Douglas X-3 Stiletto – The Flying Shoe

The Douglas X-3 Stiletto was another of National Advisory Committee for Aeronautics’ (NACA) projects that was intended to test high-speed, low aspect ratio wing designs. It was a very ambitious aircraft that aimed to grow the USA’s knowledge of machines capable of Mach 2 flight.

The first of two X-3s was built in 1952. Just 49 years after the Wright brother’s first flight of 180 feet NASA was intending to test an aircraft capable of well over 1,000 mph.

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History

With the introduction of the first jet aircraft at the end of the war, it quickly became apparent that there were inherent tactical advantages to having an aircraft much faster than your enemies. The Me-262 Schwalbe was a testament to this when Allied fighters came up against them. As we know, the Me-262 was not made in enough numbers to have any meaningful effect on the war. But, after World War II the Allies were very quick to gain as much scientific knowledge from the Germans as possible in regard to their jet engine technology.

Me-262 was the first operational jet fighter
Me-262 was the first operational jet fighter and could manage a top speed of 540 mph.
Photo by Neuwieser

New fighters were quickly introduced after the war such as the Lockheed P-80A Shooting Star and the F-86 Sabre. Jet technology was advancing at an unbelievable pace. Looking back now, it seems that almost every year a new aircraft was introduced somewhere around the world that was faster and more powerful than anything before it.

It did not take long for military jets to be knocking on the door of supersonic speeds, even in level flight. But what about beyond this? Well for the most part not much was known about what would happen once breaking the sound barrier.

F/A-18 Hortnet breaking the sound barrer
The modern F/A-18 can easily break the sound barrier in level flight. Photo credit – DVIDSHUB


By the Korean war, an aircraft that could sustain Mach 1 was incredibly sought after. But what about going Mach 2? Twice the speed of sound – that was even more unknown.

Even by the late 1940s, the boffins at aerospace companies were thinking about Mach 2 flight and NACA commissioned two X-3s to test just that. The first design was put forward in 1949, construction was approved for two X-3s and the first aircraft was built and delivered to the famous Edwards Air Force Base in California in September 1952.

Design

The very first thing you will notice about the X-3 is her appearance. Traditional aircraft had relatively large wings in comparison to the fuselage. In this case, small stubby wings were attached to a rather elongated body. The wingspan measured just 22 ft 8 in (6.91 m) and a length of 66 ft 9 in (20.35 m).

This unusual shape earned the X-3 the nickname “Stiletto” – much like a stiletto knife or the heel of a stiletto shoe. Most certainly one of the sleekest aircraft ever produced by 1952, it looks fast even when not moving. It is clear the X-3 was intended to pierce through the sky like a dagger.

X-3 on the lakebed where testing was done
The X-3 was meant to pierce the sky like a dagger.


The intended purpose was to take off under its own power, achieve and sustain Mach 2 and then land. But this did not quite go to plan.

Housed on either side of the fuselage behind the cockpit were two Westinghouse J34 turbojet engines. The J34 was actually a substitute as it was initially planned to use more powerful J46 engines. However these were unable to meet the requirements to propel the X-3.

Westinghouse’s J-34 only produced 4,900 lbs (22 kN) of pushing force with an afterburner, whereas the J46s were supposed to produce 7,000 lbs of thrust.

The wings were small, unswept and trapezoidal in shape. You may notice that they bear similarities with the F-104 Starfighter, which not even 10 years later set a world speed record of 1,404.19 mph (2,259.82 km/h) in May 1958. This type of design enabled extremely high speed when paired with powerful engines.

F-104 had a similar wing design to the X-3
The shape and size of the F-104’s wing had a striking similarity to the X-3.


Due to the effects of thermal thicket the cockpit was semi-buried into the fuselage. Thermal thicket is the heating of a solid body that passes through the air at high speed. All aircraft go through this to some degree, but it becomes an issue when traveling at several times the speed of sound.

This heat can transfer from the outer fuselage into the structure and potentially cause issues with the fuel tanks or electric and hydraulic systems – anything that might be sensitive to getting warm and needs to be accounted for in the design. By placing the cockpit inside the fuselage, it prevented the pilot from being cooked whilst flying at high speed.

Testing

The testing didn’t go to plan. The first official flight was made on 20th October 1952 and lasted less than 30 minutes. However, the test pilot had accidentally taken off in the X-3 five days previously and flew for about a mile before landing it back on the lakebed they were using.

A year later only a total of 26 flights had been made and very quickly it became apparent that the X-3 was unpowered and difficult to fly. The aircraft never got anywhere near its intended top speed and only ever reached Mach 1.2 in a fairly steep dive. Although still impressive for 1953, it was hugely underwhelming in comparison to what Douglas thought it would achieve.

The X-3's dagger like appearance
The X-3 did not live up to the promises made by Douglas, but many valuable lessons were learned.

A combination of the short wings producing a small amount of lift and low-performance engines, the take-off speed was exceptionally high at 300 mph (480 km/h). In comparison, a P-51 Mustang can rotate at around 120 mph (193 km/h). All signs were pointing to the X-3 being a failure and the second airframe was never completed.

Not wanting to have wasted their time, the scope of the project was changed. No longer would high-speed testing be a focus, instead they’d be looking at gaining experience with low aspect ratio wings. General Chuck Yeager actually flew the X-3 a handful of times too with data being gathered on the stability of the aircraft as well as pressure distribution.

General Chuck Yeager with the X-3 Stiletto
General Chuck Yeager with the Douglas X-3.


Further testing included putting the X-3 into a dive at over Mach 1 and then full hard left on the stick to roll. The aircraft inexplicably pitched up and down, producing a negative of 6.7 g and pitch up force of 7 g. The test pilot was able to regain control and land successfully, but he probably wasn’t feeling too well after experiencing -6.7 g.

After landing the X-3 was inspected and found to have been pushed to the very limit of the airframe. Any faster and it would have broken up mid-air.

The pitching up and down despite only commanding a left roll was a peculiar phenomenon that had also occurred in the F-100 Super Sabre. Known as roll inertia coupling, when an aircraft is flying at high speeds and if the design does not take it into account, can cause a maneuver in one axis to actually affect one or two other axis with potentially catastrophic results – often a total loss of an aircraft.

F-100s also had the same issue as the X-3 at high speeds
The F-100 Super Sabre also suffered with roll inertia coupling at high speeds.


After the unexpected rolling, the X-3 was grounded for about a year whilst the cause was investigated. Even afterward, the Stiletto only made a few short flights before finally being retired in 1956.

Another Article From Us: The NASA AD-1 – Flying Scissors

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Huge amounts of knowledge was gained from the X-3 and although a total failure in its intended purpose the best was made out of a bad situation to salvage the project and gain vital information that has been used to help design many other aircraft.

Specifications

  • Crew: 1
  • Length: 66 ft 9 in (20.35 m)
  • Wingspan: 22 ft 8 in (6.91 m)
  • Height: 12 ft 6 in (3.82 m)
  • Empty weight: 14,345 lb (6,507 kg)
  • Max takeoff weight: 22,400 lb (10,160 kg)
  • Powerplant: 2 × Westinghouse XJ34-WE-17 afterburning turbojets, 3,370 lbf (15.0 kN) thrust each dry, 4,900 lbf (22 kN) with afterburner
  • Maximum speed: 613.5 kn (706.0 mph, 1,136.2 km/h) at 20,000 ft (6,100 m)
  • Range: 432 nmi (497 mi, 800 km)
  • Service ceiling: 38,000 ft (12,000 m) absolute
  • Rate of climb: 19,000 ft/min (97 m/s)