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The Douglas X-3 Stiletto

 

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Size comparison with the X-15

Of all the experimental research planes ever built, Douglas Aircraft Company’s rakish X-3 was the one which, of them all, looked like it was built for speed. Which it undeniably was. During the heady era of the early 1950s, X-series aircraft were appearing one after another to explore numerous aspects of controlled human flight. Aircraft were pushing simple speed and altitude initiations on a daily basis, but also evaluating controllability, and studying various wing and tail configurations. The speed of sound was routinely being surpassed, but data was lacking on the aerodynamic phenomena of sustained high-speed flight. The "thermal barrier," in particular--the heating effects of high-speed air friction upon airframe components--remained little known.

The Douglas X-3 Stiletto was something of a disappointment. Because its planned J46 engines never materialized and its fuselage was too narrow for more powerful ones, the X-3 never achieved useful speeds or altitude. Almost by accident, however, its extremely long, narrow platform and short-span wings revealed the cause and the remedy for the deadly phenomenon of roll-coupling. All high speed aircraft since have benefited from research conducted with this craft.

A contract was issued for the design and construction of two transonic research aircraft was approved by the Secretary of War as early as 30 June, 1945, more than two years before supersonic speeds had been reached. Conceptual research and design work occupied the next six years.

The resulting X-3 was designed to reach Mach 2, and to sustain that speed for not less than 30 minutes. Unlike the contemporary X-1 bis and X-2 which were air-launched from carrier aircraft, it was designed to take off and land independently, under its own power. The X-3's final design appeared exotic, even by Edwards standards, yet it was driven entirely by the logic of its requirements. Even as early as the late 1940s, it was apparent that a straight wing with a low aspect ratio was going to be the most efficient design for sustained flight beyond Mach 1. Very short and highly-loaded, the X-3's wings mandated a proportionately longer fuselage to contain the landing gear and fuel cells. The very long, sharply-pointed nose section, designed to carry a variety of test instrumentation, heightened the effect. To cut down on drag, the fuselage was also made as narrow as possible, leaving barely enough frontal area for the pilot and a pair of turbojet engines. Very short exhaust ducts were used in order to get the maximum thrust, which in turn required that the tail surfaces be mounted on a slim boom located behind and above the engine nozzles. A cramped cockpit, beautifully faired in, contributed little drag and added to the fineness of the overall design. For the first time, titanium was used extensively in major airframe components to save weight and increase strength. By the time the final assembly of the X-3 was completed 30 September, 1951, it seemed that the nation had another superlative research aircraft.

No matter how carefully an airframe is designed and crafted, however, it will come to nothing if the engines cannot measure up. The X-3's narrow fuselage was literally designed around the early design specifications for a pair of Westinghouse J46-WE-1 engines, each anticipated to deliver some 4,200 lbs of thrust. Development of the J46 proved to be troublesome, however, and while the X-3 was taking shape, the new engine was not only falling behind schedule, but was growing in size and weight. Douglas was forced to install a pair of smaller J34-WE-17s, yielding only 3,000 lbs of thrust each. It was hoped by all that these engines would prove to be interim only, but the J46 fell ever farther behind schedule and no other engines could fit into the constricted engine bays of the new research plane.

Douglas test pilot Bill Bridgeman took the X-3 into the air for its first test flight on 20 October, 1952. Even before the jet made its first landing 20 minutes later, it was apparent that it was sadly underpowered and would never reach the performance levels necessary for its designed mission. Worse, the low power levels allowed a host of awkward handling problems to leap into prominence. Suddenly the tiny wings were difficult to control, and made the long fuselage extremely sensitive to pitch effects. The heavy wing loading, harmless enough in high speed flight, also meant a high takeoff speed--no less than a sizzling 260 mph. Naturally enough, there was no comparable effect at the other end of the speed range. With its laboring J34s, the X-3 was not only incapable of reaching Mach 2, it was firmly subsonic. The aircraft could only be nudged past Mach 1 in a power dive. The highest speed the Stiletto ever reached was a comparatively sedate Mach 1.21 after a thirty-degree dive. By the time the contractor and the Air Force had completed their initial evaluations, it was obvious that NACA’s hot new transonic jet was no more than a lumbering and treacherous dog.

