Northrop X-4 Bantam
A tiny semi-tailless twinjet built by Northrop Aircraft Company under a 1946 Army Air Forces contract, to test whether deleting the horizontal tail would cure the stability problems of transonic flight. Two aircraft were built. The first was unreliable, flew only ten times and became a source of spare parts; the second carried the NACA programme from 1950 to 1953 and uncovered a growing pitch oscillation and three-axis hunting near Mach 0.9. NASA judges the type's importance to be a proved negative: the configuration did not suit speeds near Mach 1. Both aircraft survive in museums.
Why It Mattered
The X-4 was built to test one idea: that a swept-wing aircraft could fly near the speed of sound with no horizontal tail surfaces at all. Engineers in the 1940s suspected that the stability troubles met at transonic speeds grew out of shock waves interacting between the wing and the tail, and that deleting the tail might delete the problem. The little jet gave that idea a thorough trial between 1948 and 1953. NASA sums the type up as proving a negative: the swept semi-tailless layout was not suitable for speeds near Mach 1. That answer steered designers away from a dead end, and NASA notes that designs of this kind became practical only after computer fly-by-wire systems arrived.
Design Notes
The Army Air Forces signed Northrop Aircraft Company to build two X-4s on 11 June 1946, choosing the firm for its flying-wing experience with the N9M, XB-35 and YB-49. What emerged was a very small jet: the airframe had room only for two J30 engines, a pilot, the test instrumentation and fuel for about 45 minutes. There was no horizontal tail. Pitch and roll were handled together by elevons, the combined elevator and aileron surfaces on the wing. A set of split flaps doubled as speed brakes. The aircraft sat low enough that nearly all maintenance could be done without a ladder or footstool, and a person standing on the ground could look into the cockpit.
Operational History
The first aircraft, serial 46-676, reached Muroc in November 1948 and flew on 15 December 1948 with Northrop test pilot Charles Tucker; flooding at Rogers Dry Lake then halted flying until April 1949. Mechanically unreliable, it flew only ten times before grounding; Walt Williams, who headed the NACA's Muroc flight test unit, called it a "lemon," and it became a source of spare parts for its sister. The second aircraft, 46-677, proved far more reliable and made 20 contractor flights; in February 1950 both X-4s passed to the Air Force and the NACA. The NACA instrumented the second aircraft and ran a short series with Air Force pilots, including Chuck Yeager and Pete Everest, in August and September 1950. John Griffith became the first NACA pilot to fly it on 28 September 1950; the early NACA work, to May 1951, concentrated on pitch behaviour. Griffith and Scott Crossfield found that as speed neared Mach 0.88 a pitch oscillation set in and grew stronger, likened to driving on a washboard road. Higher speeds brought nose-down tuck and a "hunting" motion in yaw, pitch and roll together, which NASA identifies as a precursor to inertial coupling. Engineers answered with a blunter trailing edge: balsa strips between the split-flap halves held them open at five degrees. Walter Jones first tested it on 20 August 1951 and reported the flight qualities greatly improved, without pitch-control problems up to Mach 0.92. With balsa on the elevons too, flights from May 1952 showed better stability in pitch and yaw and pushed the nose-down trim change from Mach 0.74 to Mach 0.91, though it still oscillated above that speed. The strips came off for a final study in the original configuration; the programme ended with the 81st and final NACA flight on 29 September 1953.
Afterlife
Both X-4s survived the test programme. The first aircraft was transferred to the United States Air Force Academy at Colorado Springs, before returning to Edwards Air Force Base, where it is now on display inside the Air Force Flight Test Museum. The second aircraft went to Wright-Patterson Air Force Base in Ohio, home of the Air Force Museum, and remains on display there in the National Museum of the United States Air Force's Research and Development gallery. The layout the X-4 tested did not vanish: NASA records semi-tailless designs appearing later on the X-36, Have Blue, F-117A and Bird of Prey, though each differed significantly in shape from the Bantam.