Aeroelastic System Modelling And Stability Analysis Of Highly Flexible Aircraft

High altitude long endurance(HALE)aircraft with high flexibility provide an effective platform for a large number of tasks,including continuous reconnaissance,surveillance,early warning and telecommunication.The highly flexible HALE aircraft is one of the most important research objects among near-space vehicles that has a wide development potential and application prospect.In order to improve aerodynamic efficiency and mission loads,HALE aircraft are usually made by lightweighted composite materials,and have large aspect ratio flexible wings.The aeroelastic problems of HALE aircraft caused by the coupling of highly flexible structures and aerodynamic loads differ from the traditional aircraft problems.HALE aircraft feature large structural deformation,system uncertainties and the coupling between elastic and rigid motions.These differences determine that it may be not appropriate to apply previous aerodynamic,aeroelastic modelling and stability analysis approaches in HALE aircraft.Therefore,it is necessary to develop new aeroelastic modelling and stability analysis methods to build the foundation for the structural design and aeroelastic analysis of highly flexible HALE aircraft.To fulfill the above requirements,the following research works are performed:1.The construction and solution of eigenvalue problems for aeroelastic stability analysis of large flexible aircraft are studied.The structured singular value theory is applied to the stability analysis of aeroelastic systems.The inflow dynamic pressure is introduced as a bounded perturbation to construct the eigenvalue problem of the aeroelastic system in the state space domain.Then the nominal aeroelastic stability boundary can be obtained.Compared with the classical flutter analysis method in the frequency domain,this method is more accurate,and the calculated stability boundary is more consistent with the experimental value.This stability analysis method is the theoretical basis for other research in this dissertation.2.A unified state space modelling approach for aeroelastic system is proposed.Several commonly used unsteady aerodynamic models are derived and combined with the state space equation of the finite element model to construct the unified form of the aeroelastic system in state space domain.The coupling between aeroelastic system and control system is studied based on this unified state space aeroelastic model,as well as the aeroelastic mode tracking.The case study shows that this unified state space modelling approach has the advantages of unified form,efficient multidisciplinary coupling and capability of both time/frequency domain analyses.The mode tracking algorithm based on left-right eigenvector orthogonal check is compared with the traditional MACbased method,results indicate that the proposed method can more accurately track the trend of flutter modes of highly flexible aircraft structures.The unified state space modeling method proposed in this chapter is applied to study the aeroelastic stability in the following chapters.3.With the unsteady vortex lattice aerodynamic theory applied,a time-marching analysis framework for flexible aircraft is established.A time-varying infinite plate spline interpolation method based on structural instantaneous deformation is proposed.By using this interpolation method,the aeroelastic analysis framework that couples unsteady vortex lattice method with both beam-based FEM and shell-based FEM is given.The analytical sensitivity computation of aeroelastic systems with UVLM aerodynamic is studied to construct the discrete-time state space model for stability analysis.The case study shows that the time-varying infinite plate spline interpolation is more practical than traditional IPS or TPS methods.The obtained distributed aerodynamic loads are more accurate.The analytical aeroelastic sensitivity analysis method based on the chain rule can acquire the sensitivities of aerodynamic loads with respect to both structural motions and circulation strengths of vortices.The proposed aeroelastic analysis framework is verified by three highly flexible aircraft models with different modelling fidelities.4.The robust flutter stability analysis of highly flexible aircraft based on the structured singular value theory is studied.Based on the derivation of the linear fraction transformation of aeroelastic systems,the robust modelling method of aeroelastic systems with structural stiffness/damping uncertainties,modal parameter uncertainties and aerodynamic parameter uncertainties is given.The case study shows that when constructing the standard p-? feedback system,the parameter uncertainty terms are extracted to form augmented equations,which can significantly reduce the cost of computation and algebraic derivation.In the case of straight wing structure with a large aspect ratio,results indicate that the aeroelastic stability is sensitive to the uncertainty of modal damping,while less sensitive to the uncertainty of aerodynamic parameters.After considering the influence of uncertainties,the robust flutter boundary is always smaller than the nominal flutter boundary.It is proved that the proposed robust aeroelastic stability analysis method based on the structured singular value theory is accurate and effective through the verification of aeroelastic stability boundary.5.The aeroelastic modelling and stability analysis considering the rigid-elastic coupling effect for highly flexible aircraft are studied.The aeroelastic modelling method with rigid-elastic coupling is developed in modal coordinates based on mean axes theory and the proposed unified state space aeroelastic model.The instantaneous center of mass is assumed to be constant under an appropriate mean axes definition.Thus the nonlinear equations of rigid motions are linearized around the equilibrium condition.The equations of motion for flexible aircraft without rigid-elastic coupling terms can be finally obtained.The system modelling and stability analysis method for rigid-elastic coupling aircraft is verified through the case study of a blended-wing-body model.Numerical results show that the first modal frequency of the free-free aircraft is smaller than that of the cantilever wing model.After considering the rigid-elastic coupling effect,the stability boundary of the aeroelastic is significantly reduced.The body freedom flutter is induced by the coupling of the short period mode and the symmetric bending mode.