During the manufacture and overhaul of gas turbines it is necessary to ensure that all blades in a stage are of an equal and known length to minimise the loss in performance that arises as a consequence of the clearance between rotor tip and engine casing. Modern compressor and turbine blades are generally loose fitting in their root fixings and only adopt their true working position when running.

In this paper, a new technique for measuring the rotor radius over individual blades is described. The measurement technique utilises a capacitance based clearance measurement system which enables rotor radius to be measured over each blade while spinning fast enough to ensure that the blades are centrifugally loaded into their true working position. The measurement technique is described, as is the system utilised to calibrate and reduce output into engineering units. The mechanical design of the “measurement head” is presented and the CNC lathe to which it interfaces described. Finally, the results of an experimental programme, utilising a fully bladed compressor disc undertaken to ascertain system performance, are presented.

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