PSI - Issue 37

Aleksey Mironov et al. / Procedia Structural Integrity 37 (2022) 241–249 Aleksey Mironov, Pavel Doronkin / Structural Integrity Procedia 00 (2019) 000 – 000

244

4

diagn ostic parameter considers the blade’s modification in any j th state summarizing the difference between computed comparable and known reference values of all modal parameters ± 2 = 1 ∑ | ̂ − ̂ 0 ± 2 | + ∑ |( ̂ − ̂ 0 ± 2 ) | + ∑ | ̂ − ̂ 0 ± 2 | (3) where , , – confidence intervals of modal parameters; Σ – integral confidence interval, c – constant, depending on the confidence level, M – number of modes. For practical application the reference values ( ̂ 0 , ̂ 0 , ̂ 0 ) can be calculated from the data measured, when the blade is initially mounted on a helicopter. Also the [ AMPV ] threshold for typical passport is determined experimentally when testing blades with the seeded faults. Periodically estimated AMPV parameters allow monitoring by calculating the trend against operating hours. The critical state of the specific blade may be predicted comparing the trend with the [ AMPV ] threshold. With this approach, the permanent monitoring of helicopter blade becomes possible in flight at any operation mode. 2.2. MP trial application to helicopter composite blade Mironov A. & Mironovs D. (2019b) applied the MP concept experimentally to the operating composite blade. The rotating data acquisition unit (mounted on the rotor hub) measured signals of embedded dynamic deformation sensors (piezo-films) spread out on the blade. The most usable OMA techniques, including Enhanced Frequency Domain Decomposition (EFDD), Canonical Variate Analysis (CVA) and Unweight Principal Component (UPC) provided the calculation of modal parameters. Computed from experimental data the modal shapes and frequencies allowed further modal identification. In the considered study this approach allowed to estimate the influence of random factors and seeded faults on the AMPV parameter of the blade. Thus, this trial application of MP techniques has identified the issues of OMA application to rotating helicopter blade. The application of OMA technique needs the object to be excited uniformly in the determined frequency range, however frequency structure of aerodynamic and mechanical loads is uneven. It is necessary to approximate blade testing conditions to OMA required ones. The typical MP of the blade must take into account the dependence of modal properties on main operational factors (rotation speed, temperature, etc.). The balance should be found between the desired resolution scale, cost and technical limitations of the measurement system should be found. External factor influences on modal parameters of a blade while testing. The optimal data development procedure has to reduce uncertainty of modal estimates. To overcome the above problems, a set of technical and methodical solutions were found and applied on the SHM demonstrator. 3. Technical basis of the demonstrator The light coaxial helicopter Ka-26 presented by Mironov et al. (2021a) became the operating platform for experimental study of the MP application to rotating blades. Each rotor blade has 20 dynamic deformation sensors (fig.1a) by 10 on each top and bottom surface. Such type of sensors was chosen because of its low cost and compatibility with typical measurement equipment applied for piezo transducers. Flexible cable connects all 60 sensors of the upper rotor to rotating Data Acqui sition Unit (DAU) that is mounted on the rotor’s shaft as it is shown on fig.1c. There are also four piezo- film sensors on the rotor’s shaft also connected to DAU and allowing dynamic behavior monitoring of the shaft. The rotating DAU includes the switchboard for 94 channels, the 12-channel DAU, the battery for autonomous power supply and the wireless data transmitter. Each engine and main gearbox have one three-axial transducer for studying the vibration structure, which additionally excites the rotor blades. These transducers are connected to another DAU (not rotating) that is located in the helicopter cockpit (fig.1b). Operational