Optical Determination of Rotating Fan Blade Deflections

Measurement of flutter motion for rotating fan and compressor blades is necessary to verify mode shape analysis and assure an accurate description of the deflection and twist distribution required for stability prediction. The static deflection of blades caused by centrifugal and gas loads also needs to be measured to improve the accuracy of performance analysis. This paper presents a new technique for making these measurements with small blade-mounted mirrors that reflect laser light once per revolution. For steady operation, in the absence of blade vibration, each mirror reflects a light beam to project a repeating spot on a display screen, once for every revolution of the rotor. However, when the blades are fluttering the reflected light moves from its stationary position and during successive revolutions describes the blade motion as a lissajous pattern. Vibration amplitude, phase, and frequency are discussed and related to analysis. Limits in accuracy and the importance of precise mode shape description for flutter analysis are presented.