## 1 Introduction  
Faster flight speed, larger aerodynamic load, and higher maneuverability and agility are the design concepts generally pursued by modern aircraft, especially military aircraft, which leads to lighter weight and greater flexibility of aircraft; thus, the aeroelastic effect caused by such structural characteristics cannot be ignored [1]. For the study of the aeroelasticity of aircraft, although the results obtained by wind tunnel experiments are accurate [2], their high cost is difficult to meet with the current design requirements; therefore, in order to balance efficiency and cost, numerical analysis methods based on computational fluid dynamics (CFD) and computational structural mechanics (CSD) have become current research hotspots. Traditionally, analysis of the aeroelastic characteristics of aircraft was carried out under two separate disciplines: flight mechanics and aeroelastic mechanics. However, with the continuous upgrading of modern aircraft, the elastic mode frequency of an aircraft is more likely to overlap with the frequency range of its rigid mode, which means that the coupling effect between the rigid body motion and the low-order elastic vibration of the aircraft is more prominent; therefore, the traditional flight dynamics research method based on elastic correction can no longer meet the requirements; meanwhile, the results obtained are inaccurate if the rigid body motion of the aircraft is ignored in the study of aeroelasticity [3;4]. Therefore, it is necessary to establish a rigid-flexible coupling model to analyze the flight dynamics of elastic aircraft. Many researchers have performed relevant research by modeling of the flight dynamics of elastic aircraft. [PERSON] [5] and [PERSON] [6], respectively, used the Lagrangian equation to derive the flight dynamics model under the assumption of the mean axis. The [PERSON] model is formally similar to the traditional rigid body dynamics model, and the elastic and rigid degrees of freedom are decoupled; the coupling between the two is achieved by aerodynamic loads. The [PERSON] model is more complex, in which the translational degrees of freedom take the same form as in the [PERSON] model, but there is a coupling between rotational and elastic deformation, and the change in the inertial tensor due to the deformation of the aircraft structure is also considered. The quasi-coordinate is fixed at the aircraft and its coordinate origin is located on the aircraft in an undeform state, which can be located on the center of mass or placed in other positions. [PERSON] [7] was the first to derive an equation for coupling rigid rotation and elastic deformation, in which the elastic deformation is described in a quasi-coordinate system, while the translation and rotation of the aircraft are described in terms of the position and attitude angle of the quasi-coordinate system in an inertial coordinate system. Although the mean axis method can greatly simplify the equation of motion of the aircraft and is easier to solve, this simplification is difficult to meet in calculation and requires a great cost; on other hand, the quasi-coordinate system can well describe the cross-coupling relationship between rigid and elastic degrees of freedom, so it has been favored by many researchers [8; 9; 10]. Based on this characteristic, this paper adopts the quasi-coordinate system method for aircraft dynamic modeling. Many other researchers have also studied the modeling of flexible aircraft. [PERSON] [11] established a rigid body structure with flexible attachments as a rigid body-cantilever beam coupling model. [PERSON] [12] studied a rigid-flexible coupling dynamic model of a new spacecraft system, and the effectiveness of the dynamic model was verified by comparing it with the results of ADAMS. [PERSON] [13] addresses the issue of load sources generated by the interaction between aircraft structures, flight maneuvers, and airflow fields, exploring how to establish mathematical models through simplified methods.