X-4, NASA history collection. Public domain image of NASA aircraft.

In this 1950 straight-on view of the NACA High-Speed Flight Research Station's X-4 research aircraft, the horizontal tailless design is quite evident. The X-4 Bantam, a single-place, low swept-wing, semi-tailless aircraft, was designed and built by Northrop Aircraft, Inc. It had no horizontal tail surfaces and its mission was to obtain in-flight data on the stability and control of semi-tailless aircraft at high subsonic speeds. The Northrop X-4, Bantam, was a single-place, swept-wing, semi-tailless airplane designed and built to investigate that configuration at transonic speeds (defined as speeds just below and just above the speed of sound, but in this case, the testing was done primarily at just below the speed of sound). The hope of some aerodynamicists was that eliminating the horizontal tail would also do away with stability problems at transonic speeds resulting from the interaction of supersonic shock waves from the wings and the horizontal stabilizers. Northrop Aircraft, Inc. built two X-4 aircraft, the first of which proved to be mechanically unsound. However, ship number 2, with a thicker trailing edge on the wings and elevon, was very reliable. Ship 1 was then grounded and used as parts for ship 2. While being tested from 1950 to 1953 at the NACA High-Speed Flight Research Station (predecessor of today's NASA Dryden Flight Research Center, Edwards, California), the X-4's semi-tailless configuration exhibited inherent longitudinal stability problems (porpoising) as it approached the speed of sound. The X-4 was a small twinjet-engine airplane that had no horizontal tail surfaces, depending instead on combined elevator and aileron control surfaces (called elevons) for control in pitch and roll attitudes. Data gathered from the aircraft's blunt elevon research were helpful in the design of the Bell X-2, which had ailerons with blunted trailing edges. The NACA X-4 program also provided substantial data on the interactions of combined pitching, rolling, and yawing motions. This interaction was soon to become critical to upcoming high-performance military fighters. The X-4, ship 2, flew 82 research flights from 1950 to 1953. With a minimal lift-to-drag ratio of less than 3, the X-4 performance was similar to the soon-to-be-developed X-15. With this similarity in mind, NACA conducted approach and landing studies of X-15-generation aircraft using the X-4. The X-4, retired in 1954, ended its days as a pilot trainer.
NASA Identifier: NIX-E-358