The response of a lift engine model compressor to inlet pressure distortion is studied in detail to determine the nature and extent of the critical area of spoiling. This is found to be a sector of width between 60 and 90 deg. On this basis a simple distortion index is proposed which implies that only the circumferential pressure gradients of a complex distortion pattern are significant. The “Parallel Compressor” model satisfactorily explains surge and this experience is related to different types of compressors and to complex spoiling patterns. Comments are made on the differences in behavior between compressors operating on rig test and in engines.

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