This illustration shows the reverse airflow on forward swept wings vs. the airflow on the swept-back wings. On the forward swept wing, the air tended to flow inward toward the root of the wing rather than outward toward the wing tip as on the swept-back wing.
Three views of the X-29 forward-swept wing aircraft.
Two X-29 aircraft, featuring forward-swept wings, were flown at NASA Dryden Flight Research Center in a program conducted from 1984 to 1992. The plane looked almost like it was flying backward.
The X-29 Ship No. 2 technology demonstrator was flown by NASA Dryden Flight Research Center in a joint NASA-Air Force program to investigate the unique design's high angle of attack characteristics and its military utility.
The lower illustration shows how the canards on forward swept-wing X-29 will share the lifting load and reduce drag as compared to a conventional aircraft in the upper diagram.
The Grumman X-29 aircraft were flown from December 1984 to 1988.
Sometimes technological developments to overcome aeronautical challenges succeed but provide such a small improvement in performance or operations that they are never widely adopted. Such is the case for forward-swept wings, a design concept that has never really caught on for any kind of aircraft.
In 1936, a German aerodynamicist first postulated developing an airplane with its wings swept forward, but nobody built any actual models at the time. During World War II, the Germans finally conducted tests of such an aircraft. The Messerschmitt company built the tailless Me 163B to explore the design. The German firm Junkers produced the jet-powered Ju 287 light bomber with forward-swept wings. They did this not because the design had any inherent aerodynamic advantages but rather to enable the wings to be mounted behind the bomb bay. In 1944, the obscure airplane manufacturer American Cornelius built one of the oddest aircraft ever to fly, the XFG-1 fuel transport glider, which was an unpowered fuel tanker with forward-swept wings. Only two of the ugly-looking craft were built.
After the war, the National Advisory Committee for Aeronautics (NACA) in the United States conducted wind tunnel tests of forward swept wings. NACA engineers even mounted a model of the Bell X-1 with forward-swept wings in a wind tunnel. But they found little inherent aerodynamic advantages to such a design. Even the Russians conducted full-scale model flights of forward-swept-wing gliders but abandoned the concept.
In 1964, the German airplane manufacturer Hamburger Flugzeugbau built the HFB-320 business jet with forward-swept wings. This design allowed the wings to be mounted behind the passenger cabin along the sides of the fuselage. Only 50 of the aircraft were manufactured and it remains the only aircraft with forward-swept wings to enter actual production. For years the primary purpose for developing forward-swept wings was structural—to allow the wings to be mounted farther back on the fuselage so that their connecting structure did not interfere with anything inside the fuselage (like bombs or people). Wind tunnel tests made it clear that there were many problems with forward-swept wings and few aerodynamic advantages. One major problem was that the wingtips tended to bend upwards and cause the plane to stallinevitable for metal wings. But in the mid-1970s, a U.S. Air Force officer noted that new composite materials then becoming available for aviation could be incorporated into the wings of a modern jet and eliminate the tendency of the wingtips to bend upward and cause the plane to stall. At the same time, several U.S. aviation companies were exploring ways to make planes that were highly maneuverable at transonic speeds (i.e., near the speed of sound).
Aircraft with forward-swept wings are highly maneuverable at transonic speeds because air flows over a forward-swept wing and toward the fuselage, rather than away from it. By the late 1970s, the Defense Advanced Research Projects Agency (DARPA) sponsored a competition to build an experimental forward-swept-wing airplane. Rockwell International proposed the Sabrebat fighter and General Dynamics proposed modifying an F-16 Falcon jet fighter. But in 1981, DARPA finally selected Grumman, which had proposed using parts from several different aircraft to develop an experimental lightweight airplane soon designated the X-29. The X-29 used the fuselage from the Northrop F-5A, the main undercarriage and other equipment from the F-16, and an engine from the F/A-18. Its wings were made of advanced composites and it was equipped with small wings called canards mounted on the forward fuselage rather than on the tail where horizontal stabilizers are usually located. These helped increase the plane's maneuverability. The reverse airflow inward from the wing tip toward the root of the wing did not allow the wing tips and their ailerons to stall at high angles of attack.
The Grumman X-29 first flew in 1984. It had a strange appearance, with the wings mounted well back on the fuselage, and almost looked like it was flying backward. The aircraft could only be flown with the help of an advanced computer control system. In numerous tests over the next several years, the X-29 demonstrated that the forward-swept wing design produced a 15 percent better ratio of lift to drag in the transonic speed region. But Department of Defense officials were not significantly impressed by this performance improvement to approve any further experimental aircraft and the two X-29 aircraft were soon retired to museums.
Forward-swept wings remained dead as a concept until the surprising appearance in 1997 of the Russian Sukhoi S-37 Berkut ("Golden Eagle") with its forward-swept wings and canards. The S-37 uses the front fuselage of the popular Su-37K fighter, but is otherwise an entirely new aircraft. It is significantly larger and heavier than the X-29 and when it first appeared, Western experts speculated that it was a prototype heavyweight naval fighter. But after several years of laboriously slow flight tests, Sukhoi did not appear ready to begin producing large numbers of forward-swept wing naval fighters and the S-37 remains a one-of-a-kind aircraft. Whether this is because the Russians have been unimpressed with their forward-swept wing airplane's performance, or simply because of lack of money is unknown. But it is clear that the forward-swept wing remains a novel solution to a problem that nobody feels the need to solve.
--Dwayne A. Day
Sources and further reading:
Braybrook, Ray. "Forward Sweep Is Back." Air International, February 1998, 119-123.
Wallace, Lane. Flights of Discovery: 50 Years at the NASA Dryden Flight Research Center. National Aeronautics and Space Administration, NASA SP-4309, 1996.
Miller, Jay. The X-Planes: X-1 to X-45. Hinckley, England: Midland Publishing, 2001.
X-29 Fact Sheet. National Aeronautics and Space Administration. Dryden Flight Research Center, April 1998. http://trc.dfrc.nasa.gov/PAO/PAIS/HTML/FS-008-DFRC.html