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X-5

The X-5 was the first aircraft capable of sweeping its wings in flight. Its mission was to study the effect of wing-sweep angles of 20, 45, and 60 degrees at subsonic and transonic speeds. With the ability to sweep its wings from 20-degrees, for optimum low-speed handling qualities, all the way to 60-degrees for maximum high-speed performance, the X-5 successfully demonstrated a concept which would later be successfully incorporated into the design of a number of combat aircraft, including the F-111, F-14, and the B-1B. Results of the research program provided a significant full-scale verification of NASA wind-tunnel predictions for the reduced drag and improved performance resulting from increasing the wing sweep as the speed of the aircraft approaches the speed of sound. The pilots found they could use the variable wing sweep as a tactical control to out-perform the accompanying escort aircraft during research missions.

Two X-5s were manufactured by Bell Aircraft Company. This single-place jet-powered aircraft, measured 36 ft in length with a wingspan of 19 ft (with the wings swept back 60 degrees). The wings could be swept back 20 to 60 degrees. The X-5 weighed 10,000 lb when fully fueled. The X-5 was powered by an Allison J-45-A jet engine with a static thrust of 4,900 lb. The maximum speed was 716 mph and the maximum altitude reached was 49,919 ft. The aircraft was equipped with and ejection seat. The first high-performance aircraft to feature a variable, in-flight wingsweep capability, the Bell X-5 completed its maiden flight at Edwards on June 20, 1951 , with Jean Ziegler, a Bell test pilot, at the controls. It was last flown on Oct. 25, 1955 by a young National Advisory Committee for Aeronautics (NACA) test pilot named Neil Armstrong.

Perhaps less well known than its more famous stable mates, the X-5 had a number of unusual features. Born from an aerodynamic concept under development in the dying days of Nazi Germany, it came to fruition in the skies over Edwards where it engendered features used by the front-line combat aircraft of several nations today. Then, its research tasks completed, the aircraft spent its remaining years as a useful and hard-working member of the AFFTC's flight line fleet.

Late in World War II, the German aircraft industry was desperately seeking new ways of improving the performance of its revolutionary new jet fighters. By 1945, German scientists were well aware that a swept-back wing was invaluable for speeds in the high subsonic range, approaching 600 mph. The new wing shape had certain inherent disadvantages, however, including instability at lower speed ranges, and it required longer runways for takeoff and landing. To better study the concept, the Messerschmitt company developed its P.1101, a research aircraft with fighter-like lines. The P.1101 was little more than a single Junkers 109-004B jet engine with a straight-through inlet and nozzle; the aircraft's cockpit, wings and tail assembly were all located above the thrust line. The sweep angle of its thin wings could be changed manually, on the ground. Nearly completed by war's end, the Messerchmitt aircraft never flew. Its design, however, and the concepts it was created to study, intrigued U.S. engineers.

Bell Aircraft Corporation, in particular, was intrigued with the possibilities offered by this strange craft. Bell was one of the most research-minded and innovative of the wartime aviation companies and, soon after V-E Day, it began to offer proposals to the Air Materiel Command to exploit and follow up on the German research. Finally, in 1949, the company received a contract to construct two variable-sweep-wing test bed aircraft based upon the wartime design.

The two X-5 aircraft which resulted resembled the general lines of the P. 1101; they were, however, advanced designs in their own right. Although the German plane was nominally a fighter design and some thought had been given to developing the Bell design into a combat aircraft, the X-5s were pure research airplanes. They were small, for one thing--36 feet in length with a (swept-back) wingspan of only 19 feet--and the 4,900 pound thrust of their single Allison J35-A-17 engine was just sufficient to move a pilot and the complicated wing assembly to a useful maximum speed of Mach 0.91--about 700 mph. But the pot-bellied little craft had a number of insuperable advantages. Unlike the more famous of its X-plane sisters, it could easily take off and land under its own power. Moreover, unlike its German-speaking predecessor, the pilot of the Bell plane could change the sweep angle of its wings in flight, repeatedly if necessary.

Like so much else in the field of aerodynamics, the concept was far more complicated than it might seem. Changing the sweep angle of a wing might be mechanically simple enough, but the act of doing so changes the aircraft's center of gravity and that in turn alters its relationship to the center of lift. Bell engineers compensated by mounting the wing pivots on the outboard edge of each side of the fuselage. A push-button on the control stick activated an electric motor which pivoted the wings by means of a gearbox and jackscrews. As this was happening, another pair of jackscrews simultaneously pushed the entire wing assembly forward (or back) on rails, up to 27 inches, to maintain the optimum center of gravity. The wings of the X-5 could be moved through its full range, from twenty to sixty degrees of sweepback, in 20 seconds.

Clearly, an aircraft which could change the sweep of its wings at will would enjoy enormous advantages in performance. Wings could be swept far back in flight for a speed advantage, yet extended nearly straight for easy takeoff and a safe landing speed. The concept came to be known as "variable-geometry" and was immediately dubbed "swing-wing" by the public. Bell test pilot Jean "Skip" Ziegler took the unusual plane aloft for the first time on 20 June, 1951. During its highly successful research program, the X-5 was flown by a succession of the Air Force's and NACA's most noted test pilots, including Joe Walker, Scott Crossfield, Lt Col "Pete" Everest and Brig Gen Albert Boyd, among others. Chuck Yeager and Neil Armstrong, later to be the first person to walk on the moon, also flew the diminutive plane.

The little white X-5, perched on its stalky landing gear, was by no means an uncomplicated aircraft, nor were its flight characteristics in any way benign. In the NACA test program, the X-5 demonstrated severe stall-spin instability. Its stall and spin qualities were best described as "vicious." Air Force test pilot Maj Raymond Popson, in fact, lost his life on 14 October, 1953, when the second X-5 crashed during a flight to study its ability to recover from a spin. Nevertheless, both the aircraft itself and its test programs were highly successful. Variable-geometry wingform was indeed a practical proposition. Not only were low- and high-speed capabilities enhanced, but test pilots soon discovered that changing the wing sweep in flight allowed them to outmaneuver their chase planes in simulated dogfights.

Although the mechanism by which the X-5 changed its wing sweep made this particular design impractical, development of a viable variable-sweep aircraft had to await Langley Aeronautical Laboratory's concept of an outboard wing pivot in the mid-1950s (Langley was a NACA research laboratory in Hampton, Virginia.) The Air Force's F-111 and B-1 bombers, the Navy's F-14 fighter and many aircraft types from other nations all owe their existence to the odd-looking research plane which emerged from the flames of World War II.

As for the surviving X-5 itself: after the completion of its research programs, the small veteran remained on the Edwards flight line as a working airplane for several years. It's dependability and proven versatility over a wide speed range made it a natural chase plane, and so after its days of glory it quietly performed line duties for later, more advanced flight programs. In 1958, it was finally transferred to the USAF Museum at Wright-Patterson AFB, where it may be seen today.



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