X-3 Stiletto

X-3 Stiletto
	275px
Type	Experimental
Manufacturer	Douglas
Maiden flight	15 October 1952
Retired	23 May 1956
Primary users	United States Air Force; NACA
Number built	1
Variants	F-104 Starfighter

The Douglas X-3 Stiletto was an experimental jet aircraft with a slender fuselage and a long tapered nose, manufactured by the Douglas Aircraft Company. Its primary mission was to investigate the design features of an aircraft suitable for sustained supersonic speeds, which included the first use of titanium in major airframe components. It was, however, seriously underpowered for its purpose and could not even exceed Mach 1 in level flight.

Development

The X-3 was sleekest of the early experimental aircraft, but its research accomplishments were not those originally planned. 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 of thrust with afterburner rather than the planned 7,000 pounds. The first aircraft was completed and delivered to Edwards Air Force Base, California, on 11 September 1952. Due to both engine and airframe problems, the partially completed second aircraft was cancelled, and its components were used for spares.

Operational history

The first X-3 "hop" was made on 15 October 1952, by Douglas test pilot Bill Bridgeman. During a high-speed taxi test, Bridgeman lifted the X-3 off and flew about a mile (1.6 km) before settling back onto the lakebed. The official first flight was made by Bridgeman on 20 October, and lasted about 20 minutes. He made a total of 26 flights (counting the hop) by the end of the Douglas tests in December 1953. These showed that the X-3 was severely underpowered and difficult to control. Its takeoff speed was an unusually high 260 knots (482 km/h). More seriously, the X-3 did not approach its planned top speed. Its first supersonic flight required the airplane make a 15 degree dive to reach Mach 1.1. The X-3's fastest flight, made on 28 July 1953, reached Mach 1.208 in a 30 degree dive.

With the completion of the contractor test program in December 1953, the X-3 was delivered to the U.S. 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. Lt. Col. Frank Everest and Maj. 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 conducting 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 m). The X-3 rolled as expected, but also pitched up 20 degrees and yawed 16 degrees. 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 aircraft in a dive, accelerating to Mach 1.154 at 32,356 feet (9,862 m), where he made an abrupt left roll. The aircraft pitched down and recorded an acceleration of -6.7g (-66 m/s²), then pitched upward to +7g (69 m/s²). At the same time, the X-3 side-slipped, resulting in a loading of 2g (20 m/s²). Walker managed to bring the X-3 under control and successfully landed.

The post-flight examination showed 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 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 US Air Force Museum. 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 phenomena. Its wing planform was used in the F-104, 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.

The principal contribution of the X-3 was its data on inertia coupling - a potentially violent divergence from the intended flight path when executing an abrupt maneuver which had afflicted the X-1 and X-2.

Production

Two aircraft were ordered, but only one was built. It made 51 flights.

In 1956, it was transferred to the National Museum of the United States Air Force, where as of 2007, it is on display in the Research & Development Gallery.

Specifications(X-3)

300px

General characteristics

Crew: 1
Length: 66 ft 9 in (20.3 m)
Wingspan: 22 ft 8 in (6.9 m)
Height: 12 ft 6 in (3.8 m)
Wing area: 166.5 ft² (15.47 m²)
Empty weight: 16,120 lb (7,310 kg)
Max takeoff weight: 23,840 lb (10,810 kg)
Powerplant: 2× Westinghouse J34 afterburning turbojet, 3,370 lbf, 4,850 lbf with afterburning (15.0 kN, 21.6 kN with afterburning) each

Performance

Maximum speed: 700 mph (1125 km/h)
Range: 497 ml (800 km)
Service ceiling: 38,000 ft (11,600 m)
Thrust/weight: 0.40

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v • d • e Lists relating to aviation
General	Timeline of aviation · Aircraft · Aircraft manufacturers · Aircraft engines · Aircraft engine manufacturers · Airports · Airlines
Military	Air forces · Aircraft weapons · Missiles · Unmanned aerial vehicles (UAVs) · Experimental aircraft
Notable incidents and accidents	Military aviation · Airliners · General aviation · Famous aviation-related deaths
Records	Flight airspeed record · Flight distance record · Flight altitude record · Flight endurance record · Most produced aircraft

X-3 Stiletto

Contents

Development

Operational history

Production

Specifications(X-3)

References

See also

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