Research On Helicopter Rotor Aeroelastic Vibratory Loads

Rotor system is the most important component of the helicopter, and it’s also the origin of the vibration and noise. The development, modification and innovation of the rotor system are the research focus in helicopter technologies. To some extent, the vibration levels of a helicopter depend on the rotor system. Therefore, estimation and optimization methods of rotor vibrationary loads need to be investigated. The rotor system dynamical model, aeroelastic model and the solution procedures have been investigated, and the structural loads of the rotor have been calculated. To verify the developed models and methods, the aeroelasticity analysis of rotors are investigated, and the results of different rotor structural loads calculated methods are also presented in this dissertation.Because of the simplicity of structure, easy maintenance and enhanced control power, new configuration rotors have been developed rapidly. In a bearingless rotor or hingeless rotor, the flap hinges, lag hinges and even the pitch bearings have been replaced by a composite flexbeam, which provides the blade motions by the elastic deformation. The deflection of the composite flexbeam is not small, so a geometrically exact beam model for analyzing new configuration rotor dynamics is deduced. This model uses nonlinear geometry formulas to describe the relationship of various blade deflections and then obtains the strain energy based on Green strain theory. To further improve the model capability of traditional articulated rotors, considering rotor hinge movements as separate rigid rotations, a new rigid-flexible coupling model is developed. In this model, no order scheme is used and all nonlinear and coupling items are untouched and kept in the final equations. For the convenience of numerical simulation, the mass matrix is extracted by using the recurrence method. Taking aerodynamic loads as generic forces, the rotor dynamic equations can be built based on Hamilton principle.The high fidelity aerodynamic models and the integration approaches with dynamic model are also presented. The effects of the airloads have been treated as generalized forces and added in the right side of the governing equations of the rotor aeroelastic model. Thus, it becomes easier to merge various aerodynamic models and more convenient to implement numerical methods accordingly. Also a new strategy for rearranging aerodynamic integrating nodes has been proposed to archieve high precision of aerodynamic load along the blade. Based on the layout of structural nodes, the strategy increases the aerodynamic elements as necessary via bisection method. The sectional airloads are obtained using nonlinear aerodynamic model, which takes the dynamic stall into account. Meanwhile the rotor inflow distribution is computed by the free wake model and updated when the attitude of the rotor changes. Furthermore, the flow field around an oscillating airfoil has been simulated using CFD method and the relationship between the corresponding aerodynamic forces and the airfoil flow separation has been investigated.Because of the stiffening effect of centrifugal forces and the difference between finite elements, the final ordinary differential equations got previously are normally non-linear and stiff. Several advanced numerical solution methods have been compared with each other, including Precise Time Integration method and explicit/implicit Euler methods. Modifications have been made for the Newton-Raphson iteration while the implicit solution methods are being implemented. By using this method, the Jacobian matrices could be calculated only if the convergence speed is slow. A Gaussian integration method based on extrapolation technique also has been deduced. The computational accuracy and efficiency are improved by using these modifications.Besides the accuracy of the rotor aeroelastic model, the structural loads calculation methods also have the affects in the results. The equivalence between the force integration method and reaction force method has been deduced, and a mixed structural loads calculation method, based on these two methods, is also presented. In the mixed method, the forces on the nodes are calculated by the reaction force method and the forces between the nodes are calculated by the force integration method. This new method not only can keep away the accumulation errors from integration method, but also overcome the predicting difficulty of reaction force method on non-node positions. Flight test results are used, in order to verify the correctness of the developed models and solution methods, and the improving computation precision has been demonstrated.