Once in a great while, however, real life mimics a Hollywood epic, and a disgraced hero is allowed to redeem himself at the last moment. So it proved with the X-3. When NACA test pilot Joe Walker, systematically following a somewhat dispirited test program, performed a routine left aileron roll and suddenly found himself wildly out of control, the way had unknowingly opened for the X-3 to redeem itself. That eventuality was the very last thing on Walker’s mind during the long seconds it took him to regain control over the slipping and rolling aircraft. What had happened was a phenomenon which was coming to be known as roll coupling, when an aircraft suddenly becomes unstable along all three axes. In this instance, the X-3's nose had abruptly pitched up at the onset of a left roll; corrective action only made the problem more extreme until the airplane was riding nose high in a heavy sideslip. Ever the professional, Walker cautiously tried the maneuver once more, and once again the X-3 slammed out of control. Recovering a second time, Walker landed and the post-flight analysis began at once.

Roll coupling, also known as inertia coupling or roll divergence, had been predicted several years earlier in design studies. Air Force test pilot Chuck Yeager had experienced it himself during the preceding December, when his X-1A suddenly flipped out of control after a high-speed test run and, as the flight analysts later reported, went into divergent angular rotations about all three flight axes. Worse was to come. Nearly two years afterward, the X-2 went into roll coupling and crashed on 27 September 1956, killing Air Force Captain Milburn Apt.

Even more troubling, the deadly phenomenon was not confined to the rarified world of flight research. Even as Joe Walker made his eventful flight, roll coupling was beginning to appear in the operational Air Force. North American’s revolutionary new F-100 Super Sabre, the world’s first fighter plane capable of supersonic speeds in level flight, was being plagued by an unexplained yawing motion which appeared at high speeds, occasionally with disastrous results. The high-tailed, long-coupled X-3, with its unexpected tendency to replicate the same aerodynamic fault, proved invaluable in studying both its cause and its remedy. Based on the new data, North American increased the F-100's fin area and wingspan and the jet went on to be one of the nation’s hardest-working and most successful fighters of the 1960s and 1970s.

As for the X-3: only the single aircraft was ever completed. After NACA completed its test program on May 23, 1956, the X-3 was returned to the Air Force, refurbished, and placed on display at the U.S. Air Force Museum at Wright-Patterson AFB in Ohio, where it remains to this day.

Global Security

 

 

"STAGGER LEE"

 
The X-3 "STILLETO"
USAF AND NACA
1952 - 1955
 

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If ever there was an aircraft that looked its moniker, it was the X-3 "Stilleto". Under direction of the US Air Force's Air Research and Development Command, and sponsored jointly by the US Navy, USAF and NACA, Douglas designed and developed a high-speed research aircraft under the designation Douglas X-3, later named Stiletto after its lines. Intended primarily for research into the problems of high-altitude, high-speed flight and the effects of kinetic heating, the X-3 began its design life in 1946. The complexity of this program is indicated by the fact that more than three years elapsed before approval was given for construction of a mockup, in August 1948, and it was not until late June 1949 that Douglas received a contract for two flying prototypes and one static test airframe; however, only one prototype was built (49-2892). 
 
The X-3 had the most highly refined supersonic airframe of its day as well as other important advances including one of the first machined structures. It included the first use of titanium in major airframe components. Its long fuselage gave the Stiletto a high-fineness ratio and a low-aspect ratio (the ratio of the wing's span to its chord; in other words, it was short and stubby). Despite this refined configuration, the maximum speed it attained was Mach 1.21, which occurred during a dive. The general consensus was that the aircraft was sluggish and extremely underpowered by its Westinghouse J-34 power plants. The X-3 also demonstrated coupling instability during abrupt rolling maneuvers, which could cause it to go wildly out of control, as happened on a flight on Oct. 27, 1954, with National Advisory Committee for Aeronautics (NACA) pilot Joe Walker at the controls. The principle contribution of the X-3 was its data on inertial coupling (roll divergence) - a tendency to diverge from the intended flight path. The aircraft also shed its small tires routinely, leading to a revision of the design criteria for tires used on high-speed aircraft. This aircraft flew 20 times between 1954 and 1956 at the NACA High-Speed Flight Station (predecessor of NASA's Dryden Flight Research Center, Edwards, California). Joe Walker was the pilot for all 20 of these missions.
                        
