The structural dynamics of a free flying helicopter in MBS- and FEM-analysis

Abstract

Since Multi Body System (MBS) codes have been proved to be potentially powerful simulation tools in the whole range of helicopter rotor dynamics, here the question of modelling the free flying helicopter in a pure MBS as well as in a hybrid FEMBS dynamical simulation model is highlighted. The objective of this research work are modelling techniques for decribing the dynamical behaviour and the struchtural interaction between helicopter rotors - main and tail rotor - and the nacelle of a free flying helicopter. Here the focus lies on the coupling of the rotating structure of the fully elastic main rotor with the non-rotating parts of the body structure via a flexible rotor-nacelle interface. As simulation platform the 9[to] generic model "Helicopter H9" has been developed. Representing the research object for this investigation it serves as a demonstrator model and as dynamic reference configuration for both the MBS and the FEM calculations. The "Helicopter H9" has a five blade main rotor with a diameter of D=16[m], a four blade tail rotor with a diameter of D=2.8[m] and a MTOW of 9118.4[to]. Investigated are modelling techniques for simulating the dynamics of the structural behaviour of the free flying helicopter in the frequency domain. On the MBS side the commecial tool SIMPACK is tested while on the FEM side the scientific rotor code GYRBLAD is used. For reasons of a better clarification of the rotor-cell coupling effects the center of gravity of the helicopter fuselage exhibits large offsets in all three coordinate directions. As a consequence we get a highly non-symmetrical dynamical system w.r.t the main rotor axis and a rotated principal axes system. By the fact that the main rotor axis does not coincide with any of the three inertial axes all three rigid body rotational modes will be coupled by the main rotor gyroscopic effect. Concerning the specific dynamic coupling effects between rotor and nacelle a survey study with topics like the main rotor suspension (lateral and vertical) or the elasticity of the drive train had been conducted. In systematic variation of the respective stiffness values (over four decades) the results of different parameter studies are presented as numerical results for single constant rotor speeds as well as in frequency fan diagrams for the overall dynamical behaviour under the change of rotor speed. By applying different blade pitch angles the influence of the blade pitch positon on the rotor eigenbehaviour has being tested. By introducing different kinematical and dynamical boundary conditions, cases of stability loss due to ground resonance could be reproduced for the isolated rotor. Even cases of stability loss of the free flying helicopter concerning elastical eigenmodes of the coupeled rotor-nacelle-system - an air resonance type - could be detected in this work. The validation of the models finally was done by comparing the eigenmodes and the eigenvalue results produced with the two elasto-mechanical methods MBS and FEM. Thus different algorithms and independent tools have been used in the examination. It has been shown that for the non-rotating as well as for the rotating test cases the coupling effects will be reproduced without any restriction in both approaches. Thus the potential of a sophisticated MBS code like SIMPACK as a powerful simulation tool for helicopter dynamics has been demonstrated with respect to the dynamics of the free flying helicopter.