Active Twist Blades: Electromechanical Damping

Active Twist Rotor Blades for helicopter main rotors 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 causing the blades to twist. At the DLR such types of active twist blades have been designed for quite some years. Several model scale blades have been manufactured to demonstrate the feasibility of such systems. This paper presents a new set of rotor blades (4 m diameter) which is designed to go into a wind tunnel experiment within the “STAR” consortium. In this paper it is discussed, what kind of other applications for such blades — equipped with piezoceramic actuators — can be considered. Besides the capability to twist at frequencies of 1 through 6/rev, there is always the option of statically changing the pretwist of the blade, which will influence the figure of merrit. The idea is to use blade integrated actuators as part of electromechanical absorbers has been discussed recently. Using certain electrical networks, the structural behavior of the rotor blade can be significantly influenced. The effectiveness of such systems was experimentally investigated using a twist blade. In this paper results of a rotating blade with aerodynamic damping is shown for the first time. Different types of shunt networks have been investigated as presented in [1] before: 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 amplitues 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 deg compared to a “regular capacity”. 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.