Exploring Nonlinear Aeroelastic And Flight Dynamics Of Solar-powered UAV

High altitude ultra long endurance solar powered UAVs(unmanned aerial vehicles) which may provide a unique cost-effective platform for a large number of tasks, from continuous reconnaissance and surveillance to early warning and telecommunication relay, is one of the most important research objects among the near space vehicles that may has a wide development potential and application prospect. Restricted by its unique form of energy, high aspect ratio wings and low structural surface density and low wing loading are the specific characteristics of this type of high-altitude long endurance solar powered UAVs. During normal flight operations, a very flexible solar powered UAV may undergo large elastic deformation, thus changing the stiffness and mass distribution as well as the propulsive system distribution relative to its undeformed state. Linear theory is impossible to meet the accuracy requirement when exploring the aeroelasticity and flight mechanics of such aircraft. It is urgent to promote the development of high precision nonlinear aeroelasticity, trim and stability analysis methods. In the design of such a flexible solar powered aircraft, aeroelasticity and fight mechanics analysis with geometrically nonlinear effects considered is one of the key technologies. Following the traction of the research needs mentioned above, the main work carried out in this paper is summarized as:(1) Large-scale, high aspect ratio wings are generally used in the design of very flexible high altitude long endurance solar powered UAVs and permit to produce moderately large elastic deformation. The classical structural model based on linear assumptions is no longer applicable. Based on co-rotational(CR) theory, the tangent stiffness matrix of the deformed wing structure and the internal force formations are derived firstly, then the tangent mass matrix and dynamic equations are presented in this paper, and a geometrically nonlinear structural model is established to describe the large deflection of flexible wings. The process of solving nonlinear static equilibrium equations based on incremental method and nonlinear dynamics equilibrium equations through direct numerical integration method are presented in this paper too. Through a test case of very flexible cantilever beam, the computational accuracy and efficiency of solving geometrically nonlinear problems based on CR theory are verified, and the validity of program written in FORTRAN language is proved. The research results show preliminarily that: with only 3 to 5 sub-iterations, the results will meet the displacement convergence criteria <10-3 with a maximum error less than 3%, when using Newton-Raphson method to solve the incremental equilibrium equations. CR FEM(finite element method) has a good accuracy and efficiency, and can satisfy the requirement to simulate geometrically nonlinear deformation characteristics of very flexible structures, and can be used to establish wing structural model of a flexible solar powered aircraft.(2) Large wing deflection of a solar powered aircraft will change the direction of the redistributed local aerodynamic loading. The exploring of aerodynamics of the deformed aircraft will has important reference values for the design of such a flexible aircraft. Coupling CR statics and CFD(Computational Fluid Dynamics) technology, we proposed a nonlinear aeroelasticity solving method with geometrically large wing deformation effects considered, and the static aeroelastic response of a solar powered UAV with a novelty layout is studied in this paper. The research results show that: the wing deformation will deteriorate the lift-drag characteristics significantly and increase the absolute value of roll and yaw derivatives by several times, but the load distribution in span wise will be improved which is beneficial to the structural design. If a linear structural model is still used to explore the aeroelastic problems of such flexible aircrafts, an opposite conclusion may be obtained.(3) With the increase of wing deformation, the stiffness and mass distribution characteristics of the structure of a very flexible solar powered UAV will change significantly. Remarkable error may occur if a linear dynamic aeroelastic stability analysis method is used. In this paper, based on linearized CR dynamic equations and unsteady linear aerodynamic strip theory, a nonlinear aeroelastic stability analysis method is proposed under the quasi-mode assumption. Based on the proposed method, the aerolestic stability of a solar powered UAV with the layout similar to Helios is investigated under variant structural stiffness, elastic axis position, mass center of the cross section, concentrated mass distribution, and so on. The results show that: with the increase of wing deformation, nonlinear flutter speed will be reduced by 8.6% or more, and the nonlinear flutter frequency will be reduced by 6.49%; Reasonably increasing the structural stiffness, moving the elastic axis toward to the leading edge and properly allocating of the concentrated mass may alleviate the disadvantageous effect of the aeroelastic stability characteristics due to the large wing deformation.(4) The solving precision of unsteady aerodynamics based on vortex lattice method, Wagner function in the Duhamel integral form and ONERA dynamic stall model as well as its improved forms are compared in this paper, and the nonlinear aeroelastic response of an airfoil is explored too. Coupling the nonlinear structural dynamic model based on CR theory and the improved ONERA dynamic stall model, an effective nonlinear aeroelastic dynamic response analysis method is proposed in time domain. The computational accuracy and efficiency of the proposed method is verified, and the effectiveness of corresponding FORTRAN program is also proved, and the limit cycle oscillation of a very flexible wing is reasonably predicted.(5) Geometrically nonlinear aeroelastic effects on nonlinear trim, flight loads and stability analysis is one of the most important problems that should be considered in the design of a very flexible solar powered UAV. Furthermore, introducing the distributed gravity and thrust model and taking the geometrically nonlinear effects into account, a nonlinear trim and stability analysis method that suits a very flexible aircraft is proposed, and the solving code written in FORTRAN language is given too. Based on the proposed method, the nonlinear flight mechanics of a very flexible solar powered UAV with distributed propulsive systems is explored in this paper. The research results show that: if a linear structure model is used for solving, the trim error will reach up to 50%, and the prediction error of wing tip displacement will reach up to 25%; the proposed nonlinear flight mechanics analysis method has a relatively good accuracy and satisfies the trim analysis precision requirement of a very flexible solar powered UAV.In conclusion, a nonlinear aeroelasticity and flight dynamics analysis method is presented in this paper based on CR theory. Using the proposed method, the static aeroelastic response, aeroelatic stability and flight dynamics of very flexible solar powered aircraft are explored. The investigation is believed to be of some reference value to the design of very flexible solar powered UAV in engineering.