Geometrically Exact Nonlinear Aeromechanics Modeling And Aeroelastic Response Of Composite Rotor Blades

Composite blades can significantly improve the aeroelastic stability,vibration characteristics and aerodynamic efficiency of rotors and have been widely used in helicopters.All of the existing aeroelastic analyses of composite blades are based on the moderate deflection beam theory and use an order scheme to neglect higher-order terms.However,the deflections of composite blades may be large.Therefore,it is very important and has a great engineering value to study the aeroealstic modeling and analysis method of composite blades with large deflections.In this dissertation,the geometrically exact nonlinear mechanics modeling of composite blades is carried out based on the geometrically exact nonlinear beam theory of Hodges et al.,and an accurate and effective aeroelastic modeling and aeroelastic response analysis method for composite blades in hover is established by combining the constructed structural model and unsteady aerodynamic model.Based on the geometrically exact nonlinear beam theory of Hodges et al.,the geometrically exact nonlinear mechanics modeling of composite beams is carried out by combining the improved variational asymptotic beam sectional analysis and the geometrically exact nonlinear beam equations of motion in the mixed variational form of Hodges.At present,the internationally used variational asymptotic beam sectional analysis transforms the second-order asymptotically correct strain energy into the generalized Timoshenko strain energy by perturbation method.The method neglects the higher-order terms in the second-order asymptotically correct strain energy,and extends the relationships between the stiffness matrices corresponding to the second-order asymptotically correct strain energy and the stiffness matrices corresponding to the generalized Timoshenko strain energy for prismatic beams to the beams with initial twist and curvature.Studies have shown that the above simplifications have large effects on some beam structures and are invalid.Therefore,in this dissertation the above simplifications are abandoned and the exact nonlinear equations about the second-order asymptotically correct strain energy and the generalized Timoshenko strain energy are solved in the process of transforming the second-order asymptotically correct strain energy into the generalized Timoshenko strain energy.The accuracy of the present mechanics modeling method and its ability for the analyses of composite beams with large deflections are verified by the analyses of composite thin-walled box beams and comparing the obtained theoretical results with the experimental and calculated results.The studies show that: the symmetric layup thin-walled box beams display extension-shear and torsion-bending couplings,the anti-symmetric layup thin-walled box beams display extension-torsion and shear-bending couplings,and the larger the deflection is,the greater the geometric non-linearity becomes.The static response and dynamic characteristic analyses of elastically coupled composite blades are carried out by using the present improved geometrically exact nonlinear structural modeling method of beams.The accuracy of the present improved beam structural modeling method for the structural analyses of composite blades is verified by comparing the obtained theoretical results with the experimental results.The influences of cross-sectional warping and transverse shear deformation on the static response and the dynamic characteristic of composite blades are also investigated.The studies indicate that: the desired elastic couplings of composite blades can be achieved by the proper layup of D-spar and the proper distribution of elastic couplings along the blade span.The cross-sectional warping has significant influences on the static deformation and the natural frequencies of composite blades and cannot be neglected.The effects of the transverse shear deformation on the static deformation and the natural frequencies of composite blades are related to the length to chord ratio of the blade.When the ratio is large to a certain value,the influence of transverse shear deformation on the static deformation and lower natural frequencies of the blade can be neglected.However,the 6×6 fully coupled stiffness matrix should be used for calculating the higher natural frequencies of composite blades accurately.The aerodynamic modeling method suitable for the blades with morphing airfoils is established by combining the Peters finite state airloads theory,the improved ONERA dynamic stall model and the Peters-He three-dimensional finite state dynamic inflow theory.According to the configuration of the morphing airfoils,the ONERA dynamic stall model is improved in the following aspects to make it suitable for calculating the additional airloads of morphing airfoils caused by dynamic stall: the static losses of morphing airfoils are used as the excitations of the dynamic stall differential equations.The static loss curves of morphing airfoils are obtained by translating the static loss curves of the corresponding conventional airfoils.The shape change effects of morphing airfoils are taken into account by the coefficients of the dynamic stall differential equations.When calculating the airloads for the two-dimensional airfoils in dynamic stall,the additional circulation caused by dynamic stall is added into the two-dimensional dynamic inflow theory.The airloads on the morphing airfoils without pitch oscillation and with trailing-edge flap in harmonic oscillation,the conventional airfoils in dynamic stall,and the morphing airfoils with pitch oscillation and with trailing-edge flap in harmonic oscillation are calculated by using the present airloads calculation method.The accuracy of the method is verified by comparing the obtained theoretical results with the experimental results.An accurate and effective aeroelastic modeling and aeroelastic response analysis method for composite blades in hover is established by combining the present improved structural modeling method and aerodynamic modeling method.The geometrically exact nonlinear equations of motion in the global frame for composite blades are used to calculate the aeroelastic response of blades under airloads.By using the present aeroelastic modeling and aeroelastic response solving method,the forces and moments of each blade cross section are directly obtained as the equation unknowns,and it is not necessary to use the conventional force or modal summation method.The present aeroelastic modeling and aeroelastic response solving method is applied to analyze the aeroelastic response of composite blades in hover and its accuracy is verified by comparing the obtained theoretical results with the experimental results.The influences of cross-sectional warping and transverse shear deformation on the aeroelastic response of composite blades in hover are also investigated.The results show that: the induced velocity rises dramatically near the blade tip which supplies the tip relief of lift and drag,matching the actual distribution trends near the blade tip.The influences of cross-sectional warping and transverse shear deformation on the aeroelastic response of composite blades in hover are related to the configuration of the blade root.Compared with articulated composite blades,the influences of cross sectional warping and transverse shear deformation on the aeroelastic response of hingeless composite blades in hover are greater.