The Bell X-5 was the first aircraft capable of sweeping its wings in flight. It was a single-place transonic jet-powered aircraft, capable of sweeping back its wings from 20 degree to 60 degree and back. Its mission was to study the effect of wing-sweep at subsonic and transonic speeds. Results from these tests provided some of the design background for the F-111 and the Navy F-14 tactical aircraft, and the B-1 bomber.
The X-5 was powered by an Allison J-45-A jet engine with a static thrust of 4900 pounds, and using standard JP-4 fuel. The maximum speed was 716 mph and the maximum altitude reached was 49,919 feet. The aircraft was equipped with an ejection seat. It weighed 10,000 pounds when fully fueled.
Two X-5’s were manufactured by Bell Aircraft Co. The first prototype with company chief test pilot Jean ‘Skip’ Ziegler at the controls made its first flight on 20 June. Ziegler ascended to 15,000 ft, with landing gear extended throughout the twenty-minute flight, and observed that the plane performed satisfactorily during several maneuvers.
During the eighth flight on 26 July, Ziegler activated the variable wingsweep mechanism for the first time, making the X-5 the first aircraft to change its wing sweep in flight. The first X-5 was officially accepted by the Air Force on 7 November, and used to carry out buffet, stall and brake effect testing. It was delivered to the NACA station at Edwards in late December, after 20 flights by Jean Ziegler and one flight by General Albert Boyd, AFFTC Commander.
Seven NACA pilots made 126 flights on this aircraft, and the results of their flights provided a significant full-scale verification of wind-tunnel predictions for the reduced drag and improved performance resulting from increasing the wing sweep as the aircraft approached the speed of sound. The pilots found they could use the variable wing sweep as a tactical control to out-perform accompanying escort aircraft during research missions. But they also realized that the X-5 was a vicious aircraft in the stall mode, and in one flight, NACA pilot Joseph Walker lost 6,000 m while recovering from a stall at 12,000 m. With dangerous spin tendencies, the aircraft was truly difficult to fly. Eventually NACA came to consider it simply as a testbed of various wingsweep profiles. After completing its test program, NACA relegated its X-5 to chase duties for other research aircraft flown at Edwards.
The second X-5 was completed in fall 1951 and transported to Edwards AFB on 9 October. Ziegler was at the controls for the first flight on 10 October, and the Air Force officially accepted this prototype eight days later. Unfortunately, this aircraft crashed on 13 October 1953 during stall tests. Its pilot, Major Raymond Popson was fatally injured.
What follows is the official account of the crash, culled from official sources, and left in the present tense, with minimal editing.
X-5, Serial Number 50-1839A, piloted by Major Raymond A. Popson, 12634A departed Edwards Air Force Base at 1429 PST for a local familiarization check out flight, escorted by F-86, Serial Number 2848, piloted by Major Arthur Murray, 14622A.
Take-off and climb to 40,000 feet were normal. At 40,000 feet major Popson performed stalls at 20, 40 and 60 degrees wing sweep and level flight accelerations. After this he made a dive (20 degrees wing sweep) and pull up, sweeping to 60 degrees wing sweep during pull up. Then a dive to .96 Mach indicated was made and level off accomplished at 35,000 feet. Major Popson then called Major Murray and informed him that he had lost his oxygen pressure. Major Murray instructed him to begin an immediate descent. During descent, dive brakes extended, Major Popson stated that he had between 25 – 50 pounds of oxygen pressure. Both aircraft leveled off at 10,000 feet where Major Popson informed Major Murray that he had 120 gallons of fuel remaining and would like to “feel the aircraft out some more” before landing. They climbed to 12,000 feet MSL (approximately 10,000 feet above terrain) and stalls were made, using 20 and 40 degrees wing sweep. A third series of stalls were begun using 60 degrees wing sweep, but during this stall the aircraft rolled slowly on its back and began a spin to the right. The aircraft crashed after completing 4 – 6 turns in the spin. Major Murray called Major Popson during the entry of the third turn and told him to “get out” but received no reply. The aircraft exploded and burned upon impact. The pilot received fatal injuries. The crash occurred at 1509 PST approximately fifteen miles east of Edwards Air Force Base.
Total disintegration of the aircraft resulted from the explosion on impact, but examination of the crater revealed that the aircraft struck the ground in a slight turn, approximately 60 degrees dive. Both wing sweep ball bearing jack screws were recovered and by comparison with like items of the NACA X-5, it was estimated that the wings were at 45 to 47 degrees sweep at time of crash. Enough of the canopy and fuselage were also recovered to determine that the canopy was not ejected from the aircraft prior to crash. All safety pins pertaining to seat and canopy ejection mechanism had been removed prior to flight.
The Aircraft Accident Investigation Board is of the opinion that this accident was the result of loss of control of the aircraft resulting in an unintentional spin at insufficient altitude for recovery but it is felt that several factors or conditions could have contributed to this loss of control.
These are as follows:
1. Wing Sweep Control – It is reasonably certain that the wings were not at 60 degrees sweep at the time of crash. From Major Murray’s statement, and from the plan of flight, it is assumed that the wings were in the 60 degrees sweep position at entry to the final stall. It is the opinion of the Board the pilot was attempting to sweep the wings back to 20 degrees during the unintentional spin. Reaction of the aircraft during spin while the degree of speed is being is changed unknown, since this condition had not been previously encountered. A noticeable trim change is known to be present during wing sweep changes at straight and level flight conditions.
2. Oxygen System and Pressurization – Examination of the Normal, Part II disclosed that the cabin pressurization of the X-5 was inoperative. The cabin pressure indicator of this aircraft usually indicated a thousand feet higher than the actual altitude. It is probable that Major Popson was using 100% oxygen from take-off until he reported the loss of oxygen pressure.
Testimony of competent witnesses reveals that by using 100% oxygen from take-off throughout the duration of a flight, the supply of oxygen would not normally be exhausted before approximately 35 minutes of flight. Total time elapsed between take-off and the time that the pilot notified the escort that he was out of oxygen was 26 minutes. Within a period of two minutes from time of initial notification of the loss of oxygen, the pilot further stated that he had between 25 and 50 pounds of oxygen pressure. The Crew Chief of the X-5 is certain that a full supply of oxygen was on board the airplane prior to this flight. This would indicate excessive leakage or partial malfunction within the oxygen system.
The medical member of the Aircraft Accident Investigation Board is of the opinion that these conditions were favorable to induce some degree of hypoxia, but to what extent is unknown. Although the pilot’s radio conversation and reactions to the stalls at 20 and 40 degrees wing sweep appeared normal, it is felt that hypoxia could have been contributing factor to this accident.
3. The intentions of Major Popson upon entering the stall at the 60 degrees sweep configuration is unknown. Full exploration of the stall characteristics at that altitude would have been unwise; however, a pilot of his known ability should have been able to effect recovery. It is possible that there was a misuse of controls during the attempted recovery from the unintentional spin; i.e., aileron against the spin rather than with, excessive elevator trim (which is highly effective) against the high elevator forces required in the 60 degrees configuration. This is considered less likely, however, than the assumption of partial hypoxia which would reduce the pilots “keenness” or “edge” of reaction.
4. If any one of the factors discussed had occurred first, the loss of control of the aircraft would have been a secondary cause of this accident.
© Henry Matthews 2012