## 1 Introduction  
The drag performance of the fuselage is one of the main concerns of modern helicopter design, which will affect the payload and range of the aircraft [1, 2]. Obviously, as the complexity of the helicopter flow field is dominated by the evolution of the three-dimensional wake flow system of the rotor, the aerodynamic interaction between the rotor and fuselage is an important consideration in the prediction of the drag performance of helicopters. Different from the hovering condition, in the forward flight condition, due to the wake generated by the main rotor sweep along the fuselage and the interfere with the wake of the fuselage and attached components, the drag performance of the whole aircraft will be strongly affected [3].  
In the early stage, the aerodynamic interactions between the rotor and fuselage in forward flight were usually studied numerically based on a simplified fuselage without most of the geometric details [4, 5]. Since the appropriate representation of rotor-fuselage interactions is critical to the prediction of aerodynamic performance, in recent years, the work has mainly focused on the verification and confirmation of the numerical method based on the high-fidelity configuration with attached components [6]. The computational study of rotor-fuselage aerodynamic interactions was performed by [PERSON] [7, 8], where severe interactions of the flow structures of complete helicopter configurations in the simulation are reported. Under the framework of the GOAHEAD project, [PERSON] [9, 10] investigated the interference effects between the rotor wake and the flow field around the fuselage, and the influence of the fuselage on the rotor load was also studied. [PERSON] [11] presented an assessment of the predictive capabilities of several CFD methods for a complete helicopter configuration. Then, [PERSON] [12] predicted the wake structures and unsteady fuselage pressures associated with rotor-fuselage interactions. [PERSON] [13] verified the efficiency and robustness of the prediction of complicated unsteady rotor-fuselage aerodynamic interaction phenomena. Unsteady compressible flow analyses of helicopter rotor-fuselage interactions in forward flight conditions were carried out by [PERSON] [14]. Time-accurate numerical predictions of the interactional aerodynamics between the fuselage and four-bladed rotor were performed by [PERSON] [15] using high-order schemes.  
However, only a limited number of numerical studies have particularity addressed the aerodynamic effects of the main rotor blade rotating on the drag performance and flow field of the fuselage, including attached components such as the skid, horizonal tail, vertical tail and so on. Thus, based on dynamic patched-grid technology coupled with a three-dimensional N-S equation solver, the numerical study of the aerodynamic interaction effects between the rotor and fuselage on the drag performance and the flow field of a light civil helicopter is carried out under the forward flight condition. The deterioration of the drag performance and the evolution of the flow field structure caused by the rotation of the blades are illustrated, and then the aerodynamic interactions between the rotor wake system and the flow field around the fuselage are investigated to provide an improved physical understanding of the blade rotating effects. A detailed analysis of the physical mechanism of the blade rotating effects on the flow field around the fuselage is carried out.  
This paper is organized as follows. Section 2 presents an overview of the methodology suitable for analyzing the rotating problems implemented in this study and the validation of the numerical approach. Section 3 provides a comparison of the drag performance between the rotor-off and rotor-on configurations in forward flight, and a detailed analysis of the aerodynamic interaction effects between the rotor and fuselage on the flow field is given. Finally, the conclusion is reached in Section 4.