The x-plane programs have yielded some fairly unusual designs over the years, and the Douglas X-3 Stiletto was no exception. The X-3 Stiletto was a prototype aircraft designed to help designers and engineers better understand what sort of features an aircraft would need in order to fly at supersonic speeds, while also being capable of taking off and land under its own power.
The Douglas X-3 Stiletto’s design was revolutionary for its day. The stubby wings and long tapering fuselage weren’t pretty, and it looked nothing like the beautifully proportioned fighter aircraft of WWII. Instead, the odd shape of the X-3 Stiletto was designed for one thing. Speed.
In the summer of 1949, the Douglas Aircraft Company received a request from the United States Air Force for two X-3 prototypes. The first aircraft was delivered to Edwards Air Force Base in California on 11th September 1952. Just four days later the first aircraft was ‘flown’ for the first time.
The X-3’s debut air time was conducted with Douglas test pilot William Bridgeman at the controls. The aircraft was just barely lifted off the ground, and Brideman kept it at the same low altitude for a mile before putting it gently back down onto the dry lakebed. On October 20th 1952, the first proper flight took place. Lasting around 20 minutes, Bridgeman took the X-3 up to operational heights before safely returning to the ground.
In total Bridgeman made 26 flights, and during his testing a serious problem was discovered with the X-3. It wasn’t anywhere near fast enough. The original goal had been to exceed Mach 2, however the X-3 couldn’t even reach Mach 1 without putting it into a 15 degree dive. There were plans to replace the turbojet engines with rocket motors but these were quickly abandoned.
In 1953 Douglas had finished their initial round of testing with the X-3 Stiletto, and the project was handed over to the USAAF and NACA (National Advisory Committee for Aeronautics). NACA pilot Joseph A. Walker made his pilot checkout flight in the X-3 on 23rd August 1954, he then conducted eight more research flights in September and October of that year. By late October, the research program was expanded to include lateral and directional stability tests. In these tests, the X-3 Stiletto was abruptly rolled at transonic and supersonic speeds, with the rudder kept centered. Despite being underpowered, the X-3 was ideal suited for these tests. The location of its engines, fuel and main structure was centered in its long, narrow fuselage, while its wings were short and stubby. As a result, the X-3’s mass was tightly concentrated along the length of its fuselage, rather than its wings. This was typical of the fighter aircraft which were being developed and tested at the time.
These high-speed roll tests would lead to the X-3 Stiletto’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 ft (9,144 m). 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. Unfazed, he then set the aircraft up for the next test. 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 upward 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). Thankfully Walker managed to regain control and successfully land the aircraft.
After analysis of the flight recorder, it was found that the fuselage had been taken to the limits of its structural integrity. Had the acceleration been a fraction higher, the aircraft could have torn itself apart. What Walker and the X-3 had experienced was “roll inertia coupling,” a phenomena in which a maneuver in one axis causes an uncommanded maneuver in one or two others.
After this eventful flight, the X-3 was grounded for almost a year. Once it returned to the skies it was never again subjected to high speed rolls and stability tests. Only 10 flights were made during this time, the last one was in May of 1956.
Although the Douglas X-3 Stiletto never led to a production aircraft, it did contribute in other ways to aircraft development. It dramatically proved the dangers of roll inertia coupling, while at the same time gathering valuable data on the phenomena, the shape of its wings were mimicked by the successful Lockheed F-104 Starfighter, it was one of the first aircraft to use and prove the exceptional qualities of titanium, and its high-take off speed forced improvements in tire technology.
Basically, it was an x-plane which helped aircraft designers literally learn from their mistakes.
If you want to see it in person, it’s on display at the National Museum of the United States Air Force in Ohio.