TRANSONIC VIBRATION INDUCED BY MISMATCHING BETWEEN ENGINE AND THRUST NOZZLE

The transonic vibration occured in the flight tests of some combat aircraft at H=3000-4000m and Ma=0.82-0.96 is analyzed in this paper. For nozzle pressure ratio of about 2.9-4.6,the vibration frequency ranged from 8 to 12Hz. Some measures were taken to discover the vibration source, such as eliminating the clearance of the aircraft structure,blocking air inlet apertures in the tail sectionof the aircraft,streamline tracing with power and strapping, and model testingin wind tunnel,but all of them failed.On the basis of extensive investigation and calculations,it was decided to make simulation test of the ejector nozzles with increased secondary flow rate or with the nozzle shroud apertured for third flow. Finally it was found out that the transonic vibration was induced by mismatching of engine with thrust nozzle. "Long skirt", "short skirt"and tail shroud with the apertures cotrolled by spring loaded differential pressure valves were tested in the flight tests. During 18 flights,the number of the differential pressure valves increased from 10 to 16,in consequence the vibration reduced to 0.1g or below,and the aircraft passed smoothly through the transonic range.