Breakthrough: Variable Sweep

Spurred on by postwar interests in the variable-wing-sweep concept as a means to optimize mission performance at both low and high speeds, the NACA at Langley initiated a broad research program to identify the potential benefits and problems associated with the concept. The disap­pointing experiences of the Bell X-5 research aircraft, which used a sin­gle wing pivot to achieve variable sweep in the early 1950s, had clearly identified the unacceptable weight penalties associated with the con­cept of translating the wing along the fuselage centerline to maintain satisfactory levels of longitudinal stability while the wing sweep angle was varied from forward to aft sweep. After the X-5 experience, military interest in variable sweep quickly diminished while aerodynamicists at

Langley continued to explore alternate concepts that might permit vari­ations in wing sweep without moving the wing pivot location and with­out serious degradation in longitudinal stability and control.

After years of intense research and wind tunnel testing, Langley researchers conceived a promising concept known as the outboard pivot.[486] The basic principle involved in the NASA solution was to pivot the mov­able wing panels at two outboard pivot locations on a fixed inner wing and share the lift between the fixed portion of the wing and the movable outer wing panel, thereby minimizing the longitudinal movement of the aerodynamic center of lift for various flight speeds. As the concept was matured in configuration studies and supporting tests, refined designs were continually submitted to intense evaluations in tunnels across the speed range from supersonic cruise conditions to subsonic takeoff and landing.[487]

The use of dynamically scaled free-flight models to evaluate the sta­bility and control characteristics of variable-sweep configurations was an ideal application of the testing technique. Since variable-sweep designs are capable of an infinite number of wing sweep angles between the for­ward and aft positions, the number of conventional wind tunnel force tests required to completely document stability and control variations with wing sweep for every sweep angle could quickly become unacceptable. In contrast, a free-flight model with continually variable wing sweep angles could be used to quickly examine qualitative characteristics as its geome­try changed, resulting in rapid identification of significant problems. Free – flight model investigations of a configuration based on a proposed Navy combat air patrol (CAP) mission in the Full-Scale Tunnel provided a con­vincing demonstration that the outboard pivot was ready for applications.

The oblique wing concept (sometimes referred to as the "switch­blade wing” or "skewed wing”) had originated in the German design studies of the Blohm & Voss P202 jet aircraft during World War II and was pursued at Langley by R. T. Jones. Oblique wing designs use a single­pivot, all-moving wing to achieve variable sweep in an asymmetrical fashion. The wing is positioned in the conventional unswept position for takeoff and landings, and it is rotated about its single pivot point for high-speed flight. As part of a general research effort that included

theoretical aerodynamic studies and conventional wind tunnel tests, a free-flight investigation of the dynamic stability and control of a sim­plified model was conducted in the Free-Flight Tunnel in 1946.[488] This research on the asymmetric swept wing actually predated NACA wind tunnel research on symmetrical variable sweep concepts with a research model of the Bell X-1.[489] The test objectives were to determine whether such a radical aircraft configuration would exhibit satisfactory stability characteristics and remain controllable in the swept wing asymmetric state at low-speed flight conditions. The results of the flight tests, which were the first U. S. flight studies of oblique wings ever conducted, showed that the wing could be swept as much as 40 degrees without significant degradation in behavior. However, when the sweep angle was increased to 60 degrees, an unacceptable longitudinal trim change was experienced, and a severe reduction in lateral control occurred at moderate and high angles of attack. Nonetheless, the results obtained with the simple free – flight model provided optimism that the unconventional oblique wing concept might be feasible from a perspective of stability and control.

R. T. Jones transferred to the NACA Ames Aeronautical Laboratory in 1947 and continued his brilliant career there, which included his continuing interest in the application of oblique wing technology. In the early 1970s, the scope of NASA studies on potential civil supersonic transport configurations included an effort by an Ames team headed by Jones that examined a possible oblique wing version of the super­sonic transport. Although wind tunnel testing was conducted at Ames, the demise and cancellation of the American SST program in the early 1970s terminated this activity. Wind tunnel and computational studies of oblique wing designs continued at Ames throughout the 1970s for subsonic, transonic, and supersonic flight applications.[490] Jones stim­ulated and participated in flight tests of several oblique wing radio – controlled models, and a joint Ames-Dryden project was initiated to use a remotely piloted research aircraft known as the Oblique Wing Research Aircraft (OWRA) for studies of the aerodynamic characteris­tics and control requirements to achieve satisfactory handling qualities.

Growing interest in the oblique wing and the success of the OWRA remotely piloted vehicle project led to the design and low-speed flight demonstrations of a full-scale research aircraft known as the AD-1 in the late 1970s. Designed as a low-cost demonstrator, the radical AD-1 proved to be a showstopper during air shows and generated consider­able public interest.[491] The flight characteristics of the AD-1 were quite satisfactory for wing-sweep angles of less than about 45 degrees, but the handling qualities degraded for higher values of sweep, in agreement with the earlier Langley exploratory free-flight model study.

After his retirement, Jones continued his interest in supersonic oblique wing transport configurations. When the NASA High-Speed Research program to develop technologies necessary for a viable super­sonic transport began in the 1990s, several industry teams revisited the oblique wing for potential applications. Ames sponsored free-flight radio – controlled model studies of oblique wing configurations at Stanford University in the early 1990s. As a result of free-flight model contribu­tions from Langley, Ames, Dryden, and academia, major issues regarding potential dynamic stability and control problems for oblique wing con­figurations have been addressed for low-speed conditions. Unfortunately, funding for transonic and supersonic model flight studies has not been forthcoming, and high-speed studies have not yet been accomplished.