Douglas X-3
The Douglas X-3 Stiletto was the sleekest of the early experimental aircraft, but its research accomplishments were not those originally planned. It was originally intended for advanced Mach 2 turbojet propulsion testing, but it fell largely into the category of configuration explorers, as it never met its original performance goals due to inadequate engines. The goal of the aircraft was ambitious—it was to take off from the ground under its own power, climb to high altitude, maintain a sustained cruise speed of Mach 2, then land under its own power. The aircraft was also to test the feasibility of low-aspect-ratio wings, and the large-scale use of titanium in aircraft structures.
Construction of a pair of X-3s was approved on 30 June 1949. During development, the X-3's planned Westinghouse J46 engines were unable to meet the thrust, size and weight requirements, so lower-thrust Westinghouse J34 turbojets were substituted, producing only 4,900 pounds-force (22 kilonewtons) of thrust with afterburner rather than the planned 7,000 lbf (31 kN). The first aircraft was built and delivered to Edwards Air Force Base, California, on 11 September 1952.
The X-3 featured an unusual slender, streamlined shape having a very long, gently-tapered nose and small trapezoidal wings. The aim was to create the thinnest and most slender shape possible in order to achieve low drag at supersonic speeds. The extended nose was to allow for the provision of test equipment while the semi-buried cockpit and windscreen were designed to alleviate the effects of "thermal thicket" conditions. The low aspect ratio, unswept wings were designed for high speed and later the Lockheed design team used data from the X-3 tests for the similar F-104 Starfighter wing design. Due to both engine and airframe problems, the partially completed second aircraft was cancelled, and its components were used for spare parts.
With the completion of the contractor test program in December 1953, the X-3 was delivered to the United States Air Force. The poor performance of the X-3 meant only an abbreviated program would be made, to gain experience with low aspect ratio wings. Lieutenant Colonel Frank Everest and Major Chuck Yeager each made three flights. Although flown by Air Force pilots, these were counted as NACA flights. With the last flight by Yeager in July 1954, NACA made plans for a limited series of research flights with the X-3. The initial flights looked at longitudinal stability and control, wing and tail loads, and pressure distribution.
NACA pilot Joseph A. Walker made his pilot checkout flight in the X-3 on 23 August 1954, then conducted eight research flights in September and October. By late October, the research program was expanded to include lateral and directional stability tests. In these tests, the X-3 was abruptly rolled at transonic and supersonic speeds, with the rudder kept centered. Despite its shortcomings, the X-3 was ideal for these tests. The mass of its engines, fuel and structure was concentrated in its long, narrow fuselage, while its wings were short and stubby. As a result, the X-3 was "loaded" along its fuselage, rather than its wings. This was typical of the fighter aircraft then in development or testing.
These tests would lead to the X-3's most significant flight, and the near-loss of the aircraft. On 27 October 1954, Walker made an abrupt left roll at Mach 0.92 and an altitude of 30,000 feet (9,100 metres). The X-3 rolled as expected, but also pitched up 20° and yawed 16°. The aircraft gyrated for five seconds before Walker was able to get it back under control. He then set up for the next test point. Walker put the X-3 into a dive, accelerating to Mach 1.154 at 32,356 ft (9,862 m), where he made an abrupt left roll. The aircraft pitched down and recorded an acceleration of -6.7 g (-66 m/s²), then pitched upwards to +7 g (69 m/s²). At the same time, the X-3 side-slipped, resulting in a loading of 2 g (20 m/s²). Walker managed to bring the X-3 under control and successfully landed.
The post-flight examination showed that the fuselage had been subjected to its maximum load limit. Had the acceleration been higher, the aircraft could have broken up. Walker and the X-3 had experienced "roll inertia coupling," in which a maneuver in one axis will cause an uncommanded maneuver in one or two others. At the same time, several North American F-100 Super Sabres were involved in similar incidents. A research program was started by NACA to understand the problem and find solutions.
For the X-3, the roll coupling flight was the high point of its history. The aircraft was grounded for nearly a year after the flight, and never again explored its roll stability and control boundaries. Walker made another ten flights between 20 September 1955 and the last on 23 May 1956. The aircraft was subsequently retired to the National Museum of the United States Air Force.
Although the X-3 never met its intention of providing aerodynamic data in Mach 2 cruise, its short service was of value. It showed the dangers of roll inertia coupling, and provided early flight test data on the phenomenon. Its small, highly loaded unswept wing was used in the Lockheed F-104 Starfighter, and it was one of the first aircraft to use titanium. Finally, the X-3's very high takeoff and landing speeds required improvements in tire technology.