Frequency domain decomposition for manual and automated tracking of modes using flight data

When flight tests are performed, no input information are available and therefore Operational Modal Analysis (OMA) approaches can be used to estimate the modal parameters. Nowadays, the main focus for these techniques lays in the possibility of real-time and automatic modal parameter estimates so as to reduce time and costs. In this paper, a new approach for the automated tracking of modes is proposed, based on the Frequency Domain Decomposition (FDD) technique and the Modal Assurance Criterion (MAC). Such methodology has been applied on a third generation aircraft, with the aim of following the evolution of modes shapes and natural frequencies, when the system is subjected to different operating conditions. For the study, firstly a GVT has been carried out, and then different stationary flight configurations have been investigated. The recorded flight data have been processed with both traditional FDD and the proposed automated Frequency Domain Decomposition Tracking technique (FDDT). From the comparison of the obtained results, a good correlation has been observed, suggesting further improvements of the FDDT method for real-time applications. A final study has been carried out, in order to assess the applicability of the FDD technique on pre-flight phases, like the taxi or the engine start segments. In this way the possibility of substituting traditional GVTs with such tests has been investigated. The presented work has been carried out in collaboration with the Experimental Flight Department of the Italian Air Force.