verter. In order to improve the quality of the LDV data the blades have been covered with retro-reflective tape allowing to increase the scattering of the laser light backwards to the photodetector. This tape allowed also a significant increase of structural damping of the blades. Fig. 2 Two blade fan 2.2 Measurement techniques The complete set of testing strategies (SLDV, TLDV, CSLDV and TCSLDV) employed, as basic component, a single point Polytec vibrometer, specifically the OFV-505 with VD-01 velocity decoder. The laser beam was moved point by point, for SLDV purpose, and continuously, for TLDV and CSLDV purpose, via a couple of galvanometric mirrors driven by a thermally controlled PID board. The mirror driving signals have been generated by a NI-PXI-6733 board (16 bit resolution). During the measurement in rotating conditions the laser beam had to move synchronously with the blade. Therefore the generation of the mirrors feeding signals was triggered with a clock provided by the output of an incremental encoder installed on the propeller shaft. A NI-PXI-4472 board has been used to acquire simultaneously sampled analog inputs with 24-bit resolution. The acquired data were: - Actual position of the horizontal mirror, - Actual position of the vertical mirror, - LDV optical signal level - LDV velocity signal. 3. Non-rotating characterization The blade characterization has been performed in static conditions by forcing it into vibration via an impact hammer and measuring its dynamic behavior with a scanning LDV. Modal analysis has been carried out in the measurement data in order to extract modal parameters (frequency, damping and modal vectors). The impulse imposed by the hammer allowed to excite almost all the natural modes of the structure up to 2kHz. Fig. 3 Scaled FRF sum from the SLDV test (red) and CSLDV output spectrum (blue) This configuration has been chosen in order to have a range of excitation wider enough to evidence the structural response of the blade to the operating excitation, which can be split in two categories of different nature: - Engine order excitation depending on unbalancing and mistuning whose energy is inversely proportional to the frequency but increases with RPM, 0 100 200 300 400 500 600 700 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 Frequency [Hz] LDV output amplitude [dB-dBref 1 m/s] CSLDV SLDV 527
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