The Institute was involved in several EU research projects on flight physics for tilt rotor aircraft. For aeroelastics, the especially critical problem of whirl flutter in these types of aircraft was the subject matter of experimental and numerical investigations.
The Institute provided the experimental modal data that was used in this project for the DNW-LLF wind tunnel model. A modal test on the TILTAERO wind tunnel model was carried out in the summer of 2006. The objective of this modal test was to determine the suitability of the model for whirl flutter experiments in the ADYN project. After a necessary reconstruction, the modified model was used in April 2007 for an additional modal test in the DNW-LLF (see figure on the top right). As such, the safe operation of the wind tunnel model before the test campaign was established using the database of the whirl flutter analysis. During the wind tunnel tests in the ADYN project, the aeroelastic stability of the wind tunnel model was monitored on the basis of measurement techniques provided by the Institute.
For the numerical whirl flutter analysis of the wind tunnel configuration for ADYN, a multi-body model for the propeller with elastic mounting was created. Here, the aerodynamics of the propeller were based on strip theory and the required aerodynamic coefficients were either derived from measured or calculated-with-CFD data records.
By comparing the results of the modelling with the values measured in the wind tunnel test, the rotational speed behaviour of the rotor depending on the blade adjustment angle was adapted, as well as the correct reproduction of the dynamic loads for the model mounting with adjustable spring stiffness in the wing torsional degree of freedom. The figure on the lower right shows examples of the frequency and damping sequence depending on the wind speed.