Chapter 33 Numerical and Experimental Study on the Modal Characteristics of a Rotor Test Rig Verena Heuschneider, Florian Berghammer, and Manfred Hajek Abstract A Mach scaled 85 kW rotor test rig for the investigation of Dynamic Stall on highly loaded helicopter rotor blades up to a diameter of 1.80 m and a tip speed up to 220 m/s is built at the Institute of Helicopter Technology, Technical University of Munich. The current state is the complete assembly of the drive train components including the engine cradle, wind tunnel adaptor, rotor shaft, and bearings as well as strain gauge load cells. To ensure a safe entry into service and operation at the nominal rotor speeds between 40 and 50 Hz, we develop a numerical finite element model of the test rig structure in order to study its dynamic characteristics. The numerical analysis results show critical eigenmodes at 44.21 and 54.98 Hz, modes 5 and 6. By means of a complementary experimental modal analysis on the steel frame structure natural frequencies, mode shapes, and damping characteristics of the critical modes are identified. The measurement results for three different excitation points and their synthetization prove the identification of ten eigenmodes between 0 and 70 Hz, which mainly emerge from the wind tunnel adaptor. Modes 6 and 7 have eigenfrequencies that are in and critically close to the nominal speed range, 46.97 and 54.88 Hz. They match with the numerically calculated eigenfrequencies but show different mode shapes, as the Modal Assurance Criterion (MAC) values for the comparison of numerical and experimental model convey. As closer investigations show, the numerical eigenfrequencies have a tendency to higher values from the fifth mode onwards. Keywords Structural dynamics · Experimental modal analysis · Finite element model · Modal Assurance Criterion · Rotor test rig · Impact excitation 33.1 Introduction Munich Experimental Rotor Investigation Testbed (MERIT) is a Mach scaled 85 kW rotor test rig for the investigation of Dynamic Stall on highly loaded helicopter rotor blades under centrifugal force influence. Designed and built at the Institute of Helicopter Technology, Technical University of Munich (TUM), its robust design allows for very high structural dynamic loads. Size and structure are due to the main design target, i.e. applicability in TUM’s wind tunnel (Institute of Aerodynamics and Fluid Mechanics) for investigations in forward flight conditions. Divisibility and compactness simplify assembly as well as maintenance. The wind tunnel adaptor is necessary for bridging the wind tunnel attachment pads and is designed with the restriction of confined space conditions in height. Its structure as well as the four load cells mounted between bearing and middle frame imply eigenfrequencies within the rotational speed range of 0–50 Hz. Closer investigations of the system’s modal parameters will ensure a safe entry into service and operation at the nominal rotor speeds between 40 and 50 Hz. The objective of this paper is to identify and discuss the frequencies and shapes of critical eigenmodes by means of an experimental modal analysis on the steel frame structure with the complete drive train installed. A subsequent comparison of the modal analysis results with respective finite element model (FEM) analysis is used for verification and validation of the numerical model. Rotor and swashplate actuation components are not included for the investigations shown here. V. Heuschneider ( ) · F. Berghammer · M. Hajek Institute of Helicopter Technology, Department of Aerospace and Geodesy, Technical University of Munich, Munich, Germany e-mail: v.heuschneider@tum.de; flo.berghammer@tum.de; hajek@tum.de © The Society for Experimental Mechanics, Inc. 2021 B. Dilworth (ed.), Topics in Modal Analysis & Testing, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-47717-2_33 315
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