## 3 Analysis of Main Rotor Rotating Effects
### Drag Performance  
The simulation is carried out under the forward flight condition as listed in Table 2 as a typical forward flight case of \(\alpha=-8^{\lx@math@degree}\). The physical time step size is set at \(\Delta\varphi=0.25^{\lx@math@degree}\). Similarly, the calculation of the rotor-on case is carried out for 50 full revolutions first to provide convergence, then for 10 full revolutions to obtain a reliable solution. The total numerical effort in CPU time is 480 h and the number of processors is 64. To investigate the drag performance of each component, the configuration is decomposed into several parts, including the fuselage, horizontal tail, left tail plane, right tail plane, landing skid and vertical tail. Table 3 compares the \(C_{D}\) of the different components of the two cases at \(\alpha=-8^{\lx@math@degree}\), which reveals that the total \(C_{D}\) increases by 0.102 (or 57.9%). Further observation shows that the \(C_{D}\) of the fuselage increases by 0.098, which contributes to the main increment. The \(C_{D}\) of the horizontal tail and vertical tail increase by 0.014 and 0.006, respectively. The \(C_{D}\) of the left tail plane increases by 0.004, while the \(C_{D}\) of the right tail plane decreases by 0.004, which cancel each other out. Notably, the \(C_{D}\) of the landing skid is reduced by 0.016. Figure 12 shows the comparison of the surface pressure distribution and streamlines between the rotor-off and rotor-on configurations, where the azimuth position of the blade is \(\Psi=0^{\lx@math@degree}\). From Figure 12a,b, it can be seen that the pressure on the afterbody and forehead of the rotor-on configuration increases due to the downwash of the main rotor, meaning an obvious low-pressure area on the left side of the cockpit can be observed. Low-pressure areas also appear on the left side and the upper surface of the fuselage. The pressure on the upper surface of the tail boom and the horizontal tail, together with the inner side of the tail plane, significantly increases. The streamlines on the rotor-off configuration distribute smoothly along the surface, while the streamlines on the tail boom of the rotor-off configuration obviously bend downward and backward, passing over the horizontal tail. From Figure 12c,d, a low-pressure region appears on the bottom surface of the horizontal tail for the rotor-on configuration, and the significant asymmetry of the pressure distribution can be observed. Due to the rotor wake induced by the advancing blade passing over the left tail plane, the rotor wake interferes with the bottom surface of the horizontal tail, resulting in the asymmetry of the pressure distribution. From the comparison of the surface pressure distribution in the symmetry plane of Figure 13, the source of the drag increase under the rotor-on condition is demonstrated clearly as the after body, where a large gradient of local pressure is observed. The \(C_{D}\) of the rotor-on configuration in one full rotor rotation is shown in Figure 14. A periodic fluctuation is observed, which indicates the aerodynamic interactions between the main rotor and the fuselage. Since the main rotor consists of four blades, the fluctuation in the \(C_{D}\) can be considered as a period of \(\Psi=90^{\lx@math@degree}\). In one fluctuation period, the \(C_{D}\) reaches the lowest value at the blade azimuth angle of \(\Psi=18^{\lx@math@degree}\), then the \(C_{D}\) gradually increases with the azimuth angle, reaching a peak at \(\Psi=72^{\lx@math@degree}\). The change in amplitude of the \(C_{D}\) is around \(\pm 20\%\) in one period. The \(C_{D}\) of each component at four azimuth angles during a period of rotation are summarized in Table 4.  
Figure 15 shows instant snapshots of the pressure distribution at several azimuth angles. At \(\Psi=0^{\lx@math@degree}\), a large area of high pressure occurs on the forehead of the fuselage