Research On Coupled Nonlinear Aeroelasticity And Flight Dynamics Of Flexible Aircraft

Solar-powered aircraft have huge development potential and broad application prospects,but their very-low wing loading,ultra-low structural area density and distributed design ideas make them have higher flexibility than conventional fuel-powered high-altitude long-endurance aircraft.Traditional static and dynamic models which based on rigid body or linear elasticity assumptions cannot describe the nonlinear deformation and complex dynamic response characteristics accurately.Besides,the coupling between structural dynamics and flight dynamics for lightweight flexible aircraft is evident;the research methods of any discipline alone cannot accurately and comprehensively describe the dynamic characteristics of this type of aircraft.Therefore,a coupled model of aeroelasticity and flight dynamics with distributed characteristics and non-linear factors must be established for this special objects,and time-domain method should be used in research.Aimed at the shortcomings of six-degree-of-freedom model and its derivative models which cannot describe the large deformations and distributed characteristics of flexible aircraft,together with the problems that conventional aeroelastic models that under fixed support conditions cannot describe the overall dynamic responses of the aircraft in flight,a comprehensive coupling model of nonlinear aeroelasticity and flight dynamics,which can describe the characteristics such as high flexibility,low wing loading,low structural area density and distributed forces,etc.,along with a time-varying discrete gust model,is proposed and established.The static and dynamic models of co-rotation-based 3D geometric nonlinear beams are established and improved.The elemental stiffness and mass matrices of fully coupled beams considering stiffness and inertial couplings among tension,bending and torsion,and the calculation methods of sectional equivalent stiffness and inertia of complex wing are established.A hybrid timemarching scheme combining the end-point and mid-point methods is proposed and applied to the implicit prediction-correction time integration algorithm for nonlinear equations,by which the efficiency,accuracy and stability of nonlinear dynamic solutions are improved.The distributed gravity,distributed follower thrust,and distributed steady and unsteady aerodynamic models are established.A simple but practical method for correcting the spanwise lift distribution of strip model is proposed.The global and elemental coordinate systems of structural finite element are analogized to redefine the ground,body and wind axes of flight dynamics on elemental scale,by which the modeling of distributed follower forces and coupling of aeroelasticity and flight dynamics are realized.The calculations of instantaneous inertia and center of mass of flexible structures are given.The nonlinear static aeroelastic trim of flexible aircraft is achieved by Newton-Raphson iteration.Meanwhile,the full-wavelength 1-cos time-varying discrete gust model with spatial distribution characteristics is established.Based on the established model,a set of complete multi-disciplinary dynamic time-domain calculation and simulation software for the coupling of nonlinear aeroelasticity and flight dynamics of lightweight flexible aircraft with independent intellectual property rights is developed.With a typical “Helios-like” lightweight flexible UAV taken as the research object,the effects of structural deformation,geometric nonlinearity,load mass and its spanwise distribution,structural area density,geometric scale,and amplitude and frequency of discrete gust on the static and dynamic aeroelastic characteristics,longitudinal stability,load capacity and gust response characteristics of lightweight flexible aircraft are studied.The process and internal mechanism of the disintegration mishap of Helios Prototype is revealed.Comparisons of static and dynamic characteristics between rigid and flexible models show that the structural deformation of flexible aircraft will enhance the longitudinal and lateral static stability,weaken the directional static stability,and reduce the phugoid and Dutch roll mode stability.Compared with time-domain method,the modal method that based on static aeroelastic deformation and small perturbation equations will underestimate the phugoid modal stability of the lightweight flexible aircraft.Meanwhile,linear structural model will underestimate the structural deformations and their effects on the overall aerodynamic and flight performances.Research on the effects of load mass and its spanwise distribution on the static and dynamic characteristics of flexible aircraft show that distributed loads can unload the wing effectively,which has better static and dynamic characteristics than the concentrated loads.Reasonable spanwise load distribution is the key to suppressing the static aeroelastic deformations of the wing and improving the overall dynamic stability and carrying capacity of highly flexible aircraft.In the given flight condition,when the load is changed from a centralized arrangement to a dispersed one,the overall carrying capacity of the whole aircraft can be increased by more than 5 times.In addition,the increased geometric dimensions of the aircraft and reduction of the structural area density can also improve its wind resistance and load carrying capacity,as well as the phugoid and Dutch roll modal stability.Research on the aircraft’s dynamic response characteristics shows that a lightweight aircraft with a high aspect-ratio,a very low wing loading and an ultra-low speed is a “gustsensitive” aircraft,which has the potential to “fly-by-wind”.The flexibility of the wings can reduce the gust overload to a certain extent.Under the same conditions,the maximum and minimum overloads of a flexible aircraft are 16.7% and 24.1% lower than those of a rigid one,respectively.The amplitude and frequency of the gusts play a dominant role in the dynamic response of this type of aircraft.The disturbance frequency that causes the maximum response amplitude of the aircraft is close to the dynamic aeroelastic response frequency or phugoid modal frequency of the aircraft at given condition,which is not directly related to the low-order natural mode of the structure.The research on dynamic response characteristics of flexible aircraft under constrained and unconstrained conditions shows that the coupling between aeroelasticity and flight dynamics of flexible aircraft cannot be ignored.Taking the cantilevered wings as the research object can neither reflect the overall motion of the whole aircraft,nor obtain the correct overload distribution,which may even overestimate the dynamic stability and carrying capacity of the aircraft to a large extent.Reproductions of the mishap of Helios Prototype HP03-2 show that the root cause of the disintegration of aircraft is the unreasonable concentrated load distribution of the aircraft,which resulted in the excessively large and continuous upward bending deformation of the wing,and the loss of longitudinal dynamic stability of the aircraft.The gusts encountered in flight further exacerbate the deformation of the wing.The research in this paper shows the necessity of considering the distributed characteristics and geometric nonlinear deformation of structure and the coupling of aeroelasticity and flight dynamics in the design and analysis of lightweight flexible aircraft,which can provide the necessary theoretical guidance and reference basis for the overall layout and characteristic analysis at the concept and preliminary design stages of lightweight flexible aircraft that represented by solar-powered UAVs.