Aerodynamic Analysis And Optimal Design Of Tilt-rotor Aircraft Based On An Unsteady Momentum Source Method

With a higher-speed cruise ability than helicopter, tilt-rotor aircraft has a vertical taking-off and landing ability compared with fixed-wing aircraft, which can satisfy the demand of a variety of flight missions. However, as an aircraft between helicopter and fixed-wing aircraft, the tilt-rotor aircraft faces more complex aerodynamic interactions which have serious impacts on the aerodynamic performance of tilt-rotor aircraft. Therefore, it has a very important academic and practical value to carry out numerical investigations on the aerodynamic interaction and optimization design. In this paper, the unsteady momentum source method is developed based on the unstructured hybrid mesh system, CFD method is used to simulate the flowfield of tilt-rotor aircraft, and parametric analyses on the effects of the aerodynamic performance and aerodynamic optimal designs are carried out. The main contents are:In Chapter 1, the developments of tilt-rotor aircraft flowfield analysis are introduced. Both domestic and abroad research conditions and technical issues of aerodynamic interaction, unsteady momentum source method and tilt-rotor aerodynamic shape optimization design research are analyzed. Finally, the necessity and significance of coupling unsteady momentum source model with the tilt-rotor aircraft numerical method are pointed out.In Chapter 2, in order to capture the viscous flow in boundary layer which affects the aerodynamic performance of tilt-rotor aircraft, considering multi-flight conditions of tilt-rotor aircraft, a hybrid mesh method is proposed in this paper, which is composed of prismatic(near boundary layer) and tetrahedral mesh(other fields). In order to employ unsteady momentum source method into CFD method, the unstructured hybrid mesh system, two dimensional airfoil mesh and Cartesian mesh for momentum source searching are coupled to establish the tilt-rotor aircraft mesh system, some mesh generation results are given.In Chapter 3, a CFD method is established to simulate the unsteady flowfield of tilt-rotor aircraft under multi-flight conditions based on the established mesh system. The Navier-Stokes equations are taken as the governing equations, Spalart-Allmaras model is used as turbulence model, dual time-stepping method is employed in temporal discretization with Runge-Kutta algorithm used in pseudo-time step, and data sharing OpenMP parallel strategy is employed to accelerate the calculation as well. The developed method is validated by analyzing Caradonna-Tung rotor, and the calculated results demonstrate the accuracy and efficiency of the present CFD method.In Chapter 4, in order to explore the unsteady aerodynamic interaction phenomenon of tilt-rotor aircraft under multi-flight conditions, the flowfields of the hovering, forward flight(helicopter mode, propeller aircraft mode) and transition flight are simulated respectively. A typical phenomenon in hovering condition called “fountain effect”, differences of aerodynamic interactions under different forward flight modes and the aerodynamic performance variation during different transition conditions are analyzed, as a result, the characteristics of unsteady interaction flowfield are revealed.In Chapter 5, the influences of different structure and aerodynamic parameters on the flowfield are analyzed. For different flight conditions, the interaction characteristics caused by rotor/wing spacing, wing flap angle, rotor rotation direction and tail configuration are simulated, and the corresponding parameter influence rules are obtained. Based on above researches, the interactions of tilt-rotor/fuselage/nacelle on wing are simulated respectively, resulting in a good foundations for the further investigations on aerodynamic optimizations on tilt-rotor aircraft with multi-objects and multi-conditions.In Chapter 6, based on the combination of the numerical simulation method and the surrogate model method, a highly-efficient optimal design method for tilt-rotor aircraft has been established. Then, in order to improve carrying capacity of tilt-rotor aircraft, taking decreasing the gravity increment in hover and drag in forward flight as optimal object, several sectional airfoils and locations of swept and forward swept are taken as the design variables, a comprehensive optimization design on the tilt-rotor aircraft in hovering and forward flight is carried out, as a result, advanced aerodynamic shape has been obtained.