Einfluss des Triebstrangs auf die Schwenkbewegung von Hubschrauberrotoren

In Simulationen der Rotordynamik von Hubschraubern wird meist lediglich der Rotor modelliert und eine ungestörte Nabendrehzahl angenommen (Basismodell). Die vorliegende Simulationsstudie hebt diese Annahme auf. Das Strukturmodell wird um den Triebstrang erweitert, um dessen Einfluss auf die Rotordynamik zu untersuchen (Rotor-Triebstrang-System). Der Vergleich der Eigenformen und -frequenzen des Rotor-Triebstrang-Systems mit denen des Basismodells zeigt, wie der Triebstrang die kollektiven Schwenkmoden modifiziert. Dazu werden die Auswirkungen der Triebstrangträgheit und -steifigkeit identifiziert und voneinander abgegrenzt. Die aeromechanische Simulation des Rotor-Triebstrang-Systems offenbart beachtliche Änderungen in den blattzahlharmonischen Amplituden der Schwenklasten gegenüber dem Basismodell. Weil die Eigenfrequenz der durch den Triebstrang modifizierten, zweiten kollektiven Schwenkmode in der Nähe der Blattzahlharmonischen liegt, bestimmt diese Mode den Triebstrangeinfluss auf die Schwenklasten im stationären Flug. Auch die Anwendbarkeit reduzierter Triebstrangmodelle zur Abbildung dieses Einflusses wird untersucht. Schließlich wird gezeigt, dass die Berücksichtigung des Triebstrangs die Korrelation simulierter Schwenklasten mit Messungen aus einem Windkanalversuch verbessert

Load prediction of hingeless helicopter rotors including drivetrain dynamics

Structural couplings between the flexible main rotor and the flexible drivetrain of the Bo105 helicopter are investigated by numerical simulation. For this purpose, the rotor hub constraint Omega = const. is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. By use of the multibody-software SIMPACK, the coupled rotor-drivetrain system is linearized and the eigenmodes are compared to those obtained with a constrained rotor hub. The drivetrain has a significant influence on the shapes and eigenfrequencies of the collective lead-lag modes. While the first collective lead-lag eigenfrequency is raised by the finite drivetrain inertia, the second is lowered due to drivetrain flexibility. To assess the influence of modeling inaccuracies on the observed couplings, the study is complemented by a sensitivity analysis. Rotor blade mass axis offset, blade pitch (causing elastic coupling) and blade precone angle have only weak influence on the coupled modes. In contrast, variations of drivetrain inertia and stiffness strongly affect the eigenfrequencies of the coupled rotor-drivetrain modes

Rotor Blade Modeling in a Helicopter Multi Body Simulation Based on the Floating Frame of Reference Formulation

The Floating Frame of Reference formulation was chosen to include the Beam Advanced Model in DLR’s Versatile Aeromechanics Simulation Tool. During the development and concurrent testing of the model in the field of helicopter rotor dynamics, some particular shortcomings have become apparent. These mainly – but not exclusively – concern inertial loads affecting the flexible motion of beams. This paper treats the related physical phenomena, and proposes enhancements to the model which remedy the deficiencies of the baseline method. Particular attention is given to the introduction of rotational shape functions to account e.g. for the propeller moment and the consideration of an accelerated Floating Frame of Reference to address the blade attachment’s radial offset from the rotor center in the centrifugal field. Furthermore, the application of external loads (e.g. airloads) away from the beam’s nodes or off the beam axis is addressed as a prerequisite for independent structural and aerodynamic discretization. Finally, the modal reduction under centrifugal loading is considered. The individual model upgrades are verified based on analytical reference results of appropriate rotor dynamics test cases. The enhancements are necessary for simulating flexible helicopter rotor blades within a Multi Body System – a feature required for sophisticated simulation scenarios in which the limitations of conventional rotor models (e.g. constant rotational hub speed) are exceeded

Drivetrain Influence on the Blade Loads of Hingeless Helicopter Rotors

The impact of structural rotor-drivetrain interaction on the blade loads of the Bo105 helicopter is investigated by numerical simulation. For this purpose, the constraint of constant rotor hub speed is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. The structural rotor-drivetrain system is coupled to an aerodynamic model consisting of an analytical formulation of unsteady blade element loads combined with a generalized dynamic wake. A time-marching autopilot trim of the rotor-drivetrain system in wind tunnel configuration is performed for a large blade loading flight state as well as a high advance ratio flight state. The comparison of the simulation results with those of a baseline case (constant rotor hub speed) reveals a major drivetrain influence on the blade lead-lag load harmonics at blade passage frequency. Beside the full drivetrain model, reduced models are shown to be capable of predicting the drivetrain influence on blade loads, if they yield the same eigenfrequency of the coupled rotor-drivetrain mode wRDL2 (second collective lead-lag mode couples with drivetrain) as the full model. In a sensitivity analysis, wRDL2 is varied by modification of the stiffness of a reduced drivetrain model. The resulting changes in blade loads are correlated to wRDL2, which serves as a simple but accurate classification of the drivetrain regarding its influence on vibratory blade loads. Finally, the improvement of lead-lag load prediction by the application of a drivetrain model is demonstrated through comparison of simulated loads with measurements from a wind tunnel test

