Model Based Blade Attachment Stiffness Evaluation of the MERIT Rotor in Hover with Photogrammetry and Digital Image Correlation

The paper focuses on the analyses of photogrammetry measurement results on the two-blade, hingeless MERIT rotor with diameter 1.8 m at the two rotor speeds 900 and 1800 RPM and collective pitch angles 0-12^◦ in hover and their comparison to simulation results calculated in CAMRAD II. Blade tip displacements in flap, lead-lag, axial, and torsional direction are shown as a function of collective pitch and rotor speed. Radial displacements in flap and lag direction depict the influence of the pitch bearing play and blade attachment. The structural blade model is validated by using static DIC deformation measurements and shows very good agreement. Calculated and measured thrust polars match very well with the use of a free wake model in the simulation. The combination of measured flapping angle and calculated flapping moment gives a stiffness estimation for a virtual flap hinge. The influence of hinge offset and stiffness are shown by parameter adjustment. Flap deformations of the rotating blades leave a tip flap offset of less than 1 mm in average, which corresponds to the statically measured effect of the bearing play at the blade tip. The study shows that photogrammetry is a valuable tool to identify and tune parameters of a numerical model.