Design and Validation of an Experiment for the Detection and Prediction of Stall and Surge in a PT6/T400 Turboshaft Engine

The aerospace industry is aggressively pursuing many avenues of engine health monitoring to improve aircraft safety and reduce operating cost. A PT6/T400 turboshaft engine has been instrumented specifically to determine if measurable compressor aerodynamic behavior can provide a warning of impending stall or surge, especially in a small (< 5kg/s), service-exposed, axi-centrifugal compressor. In accordance with a survey of experience and methods for stall testing and detection methods, the engine was instrumented with nine fast-response pressure transducers (monitored at 25 kHz) divided between the axial compressor first stage leading edge, the axial compressor exit, and the outlet of the centrifugal compressor diffuser. An automatic bleed valve was gradually disabled to induce compressor stall. The engine response to this gradual change corresponded to the predictions of a simple engine surge model. A technique for monitoring blade air-flow regularity was developed and used to prove that aerodynamic changes could be successfully detected before the onset of stall/surge. The new technique compared favorably to conventional time series analysis, fast Fourier transform and wavelet processing techniques. Recommendations are made for further improvements and study of test and analysis methods.