## 1 Introduction  
As comprehensive products of the global rapid arrival concept and high-speed flight technology, hypersonic vehicles have the characteristics of high speed, strong maneuverability, long flight ranges, and strong penetration ability [1]. As a typical hypersonic vehicle configuration, hypersonic waveiders exhibit fast time variance, strong nonlinearity, and uncertainty during flight, which pose challenges to flight control [2]. In modern scenarios, flight environments and missions are imposed with more complexity and diversity, which creates more stringent requirements for the flight performance of the aircraft [3, 4]. Due to the fixed configuration of these vehicles, the flight range and maneuverability of traditional hypersonic waveiders are often limited. By combining morphing technology with hypersonic vehicles, a hypersonic morphing waveider (HMW) can be designed. Since the advantages of both morphing technology and hypersonic vehicles are integrated in HMWs, an HMW can change its geometric configuration adaptively according to different flight environments and missions, which enables it to achieve the best performance at hypersonic speed throughout the entire flight [5, 6]. For instance, to achieve a larger downrange, the configuration with the best lift-to-drag ratio can be obtained for HMWs. Similarly, a low drag configuration can be obtained to reduce the velocity loss, thus maintaining the attack kinetic energy. In the past few decades, with the advancement of novel materials, structural mechanisms, and actuation devices, as well as the large-scale integration and application of them in aircraft design, morphing aircraft have ushered in a new development climax [7, 8]. The U.S. Naval Research Laboratory (NRL) has conducted extensive research on morphing waveiders in recent years. In reference [9], a morphing bottom surface was used to design a morphing waverider, and the aerodynamic performance was analyzed in terms of the constant dynamic pressure and constant altitude under different Mach numbers. The feasibility of the NRL morphing waverider concept was further investigated, and it demonstrated that by deforming the lower stream surface, a morphing waverider could obtain optimal aerodynamic performance across Mach numbers ranging from Mach 5 to Mach 10 [10]. In [11], control requirements and performance were investigated in a morphing waverider under a range of Mach numbers from Mach 3.5 to Mach 5. In addition, some other morphing waveriders have been designed. In [12], a waverider with a variable swept wing was designed to achieve the best flight performance. In [13], a multistage waverider was designed based on conical flow theory, and the aerodynamic performance was analyzed. The studies above mainly focus on configuration and aerodynamic characteristics, which is one basic part of HMWs [14]. Considering the strong coupling caused by deformation, HMWs have higher nonlinearity, stronger time variance, and larger uncertainty than conventional hypersonic waveriders, and it is difficult to satisfy the high stability, robustness, and control accuracy requirements with traditional control methods. Therefore, the establishment of a motion model of an HMW and the design of a controller with strong robustness are great challenges. Up to now, various control algorithms have been applied to morphing aircraft controller designs. Reference [15] introduced the modeling of a large-scale planform altering flight vehicles systematically and discussed the principles of flight control. Sliding-mode control is widely used in nonlinear control because of its robustness [16]. In reference [17], a sliding-mode controller was proposed for a tailless morphing wing aircraft to track the maneuver reference command to avoid difficulty in precise aerodynamic modeling.