The Bell X-5 experimental aircraft, a revolutionary design from the NACA High-Speed Flight Research Station (now NASA Armstrong Flight Research Center), was instrumental in exploring the advantages of variable sweep wing technology. This photograph captures the X-5 at the South Base of Edwards Air Force Base in California, equipped with a pitot-static probe for airspeed measurement.
The core concept behind the Bell X-5 was to validate the feasibility of in-flight wing sweep angle changes. This offered significant benefits for both operational and research aircraft. For operational planes, fully extended wings during takeoff would reduce takeoff speed and runway length. Once airborne, sweeping the wings back would decrease drag and boost speed. From a research perspective, variable sweep wings on the X-5 promised a wealth of data, effectively combining the capabilities of multiple fixed-sweep wing experimental aircraft like the D-558-II, XF-92A, X-2, and X-4 into a single platform.
The X-5‘s origins trace back to the German Messerschmitt P.1101, an experimental jet with ground-adjustable wings. Although the P.1101 never flew, it was captured by U.S. forces in 1945. Bell Aircraft engineers studied it, and in 1948, Robert Wood proposed a larger, modified version with in-flight sweep capability – the concept that became the Bell X-5. The NACA, already researching variable sweep wings, supported Bell’s proposal, leading to Air Force approval in 1949.
Two X-5 prototypes (serials 50-1838 and 50-1839) were constructed, mirroring the P.1101’s design with a nose intake, bubble canopy, underslung engine, and boom-mounted tail. The wings could pivot from 20 to 60 degrees during flight. The sweep mechanism was intricate; as the wings swept back, the entire wing assembly moved forward on internal fuselage rails to compensate for center of gravity and pressure shifts. A 20 to 60-degree sweep change required a 27-inch forward movement and took about 20 seconds. A manual crank allowed pilots to return wings to landing position (under 40 degrees sweep for safe landing) in case of electrical failure.
The first X-5 (50-1838) arrived at Edwards AFB in June 1951 and first flew on June 20, piloted by Bell’s test pilot Jean “Skip” Ziegler. Phase I airworthiness tests continued through August, culminating in sweep mechanism tests in July. Major Frank Everest conducted Air Force flights in late 1951 and early 1952 before the X-5 was transferred to NACA. The second X-5 (50-1839) was also delivered and flown by the Air Force.
NACA research pilot Joe Walker began X-5 program assignments in January 1952. Initial research focused on stability and control across different sweep angles, including gust response at 20 and 60 degrees. NACA pilots A. Scott Crossfield and Walter P. Jones joined the program in April 1952, contributing significantly to the research. Walker remained the lead X-5 pilot until mid-1954.
The X-5 research program was comprehensive, extending beyond stability and control to vertical tail loads, gust loads, wing and tail bending, trim effects, strain gauge temperature response, and buffeting under dynamic pressure. Pilots utilized the adjustable sweep to gather data at specific angles, such as Walker’s 59-degree sweep flight in February 1954 for stability and control data. The X-5 also participated in drag studies and served as a chase plane.
However, NACA flights revealed the X-5‘s shortcomings. Crossfield described it as “not comfortable,” citing drag and thrust axis misalignments leading to “interesting maneuvers.” A major issue was spin recovery. Walker once needed 18,000 feet to recover from a spin. Tragically, Major Raymond Popson died in a spin accident in the Air Force’s X-5 (50-1839) in October 1953 at a 60-degree sweep.
Walker’s last X-5 flight was in April 1954. Stanley P. Butchart and John B. “Jack” McKay continued stability and control research flights until October 1955. Butchart highlighted the X-5‘s spin vulnerability and the unusual nose-mounted speed brakes causing nose-down pitch. Neil A. Armstrong’s NACA checkout flight in October 1955 marked the final X-5 flight when a landing gear door separated, grounding the aircraft. The first X-5 completed 122 NACA flights and was retired to the Air Force Museum in 1958.
The X-5 program yielded mixed results. It validated variable sweep wings’ potential for drag reduction and performance enhancement, and pilots recognized their tactical advantage. Spin issues stemmed from the design, not the variable sweep concept itself. While promising, the X-5‘s complex wing sweep mechanism was impractical. NACA Langley engineers later developed a solution: pivoting wings outside the fuselage.
Variable sweep wings became prominent in aircraft design from the 1960s to 1980s, including the F-111, F-14, B-1, Su-22, Su-24, Tu-22M, Tu-160, and Tornado, demonstrating the X-5‘s lasting legacy in aviation history.
