Numerical and Experimental Study on the Modal Characteristics of a Rotor Test Rig

摘要

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.