First flown on 20 October 1952 with Douglas test pilot Bill Bridgeman in the cockpit, the X-3 had a slender needle-nosed fuselage, a low-set cantilever monoplane wing of very short span, conventional tail unit, retractable tricycle landing gear and power provided by two Westinghouse J-34-WE-17 turbojets mounted side-by-side in the fuselage. The pilot was accommodated in a pressurized cabin, on a downward ejection seat that served also as an electric lift to provide access from the ground. Design of the X-3 was of unprecedented complexity because of the high-speed requirement, involving advanced aerodynamics and the use of new constructional methods and materials. They included, in particular, the development of fabrication and construction techniques involving the use of titanium. Additionally, the airframe had more than 850 pinhole orifices distributed over its surface to record pressures, 185 strain gauges to record air loads and stresses, and 150 temperature recording points. Testing proved disappointing, the aircraft being underpowered and able to achieve only 50 per cent of its design speed of Mach 2.2. With virtually no hope of improving performance, the USAF cancelled the program after only six flights and the aircraft was handed over to NACA.
 
However, the X-3 was not regarded as a failure, for it made important contributions to titanium technology, and features of its design were used later in Lockheed's F-104Starfighter and the SR-71 Blackbird. The X-3 was transferred to the U.S. Air Force Museum in 1956, where it resides today.
 
Specifications:
 
Douglas X-3 Stiletto Type: high-altitude high-speed research aircraft
 
Power plant: two Westinghouse J-34-WE-17 turbojets, each developing 4,200 lb (18.68 kN) thrust with afterburners
 
Performance: maximum speed 706 mph (1136 km/h) at 20,000 ft (6095 m); absolute ceiling 38,000 ft (11580 m);
Endurance: 1 hour
 
Weights: empty 14,345 lb (6507 kg); maximum take-off 22,400 lb (10160 kg)
 
Dimensions: span 21 ft 8 in (6.91 m); length 66 ft 9 in (20.35 m); height 12 ft 6 in (3.81 m); wing area 166.5 sq ft (15.47 m2)

 

 

The X-3

The Douglas X-3 Stiletto was the 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.

X-3 side view in flight

 Construction of a pair of X-3s was approved on June 30, 1949. During development, the X-3's planned engines failed to meet the thrust, size and weight requirements. As a result, lower-thrust Westinghouse J34 turbojets were substituted. The first aircraft was completed and delivered to Edwards Air Force Base, Calif., on September 11, 1952. Due to both engine and airframe problems, the partially completed second aircraft was cancelled, and its components were used for spare parts.

The first X-3 "hop" was made on October 15, 1952, by Douglas test pilot Bill Bridgeman. During a high-speed taxi test, Bridgeman lifted the X-3 off the ground and flew it about a mile before settling back onto the lakebed. The official first flight was made by Bridgeman on October 20, and lasted about twenty 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 take off speed was an astonishing 260 knots! More seriously, the X-3 did not approach its planned performance. Its first supersonic flight required that the airplane make a 15 degree dive to reach Mach 1.1. The X-3's fastest flight, made on July 28, 1953, reached Mach 1.208 in a 30 degree dive.

X-3 with aircraft fleet

 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 flight. With the last flight by Yeager in July of 1954, the 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 August 23, 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.

