J-8 Flight Test and Technical Problem Solving
The development of the J-8 suffered seriously from the turmoil caused by the "Great Cultural Revolution". The Flight Test Leading Group and the Joint Flight Test Command were disbanded after the first flight of the J-8. The Chief Designer, Office was dismissed as well. The rules and regulations were put aside. Technical cadres like Wang Nanshou, Gu Songfen and Feng Zhongyue were ordered to stop their work one after another. The development work of the J-8 was repeatedly delayed and even stagnated for a time. Under the extremely difficult condition most staff and workers in the aircraft industry still tried their best to protect the J-8 aircraft program from the fate of a premature end.
Flight test is a key factor in the aircraft development. The performance and the manufacturČing quality of a new aircraft, the behavior of the vendor-furnished-equipment and components, the operational and maintainability characteristics of an aircraft, etc., have to be checked in flight. It is an iterative process of problems finding and solving, which will at last lead to a better design. That is why the flight test is a necessary step towards the type certification of a new aircraft.
During the lengthy ten years from 1969 through 1979, the flight test of the J-8 accumulated 1,025 takeoff-and-landings and 663 flying hours at the Flight Test Research Institute and the Flight Test Station of the Shenyang Aircraft Factory. In this phase a series of technical problems were solved.
- Elimination of transonic and supersonic vibration. In 1969 when the flying speed of the J-8 reached M 0.86, a sudden strong transonic vibration occured, which made the aircraft unable to exceed sound speed. The test pilot Lu Mingdong took vigorous actions to assist the designers in finding the vibration source. He repeatedly tested the phenomenon in flight but did not give a thought to his safety. Gu Songfen himself took part in the high speed wind tunnel model oil flow test and the ground resonance test. At last it was found that the vibration came from a disturbed flow. The problem was basically solved after some corrective actions were taken at that time. But eight years later the phenomenon occurred again at transonic speed. Gu Songfen cooperated with pilot Lu Mingdong for a second time. He was lifted into air in a supersonic trainer piloted by Lu Mingdong three times to follow a J-8 aircraft in order to observe and to take picture of the streamlines spectra at the tail of the J-8 fuselage. The air flow separation zone was further ascertained and the real cause of the vibration was found. After local modification to the geometry of the aft fuselage, the flow separation was completely eliminated.
- The supersonic vibration of the J-8 occurred at Mach number 1.24. Theoretical analysis and tests showed that the vibration was caused by a power control system. The designers at the Shenyang Aircraft Design Institute used an foreign product as a reference and designed a new damping cylinder. The supersonic vibration was eliminated after the damping cylinder was installed on the rudder of the aircraft.
- Working out a solution to the problem of the overheated aft fuselage. It was discovered in 1970 that the aft fuselage was overheated when the J-8 flew at high altitude and high Mach number. The problem was temporarily solved by using forced cooling. But it was found in 1967 that the drag chute and even the bay of drag chute were damaged by the high temperature in the aft fuselage when the J-8 flew at high Mach number for a longer time. The problem was completely solved by heat insulation, cooling, vibration suppression and local replacement of materials. Afterwards, The unusual phenomenon no longer occurred.
- Solving the problem of in-flight shutdown. Three in-flight shutdown accidents of the J-8 happened before 1976 and the problem was temporarily solved by the addition of a throttle retainer block. Three more in-flight shutdown accidents happened successively in October, 1976. In order to solve the problem, the MAI set up a leading group to coordinate the work. Corrective actions were taken to both the aircraft and the engine so that the problem of in-flight shutdown was completely solved. This achievement won a Second Class Prize of Scientific and Technological Achievement awarded by the MAI.
Test pilots Lu Mingdong, Hua Jun, Wang Ang and flight commander Su Guohua made great contributions in their cooperative work with design engineers during the type certification flight test. In September 1978, the Shenyang Aircraft Design Institute restored the Chief Designer Office and Gu Songfen was appointed the chief designer.
The Aero Products Certification Commission agreed to freeze the design of the J-8 on 31 December 1979 and the National Military Products Certification Commission formally certificated the design of the J-8 on 02 March 1980. Whole ten years were spent from the first flight to design certification. The reasons for the slowness were not only the turmoil caused by the ten years of the "great cultural revolution", but also the lack of knowledge in the law of the development of a new fighter as well as the underestimation of unexpected difficulties in the area of development.
- The work on the advanced R&D and ground tests were not sufficiently done when the development program was launched so that some key technical problems could not be solved in advance and, therefore, the possible solutions had to be tried or even to be found out by flight test
- Long lead time items such as new engine, new airborne equipment and avionics, new manufacturing techniques and new materials were neither developed in advance, nor in parallel with the J-8 program. They were not available when they were needed. For instance the design of the J-8 originally required the use of an A.C. electrical system and an all-weather fire control radar but later on it was temporarily changed into a day fighter with D.C. electrical system and a ranger because the A.C. electrical system and the radar was not available and could not be developed in a short period of time. Similar cases also occurred to the selection of the aerial gun, rockets and missiles.
- Lack of experience in the flight test of a new design. Too few prototype aircraft were constructed for and too less money spent on the flight test, therefore the flight test phase was prolonged. At the beginning only one prototype aircraft was available. Its flight test time was too little because of the frequent failures of its engine and equipment. In addition the flight test for adjustment should have been carried out at the Flight Test Station of the Shenyang Aircraft Factory to facilitate the handling of the problems found in the flight test by the factory and the design institute. It should be transferred to the flight certification institute for test only after it had passed the flight test at the factory. But it had not been done in this way and the duration of flight test of the J-8 was thus prolonged.
- Frequent changes of the technical management system and the administration system, especially in the middle of the development. The control, dispatch and coordination could not exercised effectively, therefore the development schedule of the J-8 was adversely affected.
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