Fig. 4 Laser path synchronized with the blade rotating at ωr The blade dynamics has been characterized in rotating conditions at different speeds, from about 2 up to 25 rotation per second [RPS]. The measurement has been performed with TLDV with the laser beam pointed to the tip right corner. By observing the built-up spectrogram, Fig. 5, it can be noticed the second order (parabolic) dependence of the resonance frequencies on the speed, caused by the gyroscopic force. This gyroscopic effect is more marked for resonance frequencies corresponding to natural modes with 10 NL, (red lines) rather than modes with 01 NL (black lines), as it has been previously pointed out. Fig. 5 Stepped run-up spectrogram zoomed around the first and second natural frequencies (left) and around the third and fourth ones (right) The same has been done by using TCSLDV. The built-up spectrogram is shown in Fig. 6 together with a zoom around the third and fourth resonances. In the last plot the characteristic sidebands are evident: notice that the scanning frequency in ydirection is varying, between 3 and 14 Hz, with the rotation speed in order to optimize the CSLDV output. Therefore the ysideband spacing (see mode at 155 Hz, black line in Fig. 6) changes. Another parameter that has been varied at the different speed was the LDV sensitivity that was of 25 mm/s/V below 11 RPS, 125 mm/s/V between 12 and 21 RPS and 1000 mm/s/V at 14 and 22 RPS. The reason of the need to set the sensitivity at the lowest value at 14 RPS was related to the presence of a resonance of the rotor support that accidently correspond to the II mode of the blade, see Fig. 6, left. To complete the characterization of the blade behavior in rotating conditions the ODSs have been extracted from the CSLDV data. Moreover it has been performed a TLDV test at each rotation speed over a grid of 7x10 points on the blade. The fourth ODS recovered from TLDV and CSLDV data measured at the speeds of 5, 10 and 20 RPS is reported in Table 2. ODSs recovered from TLDV data are given only in terms of amplitude because during the scanning it has not been used a phase reference. 2 4 6 8 10121416182022 5 10 15 20 25 30 35 40 45 50 Speed [RPS] Frequency [Hz] 2 4 6 8 10121416182022 60 80 100 120 140 160 Speed [RPS] Frequency [Hz] y y y y 529
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