The X-4 was developed for the study of flight characteristics
of swept wing semi-tailless aircraft at transonic speeds (about
Mach .85). Northrop built two X-4s. The No. 1 aircraft was first
flown by Northrop on Dec. 16, 1948, and the second X-4 made its
initial flight on June 7, 1949. The No. 1 aircraft was grounded
after its 10th flight to provide spare parts for the No. 2 aircraft.
Northrop's part of the test program ended on Feb. 17, 1950, with
the 20th flight of the remaining X-4.
Although both aircraft were turned over to the
Air Force and then to the National Advisory Committee for Aeronautics
in May 1950, only the No. 2 X-4 was used in the joint USAF/NACA
program to explore stability problems near the speed of sound.
The program ended in September 1953 with the 102nd and last flight
of the No. 2 aircraft, after proving that swept wing aircraft
without horizontal tails were not suitable for transonic flight.
Both aircraft survived the test program without
serious incident. The No. 1 X-4 is displayed at the Air Force
Academy. The No. 2 aircraft is displayed at the Air Force Museum.
The X-4 was restored by the Western Museum of Flight, Hawthorne,
Span: 26 ft. 10 in.
Length: 23 ft. 3 in.
Height: 14 ft. 10 in.
Weight: 7,550 lbs. maximum
Engines: Two Westinghouse XJ-30 turbojet engines of 1,600 lbs.
Serial number: 46-677
Maximum speed: 640 mph
Cruising speed: 480 mph
Maximum endurance: 44 minutes
Service ceiling: 44,000 ft.