Dynamic identification of helicopter structures using operational modal analysis methods in the presence of harmonic loading

This paper reports the activities performed to evaluate the effectiveness of a number of selected Operational Modal Analysis methodologies, based on both frequency- and time-domain algorithm, dealing with a class of input excitation composed by a broad band white noise corrupted with harmonic loadings, characterizing the dynamic environment of helicopters. The main objective is to evaluate how this class of excitation (that differs from the pure random noise excitation required by all the Operational Modal Analysis methods) affects the estimate of the modal parameters. Moreover, the sensitivity of the proposed approaches to the harmonic loading to the random energy ratio is investigated. An actual medium class helicopter structure, a time expired Lynx Mk7 airframe located at the University of Bristol laboratory facilities, and an AgustaWestland lightweight helicopter tail are considered for this purpose. The modal parameters of such structures are estimated in the Operational Modal Analysis framework only considering the vibration responses due to several multiple stochastic random and sinusoidal excitations, provided by electro-dynamic shakers. Such estimates are compared against those derived from the more traditional input/output-based Experimental Modal Analysis approach, which is used as a reference to assess the performance of the considered Operational Modal Analysis methods.