Additional applications of active twist blades

Abstract

Active Twist Rotor Blades have been developed for the use in secondary control such as higher harmonic control (HHC) and individual blade control (IBC). The basic principle of such blades is the implementation of piezoelectric actuators into the blades, using different types of coupling, causing the blades to twist. At the DLR model scale blades have been manufactured to demonstrate the feasibility of such systems. This paper is describing two other types of application of such blades. De-icing a rotor blade using the pre-existing actuators. The working principle for de-icing the blade is to excite vibrations with high amplitudes. These vibrations break up and shake off the ice. The centrifugal forces clear the ice off the rotor blade; however, the stiffening effect caused by the rota-tion reduces the amplitude of the bending modes, which are the most influential factor in breaking up the ice. First tests have been made using a 50 cm long blade segment. A setup was established to grow different types of ice on the blade within a freezer between -10° C and -25° C in a non-rotating system. It could be shown that glaze ice could be broken easily. Next steps will be the test of the effectiveness to remove ice in a rotating system, where more realistic ice formations will be generated. Also it has to be examined how good the system works for rime ice. Manipulation of damping using blade integrated actuators. The idea is to use blade integrated ac-tuators as part of electromechanical absorbers. Using certain electrical networks, the structural behavior of the rotor blade can be significantly influenced. The effectiveness of such systems to increase the structural damping of an active twist blade was experimentally investigated. The op-eration of different sets of actuators allowed the excitation of individual modes. At first an oscillating circuit was established coupling the capacitive piezoelectric actuators with an inductivity. If the setup is tuned right, this results in a significant decrease of the amplitudes of a single frequency, e.g. torsional eigenfrequencies. In a next step a virtual "negative capacity" was used to dissipate the vibrating energy in the electrical circuit. Such an element shows the same amplitude response as the capacity, but the phase is shifted by 180° compared to a "regular capac-ity". The advantage of this method is the effectiveness over a broad frequency range. That way several modes can be influenced at the same time. Finally, a first evaluation of the influence of these measures on the vibration level of a complete rotor was carried out.