Swing Wing: The Path to Variable Geometry

The notion of variable wing-sweeping dates to the earliest days of avi­ation and, in many respects, represents an expression of the "bird imi­tative” philosophy of flight that gave the ornithopter and other flexible wing concepts to aviation. Varying the sweep of a wing was first con­ceptualized as a means of adjusting longitudinal trim. Subsequently,

variable-geometry advocates postulated possible use of asymmetric sweeping as a means of roll control. Lippisch, pioneer of tailless and delta design, likewise filed a patent in 1942 for a scheme of wing sweeping, but it was another German, Waldemar Voigt (the chief of advanced design for the Messerschmitt firm) who triggered the path to modern variable wing-sweeping. Ironically, at the time he did so, he had no plan to make use of such a scheme himself. Rather, he designed a graceful midwing turbojet swept wing fighter, the P 1101. The German air ministry rejected its devel­opment based upon assessments of its likely utility. Voigt decided to con­tinue its development, planning to use the airplane as an in-house swept wing research aircraft, fitted with wings of varying sweep and ballasted to accommodate changes in center of lift.[110]

By war’s end, when the Oberammergau plant was overrun by American forces, the P 1101 was over 80-percent complete. A techni­cal team led by Robert J. Woods, a member of the NACA Aerodynamics Committee, moved in to assess the plant and its projects. Woods imme­diately recognized the value of the P 1101 program, but with a twist: he proposed to Voigt that the plane be finished with a wing that could be variably swept in flight, rather than with multiple wings that could be installed and removed on the ground. Woods’s advocacy, and the results of NACA variable-sweep tests by Charles Donlan of a modified XS-1 model in the Langley 7-foot by 10-foot wind tunnel, convinced the NACA to support development of such an aircraft. In May 1949, the Air Force Air Materiel Command issued a contract covering development of two Bell variable sweep airplanes, to be designated X-5. They were effectively American-built versions of the P 1101, but with American, not German, propulsion, larger cockpit canopies for greater pilot visibility, and, of course, variable sweep wings that could range from 20 to 60 degrees.[111]

The first X-5 flew in June 1951 and within 5 weeks had demonstrated variable in-flight wing sweep to its maximum 60-degree aft position. Slightly over a year later, Grumman flew a prototype variable wing-sweep naval fighter, the XF10F-1 Jaguar. Neither aircraft represented a mature application of variable sweep design. The mechanism in each was heavy and complex and shifted the wing roots back and forth down the cen­terline of the aircraft to accommodate center of lift changes as the wing was swept and unswept. Each of the two had poor flying qualities unre­lated to the variable-sweep concept, reflecting badly on their design. The XF10F-1 was merely unpleasant (its test pilot, the colorful Corwin "Corky” Meyer, tellingly recollected later "I had never attended a test pilots’ school, but, for me, the F10F provided the complete curriculum”), but the X-5 was lethal.[112] It had a vicious pitch-up at higher-sweep angles, and its aerodynamic design ensured that it would have very great difficulty when it departed into a spin. The combination of the two led to the death of Air Force test pilot Raymond Popson in the crash of the second X-5

in 1953. More fortunate, NACA pilots completed 133 research flights in the first X-5 before retiring it in 1955.

The X-5 experience demonstrated that variable geometry worked, and the potential of combining good low-speed performance with high-speed supersonic dash intrigued military authorities looking at future inter­ceptor and long-range strike aircraft concepts. Coincidentally, in the late 1950s, Langley developed increasingly close ties with the British aeronau­tical community, largely a result of the personal influence of John Stack of Langley Research Center, who, in characteristic fashion, used his force­ful personality to secure a strong transatlantic partnership. This partner­ship, best known for its influence upon Anglo-American V/STOL research leading to the Harrier strike fighter, influenced as well the course of vari­able-geometry research. Barnes Wallis of Vickers had conceptualized a sharply swept variable-geometry tailless design, the Swallow, but was not satisfied with the degree of support he was receiving for the idea within British aeronautical and governmental circles. Accordingly, he turned to the United States. Over November 13-18, 1958, Stack sponsored an Anglo – American meeting at Langley to craft a joint research program, in which Wallis and his senior staff briefed the Swallow design.[113] As revealed by subsequent Langley tunnel tests over the next 6 months, Wallis’s Swallow had many stability and control deficiencies but one significant attribute: its outboard wing-pivot design. Unlike the X-5 and Jaguar and other early symmetrical-sweep v-g concepts, the wing did not adjust for chang­ing center of lift position by translating fore and aft along the fuselage centerline using a track-type approach and a single pivot point. Rather, slightly outboard of the fuselage centerline, each wing panel had its own independent pivot point. This permitted elimination of the complex track and allowed use of a sharply swept forebody to address at least some of the changes in center-of-lift location as the wings moved aft and forward. The remainder could be accommodated by control surface deflection and shifting fuel. Studies in Langley’s 7-foot by 10-foot tunnel led to refinement of the outboard pivot concept and, eventually, a patent to William J. Alford and E. C. Polhamus for its concept, awarded in September 1962. Wallis’s inspiration, joined with insightful research by Alford and Polhamus and

followed by adaptation of a conventional "tailed” configuration (a crit­ical necessity in the pre-fly-by-wire computer-controlled era), made variable wing sweep a practical reality.[114] (Understandably, after return­ing to Britain, Wallis had mixed feelings about the NASA involvement. On one hand, he had sought it after what he perceived as a "go slow” approach to his idea in Britain. On the other, following enunciation of outboard wing sweep, he believed—as his biographer subsequently wrote—"The Americans stole his ideas,”)[115]

