## 3 Analysis of Main Rotor Rotating Effects
### Flow Field  
To identify the general flow features caused by the rotation of the main rotor, the flow fields of the rotor-off and rotor-on configurations are examined, where the azimuth position of the blade is \(\Psi=0^{\circ}\). Figure 16 shows the flow fields in the Q-criterion [24] and the streamlines of the two configurations, which are characterized by the Mach numbers. Figure 15: Surface pressure distribution of the rotor-on configuration at various azimuth angles of the rotor. From Figure 16a, the flow of the afterbody of the fuselage is attached in general, while a vortices system is generated backward at the junction of the fuselage and landing skid. The vortices system interferes with the components of the rear fuselage and affects the aerodynamic performance of the horizontal tail. Simultaneously, the pillars induce the wake of the skid, while the horizontal tail and other components also generate vortices that develop toward the rear fuselage. In Figure 16b, the blade tip vortices pulled out by the main rotor are developing downward and backward. The wake vortex from the tip and root of each blade forms a complex vortices system. The rotor wake sweeps along the fuselage and affects the flow field near the fuselage and other components. The flow field of the upper surface of the fuselage becomes more complicated due to the interference of the rotor wake in the vortices induced by the fixed components. Figure 16: Comparison of the general flow field in the Q-criterion and streamlines between the rotor-off and rotor-on configurations (Q = 10.0). However, the structures of the vortices at the junction of the fuselage/landing skid are kept unchanged as the rotor wake mainly develops backward. The influence of the rotor wake on the vortices system mainly manifests in a change in the wake flow direction, namely, the lateral deflection of the wake system. In addition, it seems that the strength of the wake is weakened by the existence of blade tip vortices, especially for the front pillars, which contributes to the decrease in the landing skid drag. The vorticity distribution of the two configurations along the X-axis is plotted in Figure 17. Figure 17(a) depicts that the vorticity magnitude of the forward skid wake is large in the vicinity of the fuselage and at the bottom of the skid. As the skid wake develops to the rear fuselage, an obvious vorticity magnitude can still be observed. At this time, the vorticity magnitude of the rear fuselage wake is low at the rear fuselage, indicating that the flow separates there due to the interference between the downward skid wake and the rear fuselage wake. Similarly, the vorticity magnitude of the wake downstream of each stake gradually decreases, while the vorticity magnitude near the top fuselage is large. From Figure 17(b), the vorticity distribution of the rotor-on configuration illustrates that the vorticity of the advancing rotor blade is greater than that of the retreating blade. By this time, the vorticity magnitude at the top of the fuselage is larger than the vorticity shown in Figure 17(a), which is caused by the blade tip vortices and can be regarded as an important source of drag increment. As the wake vortex generated by the rotor develops downward and backward, the vorticity distribution of the skid wake, the rear fuselage and the downstream wake demonstrate significant changes. The vorticity distributions at various sections aligned in the direction of the X-, Y- and Z-axis are plotted for comparative analysis, as shown in Figure 18. Here, the development of the rotor and fuselage wake in the flow field can be demonstrated. The study of the vorticity distribution along the X-axis focuses on the flow structure at the rear fuselage, while the study of the vorticity distributions aligned in the directions of the Y- and Z-axis focuses primarily on the effect of the rotor. Figure 17: _Cont._  
Figure 19 further depicts the plots of the vorticity distribution of the afterbody aligned in the direction of the X-axis.