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Beauty is in the eye of the beholder, they say, but the X-3 has looks that surely only its designer could love. It wasn’t just hit by the ugly stick, it was the ugly stick. But ugly isn’t only skin deep, the X-3 had another ugly secret waiting to be found.

Started in 1949 under a USAF contract, the X-3 was intended to be capable of taking off under its own power and reaching a cruising speed of mach 2.

Manufacturing wise, it was one of the first aircraft to make extensive use of titanium. The stubby wings pushed the take off and landing speeds to 260 kts, requiring advances in landing gear and tyre technology and materials. 260 kts is almost double the typical 130-160 kts take-off and landing speed.

Design wise, it showed the confidence engineers had built up. No longer a bullet shape, a la the X-1, the X-3 was the best aerodynamic and propulsion ideas taken to extremes. The cross section of the incredibly long fuselage was almost all intake and exhaust for the turbojet engine.

The anticipated engine turned out to be too large, so a smaller, lower thrust engine had to be used. As mach 2 cruise was a goal, this cast a cloud over the program. By the way, there’s no truth in the rumor that the lawn mower starter handle on the lower left hand side of the cockpit was used to spin up this smaller engine.

Hampered by the lower thrust engine, the X-3 was not even capable of Mach 1 in level flight. The feared thermal problems that had caused designers to incorporate flat, low incidence cockpit windows weren’t even approached. Mach numbers up to 1.3 could be achieved in dives of 30 degrees.

With its poor performance, the USAF handed the X-3 over to NACA and celebrated pilot Joe Walker (chief pilot for the X-3). Like many of the test pilots of the era, Joe flew a vast range of aircraft, but no matter how bad the others were, he regarded the X-3 as the worst he had ever flown.

And that was before he had uncovered its ugly secret.

On 27 October 1954, Walker flew a series of test points designed to investigate the aircrafts response to step inputs in the transonic region. At Mach 0.92 he made an abrupt left roll. The aircraft rolled, but also pitched and yawed by around 20 degrees. The aircraft gyrated for a number of seconds before Joe was able to get the aircraft under control.

No shrinking violet, Walker then set up for his next test by diving to Mach 1.1 and inputting the same abrupt left roll. This time the gyrations were far larger, +/- 7g in pitch and 2g laterally. To put that in perspective, people generally “red out” with accelerations of 2g upwards (pushing excess blood into their brains). Walker survived, managed to get the aircraft under control, and landed safely. Post flight inspection showed the airframe had been stressed to its physical limits.

The X-3 had given up its ugly secret, Walker had encountered inertial coupling, the same problem Yeager had dealt with in the X-1, and that had cost Mel Apt his life in the X-2.

With the long thin fuselage the roll inertia was much less than the pitch inertia. Rolling generates small forces in pitch, but it can do so at the natural frequency of the pitch axis, this means its effect becomes magnified. A classic example of the influence of the natural frequency on a system is the Tacoma Narrows Bridge (which leaves me wanting the news today to have such dramatic music!). Once rotation has been set up in two axes with widely differing responses, damping it out manually is very difficult.

At the time Walker was tacking this problem in the X-3, the Air Force’s F-100s were encountering similar problems. The knowledge gained from the X-3 helped advance the science and art of controlling this problem, an unintended success from an aircraft that started life with a low thrust cloud hanging over it.

But every cloud has a sliver lining, because if the original engine had been available, Joe Walker’s step inputs would have been done at much higher airspeeds, likely causing the airframe to disintegrate. Joe might then have had an even greater dislike for this ugly duck X-plane.

So, beautiful? Ugly? Sliver lining or downpour? What do you think of the cloud over this program?



(Images courtesy of NASA and Wikipedia)

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