Drivetrain Influence on the Lead-Lag Modes of Hingeless Helicopter Rotors

Structural couplings between the flexible main rotor and the flexible drivetrain of the Bo105 helicopter are investigated by numerical simulation. For this purpose, the rotor hub constraint Omega = const. is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. By use of the multibody-software SIMPACK, the coupled rotor-drivetrain system is linearized and the Eigenmodes are compared to those obtained with a constrained rotor hub. The drivetrain has a significant influence on the shapes and Eigenfrequencies of the collective lead-lag modes. While the first collective lead-lag Eigenfrequency is raised by the finite drivetrain inertia, the second is lowered due to drivetrain flexibility. To assess the influence of modeling inaccuracies on the observed couplings, the study is complemented by a sensitivity analysis. Rotor blade mass axis offset, blade pitch (causing elastic coupling) and blade precone angle have only weak influence on the coupled modes. In contrast, variations of drivetrain inertia and stiffness strongly affect the Eigenfrequencies of the coupled rotor-drivetrain modes

Drivetrain Influence on the Lead-Lag Modes of Hingeless Helicopter Rotors

Structural couplings between the flexible main rotor and the flexible drivetrain of the Bo105 helicopter are investigated by numerical simulation. For this purpose, the rotor hub constraint Omega=const. is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. By use of the multibody-software SIMPACK, the coupled rotor-drivetrain system is linearized and the eigenmodes are compared to those obtained with a constrained rotor hub. The drivetrain has a significant influence on the shapes and eigenfrequencies of the collective lead-lag modes. While the first collective lead-lag eigenfrequency is raised by the finite drivetrain inertia, the second is lowered due to drivetrain flexibility. To assess the influence of modeling inaccuracies on the observed couplings, the study is complemented by a sensitivity analysis. Rotor blade mass axis offset, blade pitch (causing elastic coupling) and blade precone angle have only weak influence on the coupled modes. In contrast, variations of drivetrain inertia and stiffness strongly affect the eigenfrequencies of the coupled rotor-drivetrain modes

Beam Modeling in a Floating Frame of Reference for Torsion Dynamics of Helicopter Rotor Blades

In the ongoing development of DLRs Versatile Aeromechanics Simulation Tool, an elastic beam model is integrated into the Multibody System based on the Floating Frame of Reference formulation. Although the application of this formulation for one dimensional beam models has already been addressed in the literature, the challenge remains to properly model the torsion dynamics of rotor blades - especially under high centrifugal loads. To this aim, this work suggests the consideration of rotational shape functions in the inertia shape integrals and in the application of gravitational, inertial, and external loads. This modified approach is validated based on the structural analysis of a rotor blade with complex geometrical properties

AERODYNAMIC AND STRUCTURAL MODELING IN THE ROTORCRAFT MULTI-PHYSICS SIMULATION VAST

Comprehensive aeromechanics simulation for rotorcraft is a complex field involving models from different disciplines with very different structure and complexity. VAST (Versatile Aeromechanics Simulation Tool) addresses this field with a new approach involving a generic coupling of state-space models. The paper describes the approach and focuses on aerodynamic and structural methods used in the framework. The implemented models for aerodynamics include unsteady aerodynamics based on a semi-empirical analytical model for the blade sectional airloads and a vortex-lattice model for the computation of the rotor wake. The structural modeling is based on a generic multi-body approach

A New Approach to Comprehensive Rotorcraft Aeromechanics Simulation

A new comprehensive aeromechanics code for rotary wing aircraft is being developed at the German Aerospace Center. It follows a new and very general approach in modeling all physical subsystems and numerical methods in one common interface description. The structure of the code makes no assumptions about the system to be modeled and builds the global system strictly from the logical connections of the sub-models. It relies heavily on modern language features and programming techniques like algorithmic differentiation. This paper describes the novel approach and currently implemented features. While verification and validation are a part of the paper it is not the sole purpose. The calculations serve rather as a means of verifying the general approach and its fitness for the long-term vision of the code. The description of the architectural concept is the main purpose of this paper. The description and evaluation is being underlined with a set of verification and preliminary validation cases

VAST - VERSATILE AEROMECHANICS SIMULATION PLATFORM FOR HELICOPTERS

VAST (Versatile Aeromechanics Simulation Tool) is a new multi-physics simulation environment for the highly complex field of helicopter comprehensive analysis. It is based on a general state-space approach for its physics models and solves the structural mechanics with a multibody approach. The implemented models for aerodynamics include unsteady aerodynamics based on a semi-empirical analytical model for the blade sectional airloads and a vortex-lattice model for the computation of the rotor wake. This paper describes the first version of the code which has been developed in the frame of the DLR Project VicTori