Flightline: 161 - Bell X-5
Developed in part from a captured Nazi prototype, the X-5 was the first aircraft that could change its wing sweep in flight.
As engines became more powerful the maximum potential speed of aircraft increased at an exceptional rate, with the Hughes H1 Racer setting a record speed of 352mph in 1935, a mark broken just two years later by an German Bf 109 fighter. As aircraft became faster, designers began running into aerodynamic limits imposed by the thick, straight wings in common usage, and began to research alternatives. Engineers quickly hit upon the idea of sweeping the wing backwards, which delayed the formation of drag-inducing shockwaves. They also quickly found that sweeping the wings produced poor low-speed handling and excessively long takeoff and landing runs. A proposed solution would be to develop a wing that could alter its sweep, allowing the low-speed handling of a straight wing for takeoff and landing, and the high-speed performance of a swept wing during cruise or dashes. The development and refinement of the turbojet engine during WWII put higher emphasis on researching swept or variable-sweep wings, resulting in the development of Messerschmitt Me P.1101 in 1944. The P.1101 was intended to be a research aircraft, and the sweep of the wing could only be changed on the ground. Still, the capture of the partially completed prototype at the end of the war, along with the capture of German researchers and documents under Operation PAPERCLIP proved to be a windfall for designers in the US during the post-war and early Cold War period.
An American USO troupe poses with the captured P.1101 at Oberbayerische Forschungsanstalt, Germany in 1945. | Photo: Green4life80
Delivered to Bell Aircraft's plant in Buffalo, NY, the incomplete and damaged P.1101 provided enough information to their Chief Designer, Robert Woods, to propose his own VSW research aircraft to both NACA and the USAF. Construction of two aircraft, designated the X-5 and allocated USAF serial numbers 50-1838 and 50-1839, began in 1949, with the planes being completed and trucked to Muroc dry lake in 1951.
Orthograph of the X-5, with the various positions of the wing shown in outline. | Illustration: USAF/NACA
The X-5s were small aircraft, just 33' long and a wingspan at minimum sweep of 30'. At full sweep the span was reduced to 20 feet 9 inches. The aircraft weighed just under ten thousand pounds fully loaded, and the Allison J35 engine provided enough thrust to push the plane to 705mph, just under Mach 1. Unlike the earlier P.1101, the wings sweep of the X-5 could be altered in flight, with a jackscrew assembly driving a hinge along rails and disc brakes holding the wing at sweeps of 20°, 40° and 60°. Moving from minimum to maximum sweep could be accomplished in less than 30 seconds, and articulation of the hinge and pivots compensated for shifts in the centers of pressure and gravity caused by the movement of the wings. As was common with first and second generation jets, the engine was slung under the fuselage for ease of maintenance and replacement. The X-5s were supported by tricycle landing gear while on the ground. The X-5 was equipped with a conventional tail, which as it turned out was inadequate for maintaining the craft's stability.
A multiple-exposure photograph of one of the X-5s showing the various positions of the wing. | Photo: NACA/NASA
The two X-5s took their maiden flights on 20 June and 10 December 1951, and testing continued until 1955. The aircraft were flown at speeds up to Mach 0.9 and altitudes of up to 40,000', showing that their variable-sweep wings showed potential. The aircraft were somewhat flawed however, as the poorly positioned and somewhat undersized tail could not keep the aircraft from spinning at certain sweep angles. On 14 October 1953, Air Force Captain Ray Popson died after the second X-5 entered an unrecoverable spin with the wings at 60° sweep. The test program continued on with the first X-5 for two more years, and the aircraft remained at Edwards as a chase plane until early in 1958.
The first X-5 in flight over Muroc dry lake, circa 1953. | Photo: USAF
The cockpit of the X-5. The wing sweep selection dial is prominent in the center of the instrument panel. | Photo: NMUSAF
A Lasting Legacy
The USAF had planned to further develop the X-5's basic design in to a tactical fighter for itself and for possible sale to NATO allies, but the poor handling characteristics of the plane saw those plans canceled. The data provided by the X-5 program was later incorporated into other, more successful VSW aircraft like the General Dynamics F-111, the Grumman F-14, and the Rockwell B-1, as well as some unsuccessful designs like the SST. Further advances in aerodynamics, engine design and fly-by-wire/computer controlled stability systems have rendered swing wings somewhat obsolete, as the potential gains from VSW are now available without the attendant penalty of their complex and heavy pivots and related actuators.
The remaining X-5 was transferred to the National Museum of the USAF in 1958, where it has been on display in various locations for the last 63 years. Its current home is the new R&D hangar, opened in 2016.
The X-5 on the ramp outside Wright Patterson AFB, during one of its moves around the USAF Museum. | Photo: NMUSAF
The X-5 in its former location, circa 2009. | Photo: Valder137
The X-5 in its current display. The wing of the XB-70 is just visible at the bottom of the image. | Photo: NMUSAF