Sixty-two years this week, the No. 2 USAF/Northrop X-4 experimental flight research aircraft took to the air for the first time. The flight of the second X-4 prototype originated from and recovered at Muroc Air Force Base, California.
The USAF/Northrop X-4 was an early X-plane designed to explore the flight characteristics of a swept-wing, tailless aircraft in transonic flight. It came into being as a result of recent Air Force studies which indicated that a tailless configuration might alleviate or eliminate instability issues associated with supersonic flight. The X-4’s external configuration was similar to that of the German Me163 Komet and the British De Havilland DH.108 Swallow.
The USAF contracted with the Northrop Aircraft Company in June of 1946 to construct and perform initial flight testing of two (2) X-4 aircraft. Northrop received the sole-source contract principally because of the company’s vast experience with flying-wing aircraft. Notable examples include the N-1M, XP-79B, XP-56 and the fabled B-35 heavy bomber.
The X-4 was a physically small airplane. As such, it received the nickname Bantam. It measured 23.25-feet in length and had a wing span of 26.75-feet. The wing leading edge sweep angle was 40.5-degrees. Gross take-off weight was 7,820 lbs. Power was provided by a pair of Westinghouse J30-WE-9 non-afterburning turbojets. Each powerplant had a sea level thrust rating of a paltry 1,600-lbs.
Due to the absence of a horizontal tail and an associated elevator, the X-4 was configured with wing-mounted elevons (combined elevator and aileron). These surfaces provided both pitch and roll control. The type’s split trailing edge flaps were used for low-speed lift enhancement as well as speed brake control. Aircraft directional control was provided via a standard vertical tail-mounted rudder.
The No. 1 X-4 aircraft (S/N 46-676) first flew on Wednesday, 15 December 1948 at Muroc Air Force Base, California. Northrop test pilot Charles Tucker was at the controls. The X-4’s first mission revealed that the aircraft was slightly unstable in pitch. Moving the center-of-gravity forward by 3-inches corrected this problem on subsequent X-4 flights.
The No. 2 X-4 aircraft (S/N 46-677) took to the skies over Muroc Air Force Base for the first time on Tuesday, 07 June 1949 with Northrop’s Charles Tucker once again doing the honors. The second X-4 prototype’s air worthiness characteristics and handling qualities were found to be entirely satisfactory.
This vehicle was in fact superior to the No. 1 aircraft in several respects. Not the least of which was a better flight instrumentation suite.
A total of 17 pilots flew the X-4. Northrop’s Charles Tucker piloted all 30 of the contractor flights including 10 in the No. 1 ship and 20 in the No. 2 X-4. The remaining 82 flights were all flown in the No. 2 ship by USAF and NACA pilots including such luminaries as Stanley Butchart (NACA), Scott Crossfield (NACA), Pete Everest (USAF), Jack McKay (NACA), Joe Walker (NACA) and Chuck Yeager (USAF).
The X-4 achieved a maximum altitude of 42,300 on Tuesday, 29 May 1951 and a maximum speed of Mach 0.94 on Monday, 22 September 1952. NACA pilot Scott Crossfield, who piloted the most X-4 flights (31), was at the controls in both instances.
The X-4 handled well below Mach 0.87. However, the aircraft exhibited an annoying porpoising in pitch at higher transonic speeds. Nose-down pitch changes also produced a Mach-tuck effect that worsened with increasing Mach number. The X-4 also had a nasty tendency to pitch-up as it approached sonic speed. These issues were all related in one way or another to the type’s unique tailless, swept wing configuration.
The X-4 flight test program officially ended in September of 1953. Of the 112 total flight tests conducted over the program’s 58-month duration, 102 were flown by the No. 2 ship. While the X-4 never flew supersonically, the type’s transonic flight research program revealed that the hoped-for advantages of a tailless aircraft in supersonic flight were specious.
Happily, both X-4 aircraft survived the flight test program intact. X-4 No. 1 (S/N 46-676) is currently on display at the United States Air Force Academy in Colorado Springs, CO. The No. 2 ship can be seen at the National Museum of the United States Air Force located at Wright-Patterson Air Force Base in Dayton, OH.