Fifty-nine years ago this month, the USAF/Douglas X-3 Stiletto research aircraft encountered a then little known dynamic instability during a flight test that nearly ended in disaster. NACA test pilot Joseph A. Walker was at the controls of the needle-nosed experimental aircraft.
The X-3 was designed to fly at speeds up to Mach 2. The aircraft was approximately 67 feet in length and had a wing span on the order of 23 feet. Gross weight was 23,840 pounds.
A pair of Westinghouse J46-WE-1 turbojets were intended to power the X-3. However, protracted developmental problems and installation issues with these powerplants would eventually prevent their use in the aircraft.
The X-3 was ultimately outfitted with a pair of Westinghouse J34-WE-17 turbojets. The result was that the X-3 was now underpowered and could barely fly supersonically. Maximum achieved Mach number was 1.21 and that was in a 30-deg dive!
Notwithstanding the above, the X-3 took to the air 54 times between October 1952 and May 1956 for the purpose of conducting transonic flight research. It would be on its 43rd flight that the X-3 would make its most important contribution to aviation.
On Wednesday, 27 October 1954, Joe Walker took-off in the X-3 (S/N 49-2892) from Edwards Air Force Base, California. At Mach 0.92 and 30,000 feet, Walker applied left aileron at fixed-rudder in an effort to develop a rapid roll response. To Walker’s utter amazement, the X-3 went wild in both pitch and yaw.
Although it seemed to last much longer, Walker was able to recover control of the X-3 within 5 seconds of his initial left aileron input. In true test pilot fashion, Walker again made an abrupt rudder-fixed left aileron input at Mach 1.05. The same thing happened. However, this time the aircraft’s motions were more violent.
Happily, Walker again recovered control of the X-3. Having had enough of flight test frontiersmanship for one day, Walker uneventfully recovered the aircraft to Edwards.
The phenomenon that Joe Walker and the X-3 encountered that day in 1954 is known as Inertial Roll Coupling. It is a resonant divergence in either pitch or yaw due to the presence of roll rate. Aircraft like the X-3, which have low longitudinal and/or directional static stability as well as high pitch-to-roll and yaw-to-roll moment of inertia ratios, are especially susceptible to this phenomenon.
As a postscript to our story, the phenomenon of Inertial Roll Coupling had been hypothesized by the NACA’s William H. Phillips back in June of 1948. For Joe Walker in October of 1954, engineering theory would become flight test fact in a few terrifying seconds high in the skies over Edwards Air Force Base.