Numerical Study On Unsteady Aerodynamic Force Of A Tilt-rotor Aircraft In Conversion Mode

The tilt-rotor aircraft is a new flight vehicle that combines the vertical takeoff and landing capability of the helicopter with the efficient high-speed cruise performance of the conventional turboprop aircraft.The conversion mode is one of the unique feature of this aircraft.Though the transition time is short,it’s a complex unsteady motion process.It has very important academic and engineering application value to carry out relevant numerical study on unsteady aerodynamic force of tilt-rotor aircraft in conversion mode.Based on unsteady N-S equations,studies are carried out on several key techniques of tilt-rotor aircraft in conversion mode,including: the dynamic mesh technique,high-efficiency CFD methods,the aerodynamic characteristics in helicopter mode and airplane mode,the unsteady aerodynamic force in transition mode,and the flow characteristics of thrust vector and air rudder control.A hybrid dynamic mesh generation method for multi-block structured grid is presented based on inverse distance weighting(IDW)interpolation and transfinite interpolation(TFI).Firstly,all domain vertexes with known deformation are selected as control points,and apply IDW to calculate the mesh deformation on block edges.Then,TFI is used to compute the mesh deformation on block faces and inside each block.Aiming at extremely large deformation,a novel predictor-corrector-based dynamic mesh method is proposed.This technique is intended to obtain dynamic mesh from multiple sets of pre-generated grids which provides a new idea for dynamic mesh generation.The finite volume scheme and dual time-stepping approach are used to solve the unsteady N-S equations.The SA turbulence model is applied to the simulation of turbulent flows.The rotor is modeled as an actuator disk,ignoring the effect of individual blades.A parallel algorithm is utilized to improve the computational efficiency.The field veloc ity approach is developed to accurately simulate the accelerated movement by using a fixed grid.The present numerical method and software platform are applied to all phases of flight and several examples are used to demonstrate the effectiveness.The flow mechanism in hover is analyzed.Four passive flow control strategies,namely trailing edge flap,Kruger flap,droop-nose and spoiler are used to research the download reduction for tilt-rotor aircraft in hover.The effects of different factors are discussed.The optimal assembly configuration: trailing edge flap deflection angle is 60° and Kruger flap angle is 85°.Then,the flow field in high-speed airplane mode is simulated.The increments of aerodynamic coefficients caused by slipstream are presented.The results show that the rotor slipstream can alter the pressure distributions on wing surface evidently,which increases the lift coefficients and the increment of the maximum lift coefficient is 4.6%.Further studies show that rotor slipstream has greater effect on the lower surface of the horizontal tail,while makes a small impact on the vertical tail.The conversion envelope is established according to the restrictions on wing stall and available power.As the whole unsteady effects of the aerodynamic induced by rotor tilting,aircraft acceleration,posture changing and flap deflection are considered.Quantitative and qualitative comparisons are made on the aerodynamic coefficients between quasi-steady fixed conversion mode and time-accurate continuous transition flight condition.The aerodynamic characteristics during conversion process are conducted.The general research ideas of the thesis are from two-dimensional wingtip section to three-dimensional assembly,from quasi-steady fixed tilt angle to continuous transition,from constant flight to accelerated flight,and from helicopter-airplane transition to airplane-helicopter transition.Moreover,the influence of different tilt modes and tilt time on unsteady effects are also conducted.The numerical simulation techniques about maneuvering flight integrating aerodynamics and flight control are developed,which could coupling solve the unsteady flow field of thrust vector and air rudder control.At first,the static aerodynamic characteristics of tailing edge flap are studied,and the dynamic characteristics of the different deflection rate and laws are analyzed.Then,the open-loop responses with respect to the flap deflection are investigated in detail using the types of step,sine and linear functions.The research results of this thesis can provide an advanced numerical method and lay a technique foundation for the studies on unsteady aerodynamic characteristics,thrust vector/air rudder control design and other related problems in conversion mode.