Reduction and analysis of rotor blade misalignments on a model wind turbine

Model wind turbines with rotor diameters below 1 m often make use of a collective pitch control instead of an individual pitch control. As a result it is more difficult to achieve a high precision in the rotor blade pitch angle, especially when it comes to achieving the same pitch angle on each rotor blade. For the Model Wind Turbine Oldenburg 0.6 (MoWiTO 0.6) a rotor blade misalignment between the individual blades of up to 2.5 degrees was found. Due to the design, similar blade misalignments could also occur at other model wind turbines with a collective pitch mechanism. Here, it is shown that even small rotor blade misalignments influence the experimental results of small model wind turbines and should be avoided. In addition, a new mounting procedure is presented that serves to minimize blade misalignments when assembling the individual rotor blades in the manufacturing process. This procedure makes use of 3D printed parts that enclose the rotor blade during the mounting process and guarantee a precise pitch angle. The presented procedure is easily applicable to other model wind turbines as well. The subsequent experimental investigations of blade misalignments in the range of ±2.5 degrees show a significant influence on the turbine performance and thrust. A blade misalignment of +2.4 degrees for only one blade already decreases the mean power output of the turbine by up to 9%. Additionally, the mean thrust measurements show a clear influence of the blade misalignment (up to 17% difference) in comparison to the optimal pitch reference case. Furthermore the 1P (one-per-revolution) peaks of the thrust spectrum are significantly increased with present blade misalignments which suggests cyclic loads. These results underline the relevance of a precise rotor blade attachment for model wind turbines used in wind tunnel experiments.