X-3 front view while parked

On October 27, 1954, Walker made an abrupt left roll at Mach 0.92 and an altitude of 30,000 feet. 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 X-3 into a dive, accelerating to Mach 1.154 at 32,356 feet, where he made an abrupt left roll. The aircraft pitched down and reached a g-loading of -6.7, then pitched upward to +7 Gs. At the same time, the X-3 side-slipped, resulting in a loading of 2 Gs. 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 G forces been higher, the aircraft could have broken up. Walker and the X-3 had experienced "roll coupling," in which a maneuver in one axes will cause an un-commanded maneuver in one or two others. At the same time, several F-100s were involved in similar incidents. A research program was started by the 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 10 flight between September 20, 1955, and the last on May 23, 1956. The aircraft was subsequently retired to the 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 coupling, and provided early flight test data on the phenomena. Its wing platform was used in the F-104, and it was one of the first aircraft to use titanium. Finally, the X-3's very high take off and landing speeds required improvements in tire technology.

NASA

 

 

The Douglas X-3 Stiletto

It looked fast....but looks ain't everything

 

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The Douglas X-3 (USAF S/N 49-2892), known as the Stiletto, was built to investigate the design of an aircraft suitable for sustained supersonic speeds. The X-3 was intended for sustained flight research above Mach 2, but the engines it was designed for never came. So the sleek X-3 was hampered by use of underpowered Westinghouse J34 turbojet engines of 3,370 lbs. thrust each (4,900 lbs. thrust with afterburner) which could not power the aircraft past Mach 1 in level flight; because of the combination of little engines and tiny wings, the takeoff runs were long, and the landing speeds were abnormally high. The X-3's wingspan was 22 ft. 8 in., length 66 ft. 10 in., height 12 ft. 6 in., and it weighed 22,400 lbs. max. 

 

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The X-3 had perhaps the most highly-refined supersonic airframe of its day as well as other important advances, including the first use of titanium in major airframe components. Its long fuselage gave the Stiletto a high-fineness ratio and a low-aspect ratio (the ratio of the wing’s span to its chord; in other words, it was short and stubby). Despite this refined configuration, the maximum speed it attained was Mach 1.21, during a dive. The X-3’s first flight was in October 1952 with Douglas test pilot Bill Bridgeman in the cockpit. The Air Force completed a brief evaluation of the airplane in 1953 and 1954 before turning it over to NACA in the summer of 1954.

 

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The cockpit of the X-3 was entered via a small hatch in the floor, and featured a downward-firing ejection seat. USAF Museum Photo

 

 

 

 

 

 

 

 

 

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Test pilots for the program included Chuck Yeager for the USAF and Joe Walker, the only pilot to fly it for NACA, and almost nobody who flew the X-3 liked it. The general consensus was that the aircraft was sluggish and extremely underpowered. Here, Yeager is shown standing next to the airplane on Rogers Dry Lake after a test flight; the landing probably took a couple of miles of lakebed before Chuck could get the thing stopped. USAF Photo.

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The X-Planes were all purebred performance machines, so a lot of normal utility was sacrificed in their design to optimize performance. The X-3, for example, had little dinky tires. It went through tires the way people go through potato chips. This not only led to a redesign for the X-3, but a re-thinking of the design criteria for landing gear and tires for all high-performance aircraft. And in an effort to save wear and tear on the X-3's tires, it wasn't taxied out to the runway or lakebed-it was hauled out on a trailer.

 

 

 

 

 

 

 

 

 

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The X-3 also demonstrated coupling instability during abrupt rolling maneuvers, which could cause it to go wildly out of control, as happened on a flight on Oct. 27, 1954, with NACA pilot Joe Walker at the controls. On this flight, Joe's 10th in the airplane, he performed two abrupt, rudder-fixed aileron rolls at speeds of Mach 0.92 and 1.05 (0.92 and 1.05 times the speed of sound) that led to inertial roll coupling, causing him to diverge from the expected flightpath. These two maneuvers, from which he fortunately was able to recover, yielded a wealth of valuable data on the (as yet not fully understood) phenomenon of inertial coupling. Together with data from other aircraft, such as the X-2 and the F-100, this helped the aeronautics community understand how to deal with the phenomenon of coupling dynamics. This turned out to be the X-3's most lasting contribution to the science of aerodynamics. The last flight was Joe Walker's NACA lateral control investigation mission, flown on May 23, 1956. Today, the X-3 is on display at the USAF Museum at Wright-Patterson AFB, Dayton, OH. USAF Photo.

 

 

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Last Updated

02/10/2014

 

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