The measurement of blade-tip clearances in aircraft turbines by a capacitance method

An instrument has been developed for use by airfield maintenance personnel in the checking of turbine-blade clearances, both in turbo-jet and turbo-prop engines. The measurements are made by plugging a cable into various capacitance pick-up points around the shroud ring and spinning the rotor, cither by hand cranking or by the starter motor.The instrument works on a capacitance principle, but the measurement does not depend for its accuracy on the similarity between the pick-ups; balancing, calibration and—ultimately—measurement at a pick-up point are possible without moving the attachment cable, except when changing to the next pick-up. Clearances are indicated on a peak voltmeter, scaled to show the clearance being measured as a fraction of the value to which the instrument has been calibrated. The instrument selects the longest blade and measures its clearance.The instrument has several novel features. The use of a transformer, placed part-way along the pick-up lead and designed to make the whole lead parallel-resonant, not only makes possible the measurement of minute capacitance changes at the end of a long cable but enables a simple resistance bridge to be used for this measurement. This bridge is fed by a stable crystal-controlled 50-kc/s oscillator. With the present equipment it is possible to detect capacitance changes of approximately 0.003 µµF at the end of a cable 23 ft long, requiring an overall capacitance sensitivity, δC/C, of 10−5.The method of calibrating the instrument results in the greatest accuracy of measurement for clearances near the calibration value, the estimated error being less than +5%.The underlying technique may have many other applications in measuring minute capacitance changes, especially where the pick-up is unavoidably remote from the main measuring apparatus.