Thus, by the early 1960s, multiple developments—swept wings, high-performance afterburning turbofans, area ruling, the outboard wing pivot, low horizontal tail, advanced stability augmentation sys­tems, to select just a few—made possible the design of variable – geometry combat aircraft. The first of these was the General Dynamics Tactical Fighter Experimental (TFX), which became the F-111. It was a troubled program, though, like most of the Century series that had pre­ceded it (the F-102 in particular), this had essentially nothing to do with the adaptation of a variably swept wing. Instead, a poorly written speci­fication emphasizing joint service over practical, attainable military util­ity resulted in development of a compromised design. The result was a decade of lost fighter time for the U. S. Navy, which never did receive the aircraft it sought, and a constrained Air Force program that resulted in the eventual development of a satisfactory strike aircraft—the F-111F— but years late and at tremendous cost. Throughout the evolution of the F-111, NASA research proved of crucial importance to saving the pro­gram. NASA Langley, Ames, and Lewis researchers invested over 30,000

hours of wind tunnel test time in the F-111 (over 22,000 at Langley alone), addressing various shortcomings in its design, including excessive drag, lack of transonic and supersonic maneuverability, deficient directional stability, and inlet distortion that plagued its engine performance. As a result, the Air Force F-111 became a reliable weapon system, evidenced by its performance in Desert Storm, where it flew long-range strike mis­sions, performed electronic jamming, and proved the war’s single most successful "tank plinker,” on occasion destroying upward of 150 tanks per night and 1,500 over the length of the 43-day conflict.[116]

From the experience gained with the F-111 program sprang the Grumman F-14 Tomcat naval fighter and the Rockwell B-1 bomber, both of which experienced fewer development problems, benefitting greatly from NASA tunnel and other analytical research.[117] Emulating American variable-geometry development, Britain, France, and the Soviet Union undertook their own development efforts, spawning the experi­mental Dassault Mirage G (test-flown, though never placed in service), the multipartner NATO Tornado interceptor and strike fighter program, and a range of Soviet fighter and bomber aircraft, including the MiG – 23/27 Flogger, the Sukhoi Su-17/22 Fitter, the Su-24 Fencer, the Tupolev Tu-22M Backfire, and the Tu-160 Blackjack.[118]

Variable geometry has had a mixed history since; in the heyday of the space program, many proposals existed for tailored lifting body shapes deploying "switchblade” wings, and the variable-sweep wing was a prom­inent feature of the Boeing SST concept before its subsequent rejection. The tailored aerodynamics and power available with modern aircraft have rendered variable-geometry approaches less attractive than they once were, particularly because, no matter how well thought out, they invari-

ably involve greater cost, weight, and structural complexity. In 1945-1946, John Campbell and Hubert Drake undertook tests in the Langley Free Flight Tunnel of a simple model with a single pivot, so that its wing could be skewed over a range of sweep angles. This concept, which German aerodynamicists had earlier proposed in the Second World War, demon­strated "that an airplane wing can be skewed as a unit to angles as great as 40° without encountering serious stability and control difficulties.”[119] This concept, the simplest of all variable-geometry schemes, returned to the fore in the late 1970s, thanks to the work of Robert T. Jones, who adopted and expanded upon it to generate the so-called "oblique wing” design con­cept. Jones conceptualized the oblique wing as a means of producing a transonic transport that would have minimal drag and a minimal sonic boom; he even foresaw possible twin fuselage transports with a skewed wing shifting their relative position back and forth. Tests with a subscale turbojet demonstrator, the AD-1 (for Ames-Dryden), at the Dryden Flight Research Center confirmed what Campbell and Drake had discovered
nearly four decades previously, namely that at moderate sweep angles the oblique wing possessed few vices. But at higher sweep angles near 60 degrees, its deficits became more pronounced, calling into question whether its promise could ever actually be achieved.[120] On the whole, the variable-geometry wing has not enjoyed the kind of widespread suc­cess that its adherents hoped. While it may be expected that, from time to time, variable sweep aircraft will be designed and flown for partic­ular purposes, overall the fixed conventional planform, outfitted with all manner of flaps and slats and blowing, sucking, and perhaps even warping technology, continues to prevail.