Helicopter ground resonace analysis using multibody dynamics

Kurzfassung

Helicopter dynamic response is one of the most important criterions of the helicopter design. For an accurate prediction and calculation of the vibratory responses of the helicopter structure in different flight conditions, an exact modelling is essential. The accuracy of the results returns back to the accuracies of the model and calculation procedures. The multibody dynamic simulation tool allows using modular models to build a complete detailed system. This could be used to model a complex helicopter. SIMPACK (SImulation of Multibody systems PACKage) as a multibody simulation tool has shown considerable ability to model and analyse linear and non-linear systems during different DLR-projects, however use of this tool for modelling the helicopter dynamics is something which is currently being investigated [1], [2]. This tool was originally developed by DLR (German Aerospace Center) and is now further developed and commercially distributed by SIMPACK AG. In this work the multibody simulation of the ground resonance effect is investigated. For this purpose, the interface of SIMPACK with a FEM code for modelling the elastic rotor blades is considered. This interface creates a Standatrd Input Data, SID, based on blade geometry and eigenvalue analysis. The SID-file is then used for the calculation of elastic deformation. SIMPACK provides also an interface with MATLAB, which allows performing a part of postprocessing within this program. Fig.1 shows the general view of the ground resonance analysis using multibody dynamics simulation. Helicopter ground resonance is a self exited dynamic instability, which may occur when helicopter is in contact with the ground. Elastic deformation and out of phase lagging motion of the rotor blades during rotor rotation lead to the oscillation of the rotor centre of gravity around the rotor rotation axis. Due to the interaction between rotor and fuselage, this oscillation excites the flexible fuselage and the rotor support and may cause a violent damage of the helicopter structure. This phenomenon is known as ground resonance. Modelling this phenomenon needs consideration of the following points: Linearisation, Geometric stiffening effect, Time-periodic system, Multiblade coordinates transformation. The work described within this paper is generally divided into three main parts. The first part is related to the “linearization and geometric stiffening effect”, which is a preliminary step for the dynamic stability analysis of a rotating system. Here, the simulation results are compared with analytical results. Creating the Fan-Diagram of an isolated rotor and comparison of the results with those produced with CAMRAD II belongs also to this part. The second part deals with the helicopter ground resonance analysis. Modelling the ground resonance starts with a simplified model (rigid blades with mechanically equivalent elastic lagging deformation and fuselage with two translational degrees of freedom) and is continued with more complex models comprising elastic blades and fuselage. Data used in this part belongs to DLR scaled (1:2.5) BO105 research rotor model (configuration K20) [3] and CAMRAD II model of full scale BO105 with modified skid gear provided by Eurocopter Germany. One of the features of the ground resonance model is its time-periodic characteristic, which leads to the invalidity of the direct usage of the classical eigenvalue method for the dynamic stability analysis. To perform the ground resonance analysis, the model is first linearized about the equilibrium state. Then the linear system matrix of the model is extracted and transformed from the rotating coordinates to the non-rotating multiblade coordinates. This coordinate transformation changes the system from a time-periodic to a time-invariant one. Finally, a classical eigenvalue analysis of the transformed system matrix results in the frequencies and damping of the system. To evaluate the simulation procedures, the SIMPACK results are compared with results produced with CAMRAD II model. To validate the model and calculation procedures in addition to a comparison with the CAMRAD II-results an analytical validation of the dynamic response of the system is also performed. The third part of the work deals with the parametric studies of the ground resonance effect and analysing the results and tendencies of the changes of the dynamic response.