## 2 Fuselage Aerodynamics  
The first step of this work is the simulation of the flow around the isolated \"clean\" fuselage. The Helicopter Multi-Block (HMB) solver is established in the field of rotorcraft aerodynamics and it is based on the discretization of the Unsteady Reynolds Averaged Navier-Stokes (URANS) equation on multi-block structured grids. The solver allows for sliding and overset grids and has a variety of solution methods for flows at low or very high Mach number. A mesh deformation method based on a combination of the Trans-Finite Interpolation and the Spring-Analogy method allows for aero-elastic cases to be studied. Also, a sliding-mesh method is implemented so that test cases with relative motions of different parts of the geometry can be modeled. The HMB method has been validated for a range of rotorcraft applications [13; 14; 15; 16; 17; 18] and has demonstrated good accuracy and efficiency for very demanding flows. The parallel implementation makes use of the Message Passing Interface library for inter-processor communication and of parallel I/O for saving and reading data from out-of-core storage. The HMB method has so far been used for the analysis of rotors, wind turbines, propellers and cavities and has demonstrated good scalability for up to 10 k cores. This was of course achieved using a fine mesh. A summary of the method in HMB is presented in reference [13]. The code can also use actuator disks or virtual blade models to simulate the effect of the main rotor on the fuselage. Although the solver is able to use Detached-Eddy Simulation (DES) and Large-Eddy Simulation (LES) models, the URANS equations were used in this work. This was justified by the rather limited regions of flow separation encountered during computations. In general, different models of turbulence, including the Spalart-Allmaras one-equation model, the \(k\)-\(\omega\) (SST) 2-equation model and transition models of turbulence are available in HMB solver. The geometry of the isolated fuselage corresponds to an early Ansat-P model of the Ansat helicopter produced by the Kazan Helicopters. The wind tunnel model (Figure 1a) was manufactured based on the computer model, used for CFD modeling. The aerodynamic analysis of this model was considered in references [19; 20], and were studied using the open test section (2.25 m diameter) closed circuit, low speed, wind tunnel T-1K of the KNRTU-KAI Aerohydrodynamics department. For the grid around the Ansat-P fuselage, the (ICEM)-hexa software of ANSYSTM mesh generation tool has been used. The length of the wind tunnel model (Figure 1a) was \(L_{F}\) = 1.8 m. The computational grid for this model contained 964 blocks and 13.5 \(\times\) 10\({}^{6}\) cells. The surface grid and grid details are shown in Figure 1b-d. The topology and surface grid near the area of the engine exhausts is also presented on the same figure. Care has been taken to represent the geometry of the wind tunnel models as accurately as possible, regardless of the minor edits that the laser-scans of the models needed in order to be converted to air-tight surfaces suitable for CFD. In terms of turbulence modeling, the \(k\)-\(\omega\) model [21] was employed. Experimental and CFD analyses were conducted at Reynolds number of 4.4 \(\times\) 10\({}^{6}\) and Mach number of 0.1. Figure 2 suggests a good agreement between CFD and experimental results for the lift (\(C_{L}\)) and drag (\(C_{D}\)) coefficients in the considered range of pitch angles. More detailed information about grid sensitivity studies and the HMB code validation vs. experimental data is presented in [14; 19; 20]. Figure 1: (**a**) Ansat-P fuselage model in the T-1K wind tunnel of KNRTU-KAI, (**b**) surface grid for fuselage, (**c**) multi-block topology, and (**d**) surface mesh near exhausts. Figure 2: Computational Fluid Dynamics (CFD) and experimental drag coefficients vs. lift coefficient for Ansat-P model. The next step was the simulation of the flow around the rotor model in hover [22].