-swept aircraft with sufficient structural integrity. The Nazis, with their Junkers Ju-287 jet bomber, had carried out experiments with this configuration during the last years of the Second World War, but were unable to solve this issue. They also found that a forward-swept design was inherently unstable and difficult to fly.
It wasn’t until the 1980s, with the advent of new efficient manufacturing methods for advanced computer-designed carbon-fiber composite panels, that it finally became possible to conduct full-scale experiments with forward-swept wings. At the same time, new computer-driven fly-by-wire systems became available, in which the pilot’s control inputs went to a computer which calculated the best way to move the control surfaces for the desired action. This allowed for better control stability, as the computer could make constant flight corrections far more effectively than any pilot could.
Both the United States and the Soviet Union investigated the concept, each hoping to gain an advantage over their Cold War rival. In the US, the Grumman company designed the X-29, and in the USSR, the Sukhoi design bureau produced the Su-47 Berkut. Both jets were similar in design, as they were both intended to meet the same aerodynamic requirements. The forward-swept wings were placed near the back of the fuselage and a pair of smaller canards were located at the front. The American design had a single tail fin flanked by a pair of stabilizing strakes, while the Sukhoi had twin tail fins and two strakes (to reduce costs, the Su-47 re-used the tail fins and landing gear from the Su-27 fighter).
The US produced two X-29 test aircraft, one for the Air Force and one for NASA. They conducted a series of test flights from 1984 to 1991. The Soviets began work on their Berkut project in 1983, but with the collapse of the USSR in 1989 and subsequent lack of funding for the Russian Air Force, only one Su-47 was built, and it did not fly until 1997.
During testing, both programs ran into a difficulty known as “aeroelastic divergence”, which revealed a crippling problem with the forward-swept design. In all aircraft wings, the drag caused by the forward motion of the plane produces stress, and the wings must be built strongly enough to withstand this strain. In a forward-swept wing, however, the wingtip bears a much higher proportion of this stress, which causes it to tilt slightly upwards. This tilt in turn produces even more stress, which twists it upwards even more, producing more stress. This feedback loop rapidly spirals up to a level where the wings can no longer take the strain, and they break off.
It was possible to build wings from advanced materials like carbon-fiber composites that were strong enough to survive the stresses, but this manufacturing process was complex and very expensive. In the end, it was concluded that the forward-swept design did not offer sufficient aerodynamic advantages that could not be produced by cheaper and less complicated methods like thrust-vectored engines, canards, and advanced fly-by-wire systems. Both the X-29 and Berkut programs were ended without producing any successors, and no nation has deployed any forward-swept combat aircraft.
Today, one of the X-29 test planes is on exhibit at the US Air Force Museum in Dayton OH, and the other is on display at the Research Center in Edwards Air Force Base in California. The Smithsonian Air and Space Museum had a full-scale model of the X-29 on display for many years, but it was moved to the Cradle of Aviation Museum in New York in 2011.