Forward-Swept Wings
Forward-swept wings are not a new concept, but one developed by Germans during World War II. At that point, a German engineer named Hans Wocke was the head of the bureau to explore the benefits of forward-swept wings. He was convinced that this type of airfoil configuration had distinct performance and construction advantages. The first prototype this group had designed was the Junker-287. Figure 3 shows a model of the aircraft. The plane was designed as a bomber, but the forward-swept wings concept was ahead of its time. The technology required for the proper design and building of an aircraft of this sort would not come around until years later. It was here in the early conceptualization and building of aircraft with these wings that the disadvantages were discovered.
Figure 3: Junker 287
Shortly afterwards, WWII had ended and the Cold War was at hand. Despite the Cold War, forward-swept aircraft were very low key. A few American models were made, but it was not until around the 1970s that the United States became actively involved. In 1977, the Defense Advanced Research Projects Agency (DARPA) sponsored a competition to build an experimental forward-swept-wing airplane. In 1981, the Grumman model x-29, seen in Figure 4 below, was chosen. There were two of these models made costing approximately 87 million dollars. The first aircraft was used to validate the advantages of using forward-swept wings. Most importantly the first X-29 proved an aircraft with forward-swept wings could be flown safely and reliably. The second was used to discover the planes advantages at high angles of attack. In the end, the aircraft proved to maneuver at high angles of attack better than what was predicted. The X-29 provided a path for new technologies as well as reassurance that the old technologies still worked.
Figure 4: Grumman X-29
The latest forward-swept wing airplane to be built was a Russian model, the Sukhoi 37 “Berkut” shown in Figure 5. It is more maneuverable than the American superiority fighter, aka F-22.
Figure 5: Sukhoi S-37 Berkut
Hans Wocke knew that forward-swept wings had certain advantages. The first of the many advantages include controllability and maneuverability. Maneuverability was improved in a number of fashions. While the aircraft was traveling at low speeds, the wing would begin stalling at the wing root rather than the tip. Stalling at the tip causes the ailerons to be deemed useless, but because of the design of forward-swept wings ailerons were still useful in maneuvering the plane. Maneuverability is also improved at transonic speeds. The shock wave begins by forming on the root chord. Shock waves normally generate along the leading edge of delta wings leaving the ailerons useless. By having the shock form at the root chord, the ailerons are still effective in the transonic regime. Forward-swept wings also are able to maneuver at high angles of attack. The Grumman X-29b was found to be able to maneuver and not stall up to 67 degrees angle of attack. Good maneuverability was maintained at 45 degrees angle of attack. This makes an aircraft equipped with forward-swept highly maneuverable for times of “dog-fighting.” The maneuverability allows for a high turn rate. This high turn rate proves useful to in combat for quick targeting of the next enemy aircraft.
Other advantages include a decrease in drag. The decrease in drag can be partially attributed to the flow of air over the wing. Figure 6 demonstrates the airflow over the wing. For a normal delta wing, the airflow moves out and down along the leading edge as a leading edge vortex where it eventually flows off the wing tip. The air is in essence pushed out and away from the aircraft. Airflow over a forward-swept wing is much different. The airflow from the wing tip flows down and towards the body of the aircraft. This allows for a higher lift to drag ratio.
Figure 6: Comparison of Airflows on Forward-Swept Wing vs. Conventional Wing
Also due to the configuration of the wing, there is no need for wing tip leading edge slots. This is mostly due to the improved aileron control throughout the flight envelope. This eliminates a drag causing device and some weight from the aircraft.
Hans Wocke also persisted that a forward-swept wing configuration would allow for larger internal area. This is due to a larger fuselage. This internal area could be used for a weapons bay as designed on the Su-37, cabin area or landing gear.
From Wocke’s experiment with the JU-287 he found the plane had a tendency to Dutch roll in reverse and for it to inadvertently increase g’s during a turn when nothing had been demanded by the pilot. The Dutch roll was found to be caused by yawing moments imparted by accidental rudder inputs or atmospheric conditions. The Dutch roll was fixed by advances in technology. A fly-by-wire system was implemented for the purpose of controlling the plane. The increasing g’s caused a loss in wing rigidity. The rigidity problem is solved by creation of wings with composite materials. Grumman did this with their supercritical wing. This aircraft was deemed not worth the effort, money, and did not prove to be as exceptional as made out to be by the department of defense. Despite the naysay about the aircraft, the Russian model Su-37 Berkut can out maneuver the United States air superiority fighter (F-22).