Identification of the modal masses of an UAV structure in operational environment

In the framework of Operational Modal Analysis, several methods have been developed to estimate the modal parameters, that is natural frequencies, damping ratios, and mode shapes of a structure in its operative conditions. However, it is not possible to directly estimate the modal masses associated to each mode shape due to the unknown excitation. The modal masses are usually evaluated from the analysis of the change of the modal parameters by testing the structure in correspondence of two mass configurations. In this paper the efficiency and the accuracy of two procedures for the estimate of the modal masses are assessed by performing laboratory vibration tests. Vibration response data recorded during flight tests of an unmanned aerial vehicle (UAV) are used for this purpose as well as a mass changing device was developed to produce the mass variation of the structure. Results from the traditional input/output experimental modal analysis and the operational modal analysis are compared in terms of modal parameters, modal masses, and synthesized frequency response functions.