Comparison of In-Flight Measured and Computed Aeroelastic Damping: Modal Identification Procedures and Modeling Approaches (original) (raw)
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Automatic Operational Modal Analysis for Aeroelastic Applications
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Methods of Modern Aircraft Aeroelastic Analyses in the Institute of Aviation
Journal of KONES, 2018
The aeroelastic phenomena analysis methods used in the Institute of Aviation for aircraft, excluding helicopters, are presented in the article. In industrial practice, a typical approach to those analyses is a linear approach and flutter computation in the frequency domain based on normal modes, including rigid body modes and control system modes. They are determined by means of the finite element method (FEM) model of structure or a result of ground vibration test (GVT). In the GVT case, relatively great vibration amplitudes are applied for a good examination of a not truly linear structure. Instead or apart from the measure of generalized masses, a very theoretical model is used for mode shapes cross orthogonality inspection and improvement. The computed or measured normal mode sets are the basis for flutter analysis by means of several tools and methods, like MSC.Nastran and ZONA commercial software as well as our own low-cost software named JG2 for the flutter analysis of low sp...
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Flutter is a dynamic instability caused by the interaction of the structural dynamics of the aircraft and unsteady aerodynamic forces. It occurs when one of the elastic modes of the airframe tends to negative damping above a critical speed. Prediction and flutter clearance are important problems in the design, testing and typecertification of sailplanes. From a practical point of view, Flight Vibration Testing (FVT) needs to be performed whenever a new sailplane is built or an existing type is modified. The application of Operational Modal Analysis (OMA) methods may provide improvements to identify modal parameters of multiple modes in one step without knowing the type of external excitation. If the identification is repeated for increasing flight velocities, it is possible to find the aeroelastic damping trends and to extrapolate to the stability boundary. The application of OMA needs a broadband excitation spectrum, which result from impulsive control kicks or continuing random ex...
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Flutter analysis is a major requirement to predict safe flight envelops and to decide on flutter testing conditions of newly designed or modified aircraft structures. In order to achieve reliable flutter analysis of an aircraft structure, it is necessary to obtain a good correlation between its finite element (FE) model and experimental modal data. Currently available model updating methods require construction of a detailed initial FE model in order to achieve convergence of the modes obtained from updated FE model to their experimental counterparts. If the updating procedure is not carried out by the original design team of the aircraft structure but a subsidiary company that makes certain modification on it, construction of an appropriate initial FE model from scratch becomes a tedious task requiring considerable amount of engineering work. To overcome the foregoing problem, this paper presents a new method that aims to derive dynamically equivalent FE model of an aircraft structure directly from its experimental modal data. The application of the method is illustrated with two case studies. In the first case study, the performance of the method is tested with the modal test data of a benchmark structure built to simulate dynamic behavior of an airplane, namely GARTEUR SM-AG 19 test bed, and very satisfactory results are obtained: the first 10 elastic FE modes of the test bed closely correlate with experimental data. In the second case study, the method is applied to the modal test data obtained from ground vibration test (GVT) of a real aircraft. In this application, it is observed that only the first 4 modes of the resultant FE model correlate well with experimental data. It is concluded that the method suggested works perfectly well for simple structures like GARTEUR test bed, and it gives quite promising results when applied to real aircraft structures.
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An unmanned aircraft serves as a testing platform to demonstrate the benefits of active flutter suppression. The mathematical model representing the structural dynamics and aerodynamics of the demonstrator aircraft is described. Based on flight test data the rigid body model is updated in two steps. At first the aircraft states and sensor measurements are reconstructed. Subsequently, the output error method is used to estimate the desired aerodynamic parameters. The mathematical model with updated parameters appropriately describes the longitudinal and lateral aircraft dynamics. Furthermore, a comparison of the aeroelastic mode shapes of the derived model shows good agreement, which is even improved when updating aerodynamic damping values obtained from flight tests.