Improvement of Helicopter Fuselage Modelling - Effects of subcomponents on the overall dynamic behavior

Kurzfassung

Like all aircraft, helicopters are designed to be as light as possible for the sake of low operational cost and minimum environmental footprint. Besides this lightweight construction, interaction between the rotor-blades and the air lead to strong oscillatory loads which are then introduced into the airframe of a helicopter. After significant improvement of aeromechanical modelling of helicopter rotors in the past decade, the loads can be simulated nowadays with reasonable accuracy. However, the ability to predict helicopter vibration using the simulation models of the fuselage is limited. The complexity of the helicopter fuselage structure poses challenges to both, experimental and numerical modal analysis. To achieve reasonably accurate forecast of the propagation of vibrations from the main rotor hub to different locations of interest an improvement of the simulation models is necessary. A shake test on a fully assembled helicopter is a too complex validation experiment for the targeted improvement of the simulation models. In order to address this inherent limitation, an extensive measurement campaign was conducted in the H135 helicopter production line in 2022. The vibration test team of the DLR-Institute of Aeroelasticity cooperated with vibration specialists from Airbus Helicopters Germany to plan and conduct ten modal tests on one helicopter successively built at different Assembly Stations. The modal tests took place during ongoing production operations. In conjunction finite-element models which represent the structural condition at each of the 10 production stations were created. The finite-element models of the respective helicopter components and subassemblies are used in this study to gain a better understanding of the influence of subcomponents on the overall dynamic behavior. In an experiment, a specific component can either be installed or not. Depending on stiffness and inertia properties of the components, the changes in dynamic behavior can be considerable and changes in mode shapes and eigenfrequencies are hard to track in experimental data. In numerical simulation, however, a smooth "blending in" of components is possible enabling the tracking of changes of modal parameters when installing specific components to a helicopter subassembly. This new approach to finite-element model updating is presented here. In this, stiffness and mass values are gradually increased (from 5% to 150% of the nominal value in small steps of 1%) for entire components that get mounted at the respective Assembly Station. A numerical modal analysis is performed after each alteration. The alterations and their influence on eigenfrequency and mode shapes are tracked over the different stations. This approach seems promising for the use on Helicopters (and complex structures in general) and could also be used for the development of new prototypes. The approach presented here can be considered as a pre-conditioning step of the corresponding finite-element models, to be applied in case of deviations between test and analysis being too large to conduct sensitivity-based finite-element model updating.