Wolf Krüger

AEROELASTIC EFFECTS IN MULTIBODY DYNAMICS.

To enable a realistic assessment of the aeroelastic phenomena of aircraft, a simultaneous application of computational fluid dynamics (CFD), computational structural mechanics and flight mechanics has to be performed. Each discipline has developed powerful specialized tools which have to be adapted for multidisciplinary applications. The combination of CFD and elastic multibody systems is well suited for the simulation of a range of aircraft applications, especially for aircraft ground dynamics. Approaches to a coupling of elastic multibody systems and computational fluid dynamics have been performed using close coupling, that is a modal approach, and loose coupling, that is by co-simulation. In the article the applied programs and the coupling methods are presented. Advantages and limits of using multibody simulation as compared to the direct use of FEA methods for the representation of structural dynamics are discussed. Results of coupled steady and unsteady simulations are presented. Finally, an approach to the aeroelastic trim problem is shown.

CFD-based Gust Load Analysis for a Free-flying Flexible Passenger Aircraft in Comparison to a DLM-based Approach

Gust load analysis is a relevant part of the certification process of aircraft. In need of low computing times, industrial analysis procedures often rely mainly on low-fidelty numerical aerodynamics methods, such as the Doublet Lattice method (DLM). However, their accuracy with respect to loads has not been assessed sufficiently in comparison to high-fidelity methods in the past. In this paper, simulation results of a classical DLM-based gust load analysis process are compared to the results of an alternative process in which the DLM solver is replaced by the CFD solver TAU. The investigation is performed with a realistic modern passenger aircraft. It is studied how the consideration of different levels of multidisciplinarity in the simulations affects the overall loads. The simulation methodologies exploited in the CFD-based analysis process are outlined. It is shown that the gust load factors predicted with CFD are significantly lower than those of the classical DLM-based process, not only in the transonic flow regime, where benefits from the CFD-based analysis are expected, but also in the subsonic flow regime.

Design and Simulation of Semi-Active Landing Gears for Transport Aircraft

ABSTRACT This paper describes an integrated design process for control laws for semi-active aircraft landing gears, addresses interfaces required for the implementation of different computer aided engineering (CAE) tools used in the process, and presents a multibody aircraft simulation model. Three control laws, a skyhook-type controller, a fuzzy-logic controller, and a state feedback controller, are designed, and the control parameters of all three controllers optimized using a multi-objective parameter optimization. Finally, simulation results for passive, fully active, and semi-active systems are compared. *Communicated by D. Bestle.

Multibody Simulation of the Free-Flying Elastic Aircraft

Multibody simulation is a widely used software tool to simulate the dynamic behavior of an aircraft during maneuvers, landing impacts and other highly dynamic sequences. To enable a realistic assessment, computational fluid dynamics, computational structural dynamics and flight mechanics have to be applied simultaneously. The multibody environment serves as a “virtual testbench” to investigate the overall system, or major components, in their interdisciplinary interaction. This work presents a multibody simulation-based analysis and simulation framework to study the free-flying maneuvering elastic aircraft, concentrating on the underlying problem of adequate representation of aerodynamics in a hybrid multibody system. Three interface levels coupling aerodynamic analysis and multibody simulation are described and illustrated by an application example.

Coupling Strategies for Large Industrial Models

In this article an investigation of possible coupling strategies for large numerical models in fluid-structure interaction is given. The focus is on the development and the assessment of a simplified approach that uses existing and well verified scattered data interpolation methods. The interpolation problem is addressed by a pragmatic partitioning approach of large models. Analysis of the interpolation of deflections between different discretization of the coupling models are performed, together with comparisons of static fluid-structure simulations with measured data of the elastic DLR-F12 wind tunnel model. The loads transfer between CFD mesh and FE model for different partitioning schemes is performed and assessed, finally some considerations for the use of the suggested strategies on large models are presented.

