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High requirements have been placed on the stealth performance of armed helicopters during wartime missions，in order to reduce the probability of the helicopter being found during the task，based on geometric optics method and consistent diffraction theory，the effects of different tail rotor profile thickness，curvature and tail rotor scissors angle on RCS are studied. By comparing different parameters of the tail rotor and the RCS peak value and mean value of the coated absorbing material，the conditions which are more favorable to the stealth of the helicopter tail rotor are determined. When viewed from the ground，the RCS with small tail rotor thickness and small curvature will be reduced appropriately，and the change of scissors angle will lead to the change of RCS peak phase. When the radar is irradiated in parallel，the RCS of tail rotor with smaller thickness is smaller，and the RCS peak value of tail rotor with smaller curvature is smaller. Applying wave absorbing materials on the leading edge and tip of the blade can effectively reduce the overall RCS of the blade. Comprehensive analysis shows that by using the smaller thickness and curvature of the tail rotor，it is beneficial to reduce the RCS of the helicopter tail rotor，and the blade tip surface and the blade leading edge of the absorbing material coating can effectively reduce the overall RCS.

To investigate the effects of different new types of blade-tip on transonic acoustic characteristics of a helicopter rotor, numerical analyses about the impacts of blade-tips on the high-speed impulsive (HSI) noise characteristics have been carried out by taking the UH-1H model rotor as baseline. A CFD simulation method is developed by solving the compressible Reynolds-averaged Navier-Stokes (RANS) equations with Baldwin-Lomax turbulence model, and a high-precision Roe-MUSCL scheme and high-efficiency implicit LU-SGS scheme are employed for spatial and temporal discretization respectively. Based on the validation of CFD method, a robust FW-H_pds method based on the penetrable data surface is established for predicting rotor HSI noise, and the transonic acoustic characteristics of UH-1H model rotor have been simulated to verify the effectiveness of the present method. Based on these methods, the influences of Mach number on the phenomenon named "delocalization" nearby the blade tip and the HSI noise characteristics are analyzed, and the generation mechanism of transonic noise is revealed by the comparisons of the monopole noise and quadrupole noise. Then, the influences of both sweepback and sweep-forward blade-tip on inhibiting rotor transonic acoustic characteristics are analyzed at a transonic state (Mach number 0.9) respectively. The numerical results indicate that the serious delocalization phenomenon and the HSI noise radiation occur strongly with the increase of the blade-tip Mach number. Compared with rectangular blade, the rotors with sweepback or sweep-forward blade-tip can reduce HSI noise level at a transonic state by suppressing the supersonic flow and delaying the occurrence of delocalization effects. Furthermore, sweep-forward blade-tip has a more remarkable effect on inhibiting delocalization effects by driving supersonic flow region to move to inside of blade. [ABSTRACT FROM AUTHOR]

In order to increase the prediction accuracy on helicopter radar scattering characteristics, the computational electromagnetics method (CEM) is established to analyze radar scattering characteristics, and the influence of structure parameters on radar cross section (RCS) characteristics is studied. The differential forms of Maxwell's equations are taken as the governing equations for solving electromagnetic property, computational electromagnetic grids are generated by adopting the Yee grid technology, and the governing equations are solved by temporal and spatial discretization with central difference scheme. The perfect matching layer (PML) technique is employed as the absorbing boundary conditions, and the numerical method with high precision for helicopter RCS is established by combined with the equivalence principle of near-far field extrapolation. Two-dimensional conductor cylinder with infinite length and three-dimensional metal cube are taken as numerical examples respectively. The results show that CEM has higher precision than the high frequency method obviously. On this basis, researches on the influence mechanism and variation characters of airfoil thickness, airfoil camber and the number of blades on rotor radar scattering characteristics are carried out, and RCS characteristics of fuselage with different structure parameters are computed, then the influence mechanism of short wings and horizontal tails on fuselage strong scattering characteristics is analyzed. It is demonstrated that blade tip is one of the significant scattering sources of rotor, rotor with thin airfoil can improve the radar scattering performance effectively, and short wings and horizontal tails can increase the RCS of fuselage in different angles, which should be taken into key consideration in stealth design of helicopter. [ABSTRACT FROM AUTHOR]

