Experimental investigation of an active twist model rotor blade with a low voltage actuation system

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Wierach, P.
Opitz, S.
Kalow, S.
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Smart materials that are directly embedded in the rotor blade structure are an attractive concept for active blade control. Operating as solid state actuators they can generate a twist deformation of the rotor blade without any friction and wear. A promising approach is the use of anisotropic piezoelectric strain actuators embedded in the rotor blade skin. Especially in Europe and the US this concept has been intensively investigated over the past years. A major drawback of all configurations studied so far is the high operation voltage of up to 2000V of state of the art piezoelectric actuators. Within the Green Rotorcraft Project of the European Joint Technology Initiative Clean Sky a new approach with a low voltage piezoelectric actuation system operating at 120V is investigated to demonstrate the feasibility of this technology. A first major step in this direction was done by conducting a centrifugal test with a model rotor blade. The objective of the centrifugal test was to demonstrate the performance of the actuation system and the structural concept under centrifugal loads by showing that the expected twist deformation can be achieved at the nominal rotation speed and different actuation frequencies. For the centrifugal tests a comprehensive test matrix was derived starting with the measurement of the static peak to peak twist displacement with increasing rotation speed from 280 RPM up to the nominal rotation speed of 1043 RPM for a mach scaled model rotor followed by a measurement of the tip twist at different excitation frequencies from 1/rev up to 6/rev. Since the full scale rotor is operated at significant lower rotation speeds the results of the model rotor blade are also significant for a full scale blade. The main characteristics of the blade were taken from the well-known BO 105 model rotor blade. The BO-105 model blade features a C-spar made of unidirectional glass fiber, a glass fiber skin and a foam core. It has a radius of 2m and a chord of 121mm. The new actuation system was successfully integrated into the glass fiber skin of the model rotor blade. All actuators endured the manufacturing process in an autoclave at a temperature of 120°C and a pressure of 6 bar. To allow a comparison of state of the art actuators with the new low voltage actuation system the blade was equipped with two different kinds of actuators. It was demonstrated that the new actuation system is capable to operate under high centrifugal loads. In comparison to state of the art actuators (operation voltage -500…1500V) the new actuation system (operation voltage -20..120V) exhibits higher active twist performance per active area.