IFASD2024 Paper Submission & Registration
17 – 21 June 2024, The Hague, The Netherlands

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13:30   Rotary aeroelasticity 1
Chair: Jurij Sodja
30 mins
Whirl flutter analysis using linearized propeller transfer matrices
Christopher Koch
Abstract: The transfer-matrix (TM-) method for propeller whirl flutter analysis is based on frequency-domain transfer functions from propeller hub motion to loads, which are identified from time-domain propeller models. These transfer functions can be nonlinear with respect to frequency, e.g., due to unsteady aerodynamics. This paper presents a simplified version of the TM-method based on linearized transfer function compatible with legacy whirl flutter workflows based on propeller derivatives. It will present a comparison of whirl flutter results with a full aircraft configuration obtained using either fully frequency-dependent or linearized transfer matrices using various aerodynamic methods.
30 mins
Influence of blade elasticity and thrust on the whirl flutter stability of a propeller-driven aircraft
Julia Noël, Christopher Koch, Bernd Stickan, Hans Bleecke, Jürgen Arnold
Abstract: Whirl flutter stability is an important certification criterion for propeller-driven aircraft. This stability is so far verified by introducing analytical propeller derivatives in frequency-domain flutter analysis. These derivatives are based on the assumption of rigid blades though [1]. Recent studies on an isolated propeller model with a new method using identified, frequency dependent transfer matrices for the propeller hub loads have shown that including blade elasticity in the analysis increases whirl flutter stability significantly [2]. However, this effect has not been examined on full aircraft level yet. This paper extends the transfer-matrix (TM-) method [3] to enable its application on complex aircraft models. To decouple the specific propeller model used for transfer-matrix generation from the structural model of the airframe, a few adaption procedures for the transfer matrices are introduced. These include eliminating the propeller mass influence from the transfer matrices as well as aligning the propeller orientation with the coordinate system definition of the structural model. Furthermore, an interpolation routine for the transfer matrices is introduced, reducing computational effort. Frequency-domain flutter analyses of a generic twin-turboprop aircraft are performed to demonstrate the introduced adaptions and evaluate the influence of blade elasticity and thrust on whirl flutter stability. The evaluations successfully show that the stabilizing effect due to blade elasticity also occurs on full aircraft level (see Fig. 1). Even though, it is observed that this stabilizing effect has a limit for very soft blades, the results reveal an additional flutter stability margin, that can be exploited for future aircraft designs.
30 mins
Research shake test on an Airbus helicopter technology demonstrator
Julian Sinske, Marc Böswald, Martin Tang, Ketih Soal, Johannes Knebusch, Carsten Thiem, Ralf Buchbach, Muhittin Altug, Oliver Dieterich
Abstract: This abstract comprises the work of the research shake test on an AIRBUS Helicopter full scale technology demonstrator. The test on the entire helicopter was performed by the vibration test team of the DLR-Institute of Aeroelasticity within three weeks in March/April 2023 at AIRBUS Helicopters Deutschland GmbH (AHD) in Donauwörth, Germany. In addition to the shake test, a ground resonance test with the landing skid on ground was also carried out in parallel for each main test configuration. For the shake test the helicopter was soft suspended at the main rotor hub in a dedicated test rig with a pneumatic spring system (Figure 1). The existing helicopter test rig of DLR with the associated pneumatic air suspension was further developed and completely redesigned for this test. These modifications were designed to enable fast changes between different test boundary conditions, i.e. helicopter suspended for shake test and landing skid on the ground for ground resonance test. The rotor blades were removed for the test and the main rotor head was replaced by a mechanical adapter with rotor cross to introduce force and moment excitation from electro dynamic shakers installed on vibration isolation in the test rig. The purpose of the shake test was to test new methods, hardware and to provide the necessary experimental data to achieve permit to fly of the helicopter demonstrator and to adjust the numerical models of the structure. The results of the shake test therefore mainly comprise experimental modal parameters and frequency response functions for equivalent excitation on main rotor hub and on tail rotor. For the first time in such a test, the equivalent force and moments on the main rotor hub were calculated online from a real-time controller and were recorded together with the other response signals. An equivalent modal substitute model was identified from the shake test consisting of 55 unique modes. This modal model was established with a dedicated software and database which can be used for visualization and correlation of modal analysis results, but also for the analysis of scatter and nonlinear trends in modal analysis results. The paper will focus on the tools and procedures applied during the tests, whereas the test results are subject to confidentiality and cannot be presented in detail.
30 mins
Improvement of Helicopter Fuselage Modelling - Effects of subcomponents on the overall dynamic behavior
Johannes Knebusch, Keith Soal, Tobias Meier, David Meier, Muhittin-Nami Altug, Renata Nepumoceno-Merce, Oliver Dieterich, Marc Böswald
Abstract: Update: Approval by Airbus Helicopters is now granted and the extended abstract is uploaded. 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, strong oscillatory forces are introduced into the airframe of a helicopter due to rotor blade control. After significant improvement of aeromechanical modelling of helicopter rotors in the past decade, the loads can be simulated with reasonable accuracy. However, the predictive capability of helicopter vibration using the fuselage simulation models is limited. The complexity of the structure poses challenges to both experimental and numerical modal analysis. In order to calculate the propagation of vibrations from the main rotor hub to different locations of interest already in the design phase 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 a helicopter production line. The vibration test team of the DLR- Institute of Aeroelasticity cooperated with vibration specialists from Industry to plan and conduct ten modal tests on one successively built helicopter 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 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 0% to 100% of the nominal value) for entire components that get mounted at the respective assembly station and a 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.

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