Variable-Sweep Wings

Presumably, after decades of designing and building airplanes, a person would suspect that all airplanes with a similar purpose would look the same. All fighters would look alike, all passenger jets would look alike, and so on. But this has not happened. Even more than five decades after the dawn of the jet age, jet fighters designed around the same time can look radically different. The level of technological capability can explain most of these differences; cultural factors are also a factor, although generally of less importance. Aeronautical engineers, like any other group of people sharing similar interests, can become enthusiastic about certain ideas at the same time and overlook their drawbacks. That is what happened with variable-sweep wings.

The swept-back wing used for the B-47 and common to many aircraft that followed demonstrated that airplanes could use wing designs other than the straight wing to achieve certain performance goals. Straight wings were clearly advantageous for short takeoffs and landings, low speed, and fuel-efficient flight, but swept wings were ideal for high-speed, particularly supersonic, flight.

All aircraft represent numerous compromises made by their designers. Some aircraft need to fly at very high speeds whereas others need to fly very slowly. Some have to be highly fuel-efficient whereas others have to accomplish their missions without regard to cost. Designers, therefore, try to optimize each aircraft so that it does its required mission as best it can. But sometimes aircraft are required to do things that demand design features that oppose each other. The best example is an aircraft that can fly at high supersonic speeds but still needs to be able to land at relatively low speeds, such as on the deck of an aircraft carrier.

Experiments with variable-sweep wings began in France about 1911. Dr. Adolf Buseman, a German designer, presented a theoretical concept for a practical moveable wing at a convention in Rome just before World War II. His theory, supported with research by Dr. Albert Betz of the Göttingen Aerodynamics Research Institute, led Messerschmitt to begin developing a variable-sweep wing design, the P-1101, in 1942. The war ended before the aircraft could be produced. It is also doubtful that existing engines provided high enough speed for the design to make an appreciable difference in performance.

Beginning in the late 1940s, as technological capabilities improved, designers in the United States began to examine the possibility of moving the wing of an aircraft while it was in flight, so that it extended straight out from the fuselage for takeoff and landing and swept back for high-speed operations. Such an airplane might be able to accomplish its demanding missions without paying a performance penalty.

In the early 1950s, Bell Aircraft built the experimental X-5 aircraft for the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA). It had a wing that could be moved backward and forward in flight-a variable-sweep wing. Grumman Aircraft built the F-10-F for the U.S. Navy, also with variable-sweep wings. Both designs proved that the concept of movable wings worked.

A few years later, in 1959, engineers at NASA's Langley Research Center discovered the two-pivot variable-sweep concept-as opposed to the single-pivot used in earlier experiments. This development led to the success of the variable-wing idea.

In the late 1950s, the U.S. Air Force submitted specifications for a plane with variable-sweep wings that could fly at supersonic speeds and could also cruise for long distances at high altitudes to reach its distant targets. Secretary of Defense Robert McNamara wanted to combine this requirement with Army needs for close air support and a Navy requirement for air defense of the naval fleet. This soon led in December 1960 to the Tactical Fighter Experimental (TFX) program. This was to consist of aircraft suitable for use by the Air Force, which would operate it in the close air support role for Army ground forces and also use it in its own roles, and by the Navy in aircraft carrier roles.

The Department of Defense selected General Dynamics to develop two versions of the TFX, the F-111A for the Air Force and the F-111B for the Navy. Roll out took place on October 15, 1964, and Secretary McNamara stated that the Air Force and Navy now had an aircraft with the range of a transport, capacity and endurance of a bomber, and agility of a fighter. However, the Navy canceled its portion of the program, in August 1968, finding that the plane was too heavy for use aboard aircraft carriers. Commonality between the aircraft used by the two services had also dropped significantly by that time with the adoption of different engines. The Navy subsequently selected Grumman Aircraft to develop the F-14 Tomcat interceptor, which also had a variable-sweep, or "swing-wing."

The F-111 Aardvark, as it become known, was a medium-size tactical fighter-bomber capable of flying at high speeds at very low altitudes, hugging the terrain. It achieved this by using a variable-sweep wing. For takeoff or long flights to and from its targets, it operated as a straight-wing aircraft. For high-speed dashes at low altitudes toward its target, the wings swept back and acted much like a delta wing. Several versions of the F-111 were developed-a supersonic bomber for dropping conventional and nuclear bombs, a strike aircraft, and a version for electronic warfare.

The F-111 had numerous problems during its early service, including problems with cracks in the large gearbox used to move the wings. The plane, though, eventually saw service in Vietnam, and the F-111F saw considerable action during the Gulf War. Perhaps its greatest success occurred in a combined U.S. Air Force and U.S. Navy attack on Libya and its terrorist government in mid-April 1986, at El Dorado Canyon.

The F-14 adopted by the Navy incorporated a swing-wing that could be manually controlled by the pilot or shifted automatically according to the plane's speed. It moved forward to allow the plane to land on tiny aircraft carrier decks at relatively low speeds and backward as the plane dashed out to intercept Soviet bombers. More than 700 F-14s were produced, in several variants, and more than 70 of them were exported to Iran in the 1970s. It first entered service in the mid-1970s, and still serves today in 2001, although it is being retired. Despite its long service, the F-14 has been the most expensive interceptor aircraft to operate in the U.S. military.

During the 1970s, an Israeli Air Force (IAF) pilot evaluated the Navy F-14 and the Air Force F-15 Eagle for service in the IAF. He walked around both airplanes and counted their control surfaces such as ailerons, flaps, slats, and speed brakes. The F-14 had more control surfaces and the pilot determined that this would make it more difficult and expensive to maintain; for this and many other reasons, the IAF subsequently purchased the F-15 Eagle. In many ways this was an omen, for the variable-sweep wing, the largest moving part ever developed for an aircraft, proved to be more trouble than it was worth for many aircraft.

But for a period, variable-sweep wings were in vogue and more than a half dozen major military aircraft were designed with variable sweep wings during the 1960s and 1970s, with the number of swing-wing aircraft numbering in the thousands. The Soviets developed the Su-24 and MiG-27 attack planes and the MiG-23 fighter, all with swing-wings. The European consortium Panavia developed the Tornado, produced in both ground attack and interceptor versions. It too had a variable-sweep wing. Aircraft designers also applied variable-sweep wings to large bomber-size aircraft. North American Rockwell began the B-1A bomber in the early 1970s as the U.S. Air Force's new strategic bomber (it was canceled in the late 1970s and revived a few years later). The Soviet design firm Tupolev also developed the Tu-22M Backfire naval attack bomber and the Tu-160 Blackjack strategic bomber.

By the 1980s, however, no one was designing variable-sweep aircraft and no new work on this technology has been incorporated into any new production military aircraft in at least the last 15 years, although work is still being carried out on wings that move in other ways. The technology of variable-sweep wings lasted little more than 20 years before being phased out, although hundreds of the aircraft continued to fly for years more.

There were several reasons for the move away from this technology, but the primary reason was that the large metal gearbox needed to move the wings was complicated and heavy. This increased maintenance requirements and decreased fuel performance. An aircraft capable of moving its wing forward for fuel-efficient flight could never be as efficient as an airplane equipped with a straight wing. The same was true for aircraft with swept-back wings; they would always be more efficient than aircraft with swing-wings. The B-1B Lancer, for example, has never been able to achieve its original range requirements and has to refuel in the air more often than planned. It also rarely flew at the high speeds that sweeping back the wings allowed it to do. Ultimately, aircraft designers decided that the flexibility of the variable-sweep wing was not worth the compromises it demanded.

--Dwayne A. Day

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