Two X-4s were built by the Northrop Corporation, but the first was found to be mechanically unsound and after ten flights it was grounded and used to provide parts for the second.

[1] While being tested from 1950 to 1953 at the NACA High-Speed Flight Research Station (now Edwards Air Force Base), the X-4's semi-tailless configuration exhibited inherent longitudinal stability problems (porpoising) as it approached the speed of sound.

The United States Army Air Forces signed a contract with the Northrop Aircraft Company on 11 June 1946, to build two X-4s.

The resulting aircraft was very compact, only large enough to hold two Westinghouse J30 jet engines, a pilot, instrumentation, and a 45-minute fuel supply.

These included Chuck Yeager, Frank Kendall Everest, Jr., Al Boyd, Richard Johnson, Fred Ascani, Arthur Murray and Jack Ridley.

NACA pilots Griffith and Scott Crossfield noted that as the X-4's speed approached Mach 0.88, it began a pitch oscillation of increasing severity, which was likened to driving on a washboard road.

This combined yaw, pitch and roll, which grew more severe as the speed increased, was a precursor to the inertial coupling which would become a major challenge in the years to come.

To correct the poor stability, project engineers decided to increase the flap/speed brake trailing edge thickness.

Balsa wood strips were added between the upper and lower hinged "clamshell"-style flap/speed brake halves, causing them to remain open at a 5° angle.

The results were positive, with Jones commenting that the X-4's flight qualities had been greatly improved, and the aircraft did not have pitch control problems up to a speed of Mach 0.92.

In May 1953, the balsa wood strips were again removed, and the X-4's dynamic stability was studied in the original flap/speed brake and elevon configuration.

Related lists This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.