Comparison among rotor blade structural load calculation methods

Based on finite rotation hypothesis, a rigid-flexible coupled rotor dynamic model was developed. Comparing with classical finite element model, in this model three rigid DOFs were introduced for hinge rotations coupled with blade elastic deformations and thus the model had potential advantages over the small rotation beam model. Generalized aerodynamic forces were tightly coupled with rotor blade structural rigid rotations and elastic deformations. Structural dynamic loads were computed using three load calculation methods including force integration method, reaction force method, and curvature method on each time step for solving the dynamic equations. The loads were examined with the analysis results of BO105 model blade and the flight test data of SA349/2 Gazelle helicopter. All load methods could handle the structural load calculation without aerodynamic forces applied. Force integration method's predicting accuracy was not enough at sections near the blade root, especially, when transient aerodynamic forces were taken into account. The results with the curvature method and the reaction force method were nearly the same at the rotor blade's nodes of finite elements. The accuracy of the reaction force method depended on the finite element modeling accuracy and was only efficient to predict loads at nodes. Since the curvature method only considered the bending curvature of the elastic rotor blade, it required higher order shape functions to satisfy the continuity of second order derivatives. To speed up convergence and decrease accumulated errors, the extrapolation technique was introduced to implement a numerical integration algorithm.