Modal Investigation of an Active Twist Helicopter Rotor Blade

Abstract

The capability of hovering makes the helicopter one of the most fascinating aircrafts and allows covering a wide range of operational scenarios. The principle of the rotating wings which enables to lift without forward motion is also responsible for the very complex aerodynamic environment of the main rotor. Consequently the main rotor is one of the major sources for helicopter noise and vibra-tion which are diminishing the ride quality and the public acceptance. Being boon and bane at the same time the main rotor is subject to continuous investigations aiming on the improvement of noi-se and vibration characteristics. But due to the combination of strong centrifugal forces and the complex aerodynamics the rotor of a helicopter also presents a very challenging environment for the application of adaptive systems. One opportunity to influence the flow field favorably is the acti-ve rotor control. The idea behind this concept is to superimpose a small blade motion to the prima-ry rotor control. This motion can change the trajectories of the tip vortices to reduce noise or gene-rate additional aerodynamic forces to cancel vibrations of the rotor hub. During the last decades the mechanisms used for active rotor control evolved up to the twist deformation of the rotor blades called Active Twist. The Active Twist rotor blades built at the German Aerospace Center (DLR) incorporate Macro Fiber Composite (MFC) actuators to twist the blade. These piezo ceramic actua-tors are arranged in span-wise segments that can be activated separately. The morphing of the rotor blades is performed with frequencies up to six times the rotational frequency of the rotor. Sin-ce this range already comprises several eigenfrequencies of the blade, the dynamics of the system has to be regarded in order to achieve the intended tip twist. Within this paper the dynamic behavi-or of an Active Twist Rotor Blade will be investigated. Results of an experimental modal analysis in the non-rotating system will be presented. The experimental determination of bending stiffness, torsional stiffness and elastic axis will be shown. The measured blade characteristics will be used to build up a numerical model of the blade. The effects of the centrifugal forces to the eigenmodes and natural frequencies of the system will be elucidated using results from numerical investigations as well as experimental data. Finally the segmented actuation will be used to determine a generali-zed excitation for the first two eigenmodes in torsion. The results of this modal investigation will establish the basis for a modal control of the active twist rotor blade which is planed for the future and will not be presented in this paper.