Considering the characteristics of several kinds of rotor movements and complicated fuselage geometry shape of rotorcraft, a robust generation method for 3-D structured embedded grid is established. 3-D preliminary grids are generated by using the Poisson equations for iterations, then 3-D structured grid around the rotor blade with high quality is acquired. Based on this method, a parabolic marching method along the normal direction is established, which uses surface grid around complicated geometry shape as its preliminary grid. In the marching process, new Poisson equations modified by parabolic means, which source terms are determined by the Hilgenstock way, are used for smoothing iterations, and the difficulties of adjusting points on boundaries by using elliptic grid generation method are overcome effectively. The advantages of the parabolic method is proved by using examination of minimum orthogonality of grid. The efficient way of hole cutting ("Top map") and donor cells searching ("Inverse map") are introduced further to 3-D structured grid generation above, and the method for generating moving-embedded grid around rotorcraft is established. It is demonstrated by conducting computational fluid dynamics (CFD) calculations on the flowfields of Caradonna-Tung rotor and V-22 tilt-rotor aircraft respectively using the RANS equations that the grid generation method could meet the demands of CFD calculations. [ABSTRACT FROM AUTHOR]

Aiming at reducing the calculation amount of the CFD simulation on aerodynamic force of tiltrotor in transition flight, a new highly effective hybrid CFD method is proposed. An embedded grid system with the corresponding digging hole and donor element searching method is presented. By combination of blade element theory and momentum theory, a simplified momentum source technology, i.e. virtual rotor model (VBM) is established. In order to analyze the details of aerodynamic force of the tilt rotor in transition flight, the multilayer embedded grid system and the single program multiple data (SPMD) model parallel acceleration technology are adopted in the real blade model (RBM).Then, a highly efficient hybrid blade model (HBM) is established by combining VBM and RBM methods. Finally, a typical rotor and 7A rotor are taken as numerical examples to verify the validity of VBM and RBM respectively, and the aerodynamic force of tiltrotor in transition flight with different tilt angles is analyzed by three methods also. It is demonstrated that VBM shows the most optimal efficiency in the simulation of the aerodynamic characteristics of the tiltrotor in conversion. One third computation time is saved by the HBM method compared with the RBM method with the guaranteed accuracy of the flow solver. [ABSTRACT FROM AUTHOR]

Aiming at improving the CFD simulation efficiency on the transonic viscous flow around rotor, the implicit LU-SGS algorithm and OpenMP parallel strategy are employed for highly efficient predicting the unsteady flowfield of rotor. Firstly, the O-C-O type orthogonal and body fitted grid around rotor blade is generated by solving Poisson equations for the airfoil, interpoloting and folding approach. Then the ″disturbance diffraction method″(DDM) is utilized in the identification of hole cells, and the parallel ″inverse map″(IM) method is adopted for searching the donor cells, thus constructing the rotor moving embedded grid system. On these bases, the unsteady RANS equations coupled with SA turbulence model are used as the governing equations, the high-order scheme of Roe MUSCL is used for spatial discretisation of convective fluxes, a dual time based implicit scheme of L-SGS is adopted for temporal discretisation, and the data sharing OpenMP parallel strategy is employed to accelerate the calculation as well. Finally, aerodynamic characteristics and blade tip vortex characteristics of several rotors, i.e. hovering ONERA 7A rotor, forward flight non-lifting Caradonna Tung(C-T) rotor and lifting SA349/2 rotor are simulated by the presented method. By comparison of numerical results with experimental data, it demonstrates the effectiveness and high efficiency of the presented method on the CFD simulation of the unsteady rotor flowfield. [ABSTRACT FROM AUTHOR]

