Model Updating and Determination Of Structural Dynamic Properties In The Presence Of A Second Source Of Excitation

Predictions of the dynamic behaviour of helicopters usually involve finite element (FE) models of the structure. These can be easily modified to determine the effect of different stores configuration or structural modification. The FE models are necessarily simplified models and are often validated or updated on the basis of experimental data. Experimental modal testing of a grounded helicopter or a helicopter suspended from the rotor hub neglects the effects of rotating components, for example the main rotor, and aerodynamic loading. Therefore it is preferable to obtain accurate experimental data during flight. This paper discusses the use of experimental data to update an FE model of a helicopter-like structure. In order to examine the feasibility of obtaining dynamic properties of a helicopter during flight, periodic impulsive excitation and synchronous averaging in the presence of a second source of excitation was applied to a simply supported beam. Model updating has been found to increase the accuracy of dynamic properties predicted by the FE model. Periodic impulsive excitation and synchronous averaging has been shown to improve the quality of modal properties extracted by experimental modal analysis in the presence of a second source of excitation.  Copyright of this paper retained by authors. Compilation copyright Commonwealth of Australia. INTRODUCTION The accurate prediction of structural dynamic properties of helicopters is a critical requirement in effective design and modification of helicopter structures. Experimental and analytical techniques are currently used to construct models describing the dynamic properties of helicopter structures. These models can then be used to aid the prediction of component loads, which in turn can be used for calculation of component retirement times. The models also serve as powerful tools when trying to prevent adverse dynamic behaviour resulting from non-standard flight regimes or modification of the helicopter structures. Finite element (FE) models are often used in the design process of helicopter structures, however, it has been shown that dynamic analysis carried out on these models produces results of limited accuracy when compared to experimental data[1, 2]. Some factors that contribute to poor accuracy include the difficulty of modelling some components, for example welded joints, as well as the accuracy of element parameters used in the FE model. In addition, the time and computational power required to solve complex models can often be prohibitive, consequently simple accurate models are most desirable. Nevertheless, FE models present several advantages over experimentation. Experimentation is usually case specific and therefore many experiments are required to obtain results that fully describe the behaviour of the helicopter structure in a number of configurations. Furthermore, experiments are often expensive, time consuming and in some cases not practical due to safety issues or availability of a suitable test platform. A solution that aims to improve the accuracy of FE models involves the use of experimental results to modify or update the FE models. This can be carried out manually by using engineering judgement to modify model parameters. Recently, a number of automated techniques have been developed based on optimisation algorithms [3-5] allowing a more systematic model updating procedure to be carried out. Model parameters are modified within physically realistic bounds such that the dynamic behaviour determined by the FE model correlates with that obtained from experimental results. Validation of the model updating procedure can be achieved by testing a modified physical structure and comparing results with those obtained from a similarly modified FE model. A successful outcome to the