The rotor tip clearance in a gas turbine engine varies throughout the engine operating regime. It has considerable influence on the engine performance. Blade to casing rub is imminent at certain operating points of the engine. Mechanical rub at high speeds could damage the total engine hardware. A precise measurement helps to obtain the optimum engine performance with safe engine operation. In this article a typical case study related to fan clearance measurement is discussed, where indications of a proven measurement system is not in agreement with the physical event during engine test. Centrifugal, thermal, assembly and wear effects can affect tip clearance measurement. Centrifugal forces untwist the blade tip, resulting in change in the effective area of the target that is seen by capacitance sensor. Relative component growths due to thermal effect result in the displacement of the sensor from its original position. This could induce error into this measurement. Assembly errors are seen during blade to disc assembly. Wear occurs under the action of centrifugal loading and vibration in compressor blades dovetail roots that are attached to the disc. This leads to wear in involved metal surfaces and it could be a source of error in this measurement. Measurement system also has its own uncertainty. During the current work all sources of errors were evaluated. Probable actual running clearance on the engine and reasons for the mismatch in indication were successfully arrived at through analytical and experimental studies. This work has provided an insight into probable sources of errors and their treatment methodologies using analytical and experimental techniques. This has helped in identifying the changes needed in the calibration procedure, methods to reduce the measurement system uncertainty band and measurement procedure.

The blade tip timing (BTT) method uses the differential arrival timings of the blades at case-mounted sensors to effectively characterize the vibrations of all blades in a rotor. This paper studies the use of the BTT method for pre-emptive prediction of rotor blade damage; through a careful monitoring of blade natural frequencies in conjunction with the blade tip position during an engine test. In the current study, the low pressure turbine stage of a developmental aero engine is instrumented with a combination of eddy current and optical sensors located circumferentially on the casing. This instrumentation effectively captures the engine order resonances of interest for the blade bending mode. During one of the normal engine tests, one of the blades in the LPT stage suddenly showed a drop in natural frequency beyond the allowable scatter and an abrupt change in the blade tip position. As the engine test was continued further, this drop in blade natural frequency and change in blade tip position progressively increased towards blade failure limits. Suspecting a propagating crack in the particular blade, the test was aborted and the engine was withdrawn for detailed inspection. Inspection of the rotor blades confirmed the presence of significant aerofoil crack in the suspect blade.

The Blade Tip Timing (BTT) method uses the differential arrival timings of the blades at case-mounted sensors to effectively characterize the vibrations of all blades in a rotor. This paper studies the use of the BTT method for preemptive prediction of rotor blade damage; through a careful monitoring of blade natural frequencies in conjunction with the blade tip position during an engine test. In the current study, the low pressure turbine stage of a developmental aero engine is instrumented with a combination of eddy current and optical sensors located circumferentially on the casing. This instrumentation effectively captures the engine order resonances of interest for the blade bending mode. During one of the normal engine tests, one of the blades in the LPT stage suddenly showed a drop in natural frequency beyond the allowable scatter and an abrupt change in the blade tip position. As the engine test was continued further, this drop in blade natural frequency and change in blade tip position progressively increased towards blade failure limits. Suspecting a propagating crack in the particular blade, the test was aborted and the engine was withdrawn for detailed inspection. Inspection of the rotor blades confirmed the presence of significant aero-foil crack in the suspect blade.