Determination of dynamically equivalent fe models of aircraft structures by using modal test data

Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Mechanical Engineering, Turkey

Approval Date: 2010




Reliable flutter analysis of aircraft structures is a major requirement to determine safe flight envelops. Dynamically equivalent finite element model of an aircraft structure correlating well with experimental modal is a major requirement for a reliable flutter analysis. Currently available model updating techniques require enormous time and engineering work to achieve appropriate finite element models of aircraft structures. The method developed within the scope of this thesis work aims to remove important disadvantages of common model updating procedures. In doing this, the method starts with a simple finite element mesh obtained by connecting measurement points, used in the Ground Vibration Test of an aircraft structure, with 3 D Euler-Bernoulli beam elements. Initial estimates of the geometric and material properties are determined by solving structural identification equations derived from the mass and stiffness orthogonality of experimental modes. By using those initial estimates, an initial finite element model is constructed. Starting from this initial finite element model, structural identification equations are updated and solved iteratively by using experimental natural frequencies and eigenvectors of the v updated finite element model representing the same mode shapes with measured normal modes. Iterations are continued until eigen solution of the updated finite element model closely correlates with experimental modal data. The applicability of the method is illustrated on a scaled aircraft model and a real aircraft structure. The results are quite satisfactory but the method requires further improvements to achieve a much better correlation level in case of real aircraft structures.