An Enhanced Modal Approach for Large Deformation Modeling of Wing-Like Structures

A new modal-based method that captures the geometric nonlinear effects that arise in the regime of large deformations of wing-like structures is presented. The most limiting factors of the modal approach are the linear force-displacement relationship and the representation of the nodal displacement field based on normal modes. The proposed extension includes stiffness terms that cubically depend on the generalized coordinates. The structural deformation is calculated not only by normal modes but also by higher order mode components that account for the foreshortening effect at beam-type structures. The approach is applied to a cantilever slender wing. Static and dynamic results are presented together with results from a commercial finite element solver and from the UM/NAST aeroelastic solver from the University of Michigan. The numerical study highlights the capability of the new approach to capture nonlinear effects while keeping the simplicity of the modal approach.

Multi-Fidelity Simulation of Cargo Airdrop: From the Cargo Bay to the Ground

Future military transport aircraft (FMTA) have to cope with the increasing demands in tactical and strategic airlift. Within the DLR-internal project MiTraPor II simulation and assessment tools have been developed that can be used as a virtual test bed for the evaluation and optimization of new technologies and procedures applied to military transport aircraft. This paper gives an overview over the three different approaches that have been developed for the simulation of cargo airdrop, namely a flight dynamical simulation of the entire airdrop sequence, a high-fidelity CFD-simulation accounting for air-flow influence in the near field of the aircraft, and a multi-body simulation for the evaluation of the impact on the payload during landing.

Multibody analysis of whirl flutter stability on a tiltrotor wind tunnel model

Tiltrotor aircraft are equipped with large rotors, usually driven by heavy engines in nacelles located at the tip of the wings. This combination of large rotors and high masses on wings which form a relatively elastic support makes whirl flutter a critical phenomenon for the design of the aircraft and its performance. In this article, a multibody-based simulation model of a tiltrotor wind tunnel model is presented, combining a mixed finite element/multibody representation of the support, a detailed kinematic model of the rotor hub and a strip-wise definition of air loads on the blades. Investigations on the effect of parameter changes and non-linearities on the calculation of the aeroelastic stability boundary are executed. The selected operational points are taken from wind tunnel experiments performed by a consortium of partners in the course of the European ADYN project. The regarded wind tunnel model is a half model of a tiltrotor wing, simplified for the use in whirl flutter investigations. The rotor is four-bladed; the rotor hub is of a gimbal type. Structural dynamics properties of the model are known from detailed experimental investigations. Numerical analyses of the dynamic behaviour of the model are performed in the frequency and in the time domain. Results show considerable differences for linear and non-linear models. This paper describes the simulation approach, the model setup and the comparison of the data with selected experimental results.

Flight Loads Analysis and Measurements of External Stores on an Atmospheric Research Aircraft

The research aircraft DLR HALO (High Altitude and Long Range Research Aircraft) is able to carry external stores which are attached a t wing hardpoints. The external stores are designed to house several measurement instruments for atmospheric research. However, each modification on the aircraft has to be investigated with numerical analyses and / or experimental data to ensure the structural integrity of airframe and stores. The DLR project iLOADS aims at the development of an internal DLR loads process and being able also to support certification capabilities for the DLR aircraft fleet. To assist the DLR HALO operations, a simulation model of the aircraft was set up and loads analyses have been carried out in the Institute of Aeroelasticity at DLR Göttingen. For the experimental part, flight tests with DLR HALO with 14 flying hours in total have been performed. In the flight tests strain data of wing external stores, acceleration data of installed sensors and turbulence data were collected. First analyses have been carried out and the findings can be utilized in the further development of the DLR loads process.

Aeroelastic Simulations of High Reynolds Number Aero Structural Dynamics Wind Tunnel Model

This article presents results of numerical simulations of the High Reynolds Number Aero Structural Dynamics wind tunnel configuration. The activities consist of the selection of relevant transonic test cases, the setup and simulation of the numerical models, and the comparison of computational and experimental results. Two steady (static coupling) and three unsteady (forced motion) test cases were selected, all in the transonic flow regime at Mach numbers up to 0.85 and Reynolds numbers up to 23 million. Good agreement between numerical and experimental data was obtained for both the steady and the unsteady test cases. It is shown that the consideration of the static elastic deformation of the wing is indispensable for both the steady and unsteady simulations. The unsteady simulations were executed in such a way that the elastic motion of the wing in a selected structural eigenmode was prescribed in a forced motion computational fluid dynamics simulation using grid deformation. This approach yields very s...