Structural modeling and validation of an active twist model rotor blade

Abstract

DLR has been researching on active twist rotor blade control for at least 15 years now. This research work included the design and manufacturing of model rotor blades within the blade skin integrated actuators. As a main subject, numerical benefit studies with respect to rotor noise, vibration, and performance were carried out with DLR's rotor simulation code S4. Since this simulation code is based on a modal synthesis, it uses the natural blade frequencies and mode shapes to model the blade dynamics. Both, natural blade frequencies and mode shapes, are computed in advance employing a finite element beam model of the blade. Each beam element possesses certain structural properties that are derived from an ANSYS model for certain cross sections of the blade. Since model rotor blades are built for wind tunnel testing, they are highly instrumented with sensors and therefore vary in their structural properties along span. Modifications in the structural properties due to the instrumentation are not included in the ANSYS model. However, to account for these variations, two experimental methods have been developed. They allow the determination of the real values for the most important structural blade properties such that the structural blade model is improved. The paper describes the experimental methods, as well as the development of an advanced structural blade model for rotor simulation purposes. It shows a validation of the structural blade model based on the measured non-rotating and rotating frequencies.