Herman Van der Auweraer

Ground Vibration Testing in the Aeroelastic Design and Certification of a Small Composite Aircraft

†† Recently, a new composite four-seater aircraft was developed. As soon as the first prototype became available a Ground Vibration Test (GVT) was carried out. The aim of the GVT was to obtain an experimental structural dynamics model of the aircraft. Hereto, two shakers were exciting the aircraft with broadband noise, the Frequency Response Functions (FRFs) were computed from the force and acceleration signals and, finally, the aircraft modes were identified by applying the new PolyMAX modal parameter estimation method to the FRFs. Before performing flight tests, it is desired to predict analytically the flutter behaviour. Since no detailed Finite Element Model (FEM) of the aircraft was available, the experimental modal model from the GVT was used for this purpose.

Experimental modal analysis using camera displacement measurements: a feasibility study

Recently, a mobile coordinate measurement machine consisting of three CCD cameras was expanded with dynamic measurement capabilities. The system is able to track three LEDs in three directions with a maximum sampling frequency of about 1000 Hz. This offers the possibility to use the measurements for dynamic system identification. To this aim, a vibrating structure is equipped with multiple lightweight infrared LEDs and excited by dynamic excitation sources. Transfer functions between force and displacements are estimated from which the modal parameters of the structure can be identified. The benefits of using the camera displacement measurement system is that information down to 0 Hz can be obtained, that mounting LEDs is much easier than installing traditional displacement transducers (LVDTs) and that the coordinates of the measurement points are also available from the measurements. In this paper a dynamic testing study is performed using a scale model of an airplane. Both LEDs and accelerometers are mounted on the structure allowing a comparison between displacement and acceleration transfer function modal analysis results. The main conclusion is that it is possible to successfully identify the modal parameters of the airplane scale model from the displacement measurements in the complete frequency range of the excitation.

CORRELATION OF MEDIUM FREQUENCY TEST AND FE MODELS FOR A TRIMMED AIRCRAFT TEST SECTION

In the context of the European co-operative research project ASANCA II. methods for deriving a global vibro-acoustical model of the aircraft fuselage-cabin cavity system are investigated. Firstly, an extensive experimental study was performed on a trimmed SAAB 340 test section. A full frequency response function matrix was measured between a large number of structural and acoustical degrees of freedom. Based on these data, a non-parametric (vibro-acoustical principal field shape) system model was identified. Furthermore, a full FE model was derived for the SAAB 340 acoustic test section. First a model of the fuselage structure was formed and evaluated by means of eigenvalue analysis. Based on prior test results this model was updated. The remaining parts of the fuselage section, containing the cabin air cavity, trim panels and the material between the cabin structure and trim panels, was formed into a second model. This model was also analysed by means of eigenvalues followed by connecting the two (sub-) models, by means of modal coupling, to form the complete description of the cabin section. For this model, the frequency response functions were evaluated at the nodes corresponding to the experimental analysis. Finally, a correlation analysis between the FE and the test model was performed. Hereto, direct FRF correlation between the FE and test models, as well as a correlation of the principal field shapes corresponding to both data sets was executed.

USE OF MULTIBODY DYNAMICS AND EXPERIMENTAL IN- FLIGHT DATA FOR LOAD ANALYSIS AND DESIGN OPTIMISATION OF AIRCRAFT COMPONENTS

In aerospace industry, safety is a crucial concern affecting the design of critical structural components. Yet, the design of safe components entails a number of compromises that have to be taken to cope with the difficulty of meeting technical requirements leading to opposite design solutions. In particular, structural integrity poses a challenging task in finding the optimal balance between conflicting needs such as structural strength and weight reduction. Industry and research institutions are dedicating great efforts in the development of new techniques to improve reliability of aircraft components. However, improving durability without any loss in safety and functionality is a multifaceted research task that encompasses several different research topics ranging from the structural dynamics to material properties and from load identification to fatigue analysis. Such interdisciplinary competence is not always available with single experts and a strong effort is to be placed to fasten together competences of different research teams and identify the common optimal solution. In this paper, we have partly reproduced the design process requested for the optimisation of an aircraft component. A variety of different problem analyses are performed in a unique software environment 18 . This allows focusing on the overall optimisation goal without dispersion of time and resources in transition of data format or endless discussions among different teams to conciliate conflicting solutions. A slat track of an Airbus A320 is studied to the aim of assessing the margins for design improvements with special attention to durability and weight reduction. The design improvement process is described starting form the prototype testing to the identification of the locations on the component where fatigue damage could occur and the assessment of the impact of weight reduction to the component durability. In the Motion module of LMS Virtual.Lab ® a multibody model of the slat mechanical system was made to calculate the internal loads acting on the slat track for several boundary conditions related to the slat setting angle. Resulting loads are validated by a comparison with loads measured with an experimental test campaign of the aircraft and a FEM numerical simulation. From the complete load time history, time segments where critical loads appear are selected, extracted and given as input to the Fatigue module in order to identify crack initiation locations on the slat track. In these locations, local stress tensor histories are computed and suitably processed for further investigations of crack propagation. This allows estimating the remaining life of the component and assessing whether or not weight reduction can be achieved.