## 1 Introduction  
With the rapid development of aviation technology, modern aircraft such as long-endurance unmanned aircraft, large transport aircraft and large civil airliners are characterized by light weight, low damping and large flexibility due to the high flight performance requirements [1; 2; 3; 4; 5; 6]. As a result, aeroelasticity has become an important factor that cannot be ignored in the design of these aircraft; rigid-elastic dynamics coupling is one of the aeroelasticity problems of aircraft, and this problem is pronounced in HWB civil airliners due to their unique structural layout and mass distribution. On the one hand, the structure of HWB aircraft is quite flexible, which leads to a low natural frequency of the first-order elastic mode of HWB. For example, Boeing's preliminary high-speed civil transport has a first-order symmetric bending mode with a natural frequency of 1.4 Hz [7]; on the other hand, the HWB aircraft has a relatively small rotational inertia about the pitch axis, resulting in a higher short-period mode frequency [8; 9; 10; 11]. The above two facts cause the difference between the frequencies of the rigid body motion mode and the aircraft structure elastic mode of the HWB to become smaller, which eventually leads to the coupling problem between the rigid body motion and the structural elastic motion of the HWB configuration to be more obvious than that of the conventional configuration. The decoupling relationship between the rigid body motion and the elastic structure motion of HWB aircraft is no longer obvious or even non-existent. For the above reason, the traditional analysis approach based on the rigid-body assumption is no longer applicable when analyzing the flight dynamics of an HWB aircraft, and a new analysis method considering aeroelastic factors must be developed. To this end, a series of studies on the rigid-elastic coupling flight dynamics for flexible aircraft have been conducted in the past years [12; 13; 14; 15]. The computation of the unsteady aerodynamic force caused by the rigid-body motion of the aircraft superimposed on the elastic deformation motion of the aircraft structure is an essential part of the rigid-elastic flight dynamics, and the main work of this paper is to establish a rigid-elastic coupling flight dynamics analysis platform from the perspective of using new unsteady aerodynamic force computational method. For the convenience of studying the dynamics and control problems, the double-let method is currently used in studies related to the analysis of rigid-elastic coupling flight dynamics. The state space form of unsteady aerodynamic force by DLM is conducive to being integrated into linear dynamics equations, which is convenient for dynamics analysis and related aeroelastic servo system design [16; 17; 18; 19; 20; 21]. However, the DLM has the following two deficiencies. Firstly, DLM is more suitable for unsteady aerodynamic calculation in critical states such as flutter motion, while the accuracy of unsteady aerodynamic force in the non-critical state cannot be guaranteed. Secondly, due to the limitation of theoretical assumptions, the computational accuracy of aerodynamic components (such as drag components) on some rigid body degrees of freedom is poor. In order to avoid the two deficiencies of DLM, the CFD method is considered in this paper to compute the unsteady aerodynamic force. The CFD method is directly based on the basic equation of the flow, and with fewer additional assumptions, it can more accurately describe the unsteady aerodynamic force caused by the rigid-elastic coupling motion. However, the unsteady aerodynamic forces obtained by the CFD method are in time-domain form, which cannot be converted into a state-space form such as DLM to facilitate the study of rigid-elastic coupling dynamics. For this reason, this paper plans to rely directly on the time domain analysis to achieve the research on rigid-elastic coupling flight dynamics, and a multidisciplinary co-simulation program including a CFD module is thus needed.