Severe aerodynamic interaction between the fan core stream section and the high-pressure compressor of a three-shaft low-bypass-ratio engine is described. At high fan running lines a heavy single-cell rotating stall was found in the fan core stream even at high aerodynamic speeds between 90–98%. The rotating circumferential distortion with 180–200 deg sector angle is swallowed by the intermediate pressure compressor but erodes the high-pressure compressor surge margin by about 22%, leading to steady-state surges. A remotely mounted transducer in a specific arrangement was used successfully for measurements in the hot environment behind intermediate and high-pressure compressor using a so-called “long-line” system with a closed end at the downstream pipe.

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