A high-efficiency and high-precision CFD method for simulating the unsteady dynamic stall of rotor airfoil has been established based on moving-embedded grid and compressible RANS equations. Firstly, the generation method of viscous and orthogonal body-fitted grid around the rotor airfoil is developed by solving Poisson equations. Meanwhile, aiming at overcoming the shortcoming of spring simulation approach which may result in the distortion of grid, an improved Minimum Distance Method is proposed to generate the embedded grid around airfoil. Secondly, in order to simulate the hysteresis effect of aerodynamic forces caused by the turbulence separation and re-attachment of the flow, a high-precision method on the analysis of unsteady aerodynamic characteristics of rotor airfoil is developed by employing RANS equations and dual-time method. The S-A turbulence model is employed to capture the separation phenomenon of flow around airfoil. Thirdly, according to the conditions of low-speed inflow and high AOAs of the retreating blade, together with the limitation of L-B semi-empirical model on the calculation of unsteady dynamic stall of airfoil, a combination method of Pletcher-Chen preconditioning, FAS multigrid approaches and implicit LU-SGS scheme is established to overcome the problems of convergence difficulty and insufficient precision of compressible equations. The steady, mild and deep dynamic stall cases of NACA0012 and SC1095 rotor airfoils are calculated using this previously mentioned method, the hysteresis effect and the formation, convection, shedding of the vortical disturbance are well captured, the effectiveness of numerical simulation method on dynamic stall is verified. Finally, focus on the deep stall of NACA0012 airfoil, the influence analyses of parameters on the unsteady aerodynamic forces of rotor airfoil are carried out, and the results demonstrate that the exchanges of averaged AOA, amplitude and reduced frequency may cause a variational hysteresis effect and regularly changes of peak value of aerodynamic force. [ABSTRACT FROM AUTHOR]

The rotor airfoil has important influence on aerodynamic characteristics of helicopter and its rotor. Incorporating with the flight characters of helicopter and aerodynamic circumstance of helicopter rotor, a new design concept of rotor airfoil is proposed, which is the optimization design method considering both steady and unsteady states for helicopter rotor airfoil design. Firstly, the high accuracy CFD method is established to simulate the unsteady aerodynamic characteristics of rotor airfoil. Considering the characters of optimization condition for rotor airfoil, the genetic algorithm (GA) and the sequential quadratic programming (SQP) method are employed in steady optimization and unsteady optimization process respectively. In order to obtain an initial airfoil, the optimization design is firstly accomplished under steady condition. Then the optimization design under unsteady condition is implemented to form a new airfoil with unconventional configuration which is suitable as the rotor airfoil. The optimized airfoil has remarkable improvements on dynamic stall characteristics with obviously weakened separated vortex while it maintains considerable aerodynamic characteristics of steady condition. [ABSTRACT FROM AUTHOR]

The relationships between the section normal mach number and the equivalent angle of attack of the rotor with a swept tip and the mach number and angle of attack of the rotor with a rectangular tip are derived, and a new conclusion that the normal incoming flow on a swept blade tip are not always less than that on a rectangular tip in forward flight is presented and proved. As a result, an analytic azimuthal interval where the normal incoming roach number on a swept tip is bigger than that on a rectangular tip is given. On the contrary, in order to avoid the emergence of the analytic azimuthal interval, a sweepback angle distribute regulation is presented. A design program of the rotor with a curvilinear swept tip for keeping the velocity of leading-edge normal incoming flow constant is raised. Then, numerical simulations of transonic flowfield for a helicopter rotor with a swept tip in both hover and forward flight are performed by using CFD. Based upon the theoretical analysis results and numerical simulated results, the effects of a swept blade tip on the flowfield and aerodynamic characteristic of the rotor, especially the changing of shock wave location, scope and strength nearby the blade tip are analyzed thoroughly. [ABSTRACT FROM AUTHOR]

In order to enhance the analytical accuracy of advanced rotor aerodynamic characteristics, the aero-elastic effects are coupled into high fidelity CFD analysis method instead of rigid blade assumption. Considering the quasi-steady characteristics of rotor flowfield in hover, a CFD/CSD coupling analysis model for the helicopter rotor is developed based on the non-inertia frame. Airloads are obtained through solving 3-D Navier-Stokes equations with Jameson central difference scheme used in spatial discretization, five-step Runge-Kutta method adopted in temporal integration and B-L turbulent model. Based on the Hamilton principle, nonlinear differential equations which describe the elastic motions of the rotor are formulated and then calculated by Raphson iteration method. During the process of coupled CFD/CSD calculations, airloads and deformation information are exchanged between CFD and CSD module, and grid deformation method based on the direct coordinate transformation is adopted to improve grid deformation efficiency. Finally, the surface pressure distributions and airloads of UH-60A helicopter in hover are calculated, and it is demonstrated that the present CFD/CSD coupling model can predict rotor airloads more accurately compared to CFD method with rigid rotor assumption. [ABSTRACT FROM AUTHOR]