Ground Vibration Testing of Large Aircraft - An Overview of DLR Activities

Abstract

Modal identification is a common and necessary procedure performed on large aerospace structures. Aeronautical structures like the prototype of new, large aircraft have to be investigated within a ground vibration test (GVT) in order to validate the analytical Finite Element Model and to enable reliable flutter predictions for flight tests. The dynamics of such aircraft are characterized by their modal parameters like natural frequencies, mode shapes, modal damping and generalized masses. These modal parameters can be obtained using two different philosophies. One of these is the phase resonance method in which modes, which are important within the scope of the certification process, can be identified. On the other hand, not all modes have to be known with the same precision and to the same extent, i.e., including linearity checks. This is where the second method, the phase separation technique, comes in. Nowadays, it is the standard method in modal analysis of large aerospace structures and can save time compared with the phase resonance method. As described above, modal identification within ground vibration testing on large aircraft is more or less standard in order to gather data (modal parameters) which are required within the aeroelastic certification process. This data is also used for the assistance of the flight tests or for the prediction issue with respect to structural modifications. Nevertheless, the scope of requirements of the aircraft manufacturers regarding the test program has increased in the last years in order to cover new issues like passenger comfort, fan-blade-off (wind-milling) or aero-servo-elasticity. Therefore, the test procedures, soft- and hardware and also the test strategy had to be enhanced. In this lecture the theory of modal identification methods is presented in brief. After that the lecture focuses on an advanced test strategy, which is used for the GVTs on large aircraft. Recent developments in terms of identification methods, soft- and hardware applications and a vision with respect to ground vibration testing in the future will complete the seminar. Examples from vibration tests that were recently performed on large aircraft (e.g. Airbus A380) will be used to underline the applicability of the developed procedures.