Operational Modal Analysis of a Helicopter UAV from Flight Tests

The Operational Modal Analysis (OMA) could be used for the identification of the dynamics of aerospace vehicles, in terms of natural frequencies, damping ratios and mode shapes, during flight tests, using only the output responses due to the aerodynamic envinronment. The loading acting on the flying helicopter is composed by the contributions of both the white noise excitation and the aerodynamic harmonics acting at discrete operational fre-quencies equal to the multiple of the fundamental angular velocity of the rotor. As a result, the system could not be considered excited by a pure white noise input and then the OMA methodologies could not be theoretically applied to the output responses to estimate the dynamical behavior of the helicopter. In this paper an approach is developed to cope with this problem. First, a method based on the statistical index, called Entropy, is introduced to identify such op-erational frequencies by means of a statistical characterization of the responses of the structure due to deterministic input superimposed to stochastic noise. Then, the effects of the removal of the previous identified harmonic contributions from the Frequency Response Functions (FRF) gained, in turn, by applying the procedure known in literature as Hilbert Transform Method, HTM, will be evaluated. In order to assess the overall accuracy in the modal parameter esti-mates, an experimental investigation, carried out on a helicopter UAV flying at different condi-tions, i.e. in- And out-of-ground effect hovering and forward flight at several advance ratios, will be presented. Copyright © (2011) by International Operational Modal Analysis Conference (IOMAC) All rights reserved.