## 1 Introduction  
The tilt-rotor UAV (TRUAV) can take-off and land vertically, and also has the advantage of long endurance, which has drawn considerable interest from interested military and civilian parties due to its potential applications. According to the rotor number, the tilt-rotor UAV can be divided into two types: Dual-TRUAV and Multi-TRUAV [1]. The Engle eye UAV, which has two tilt rotors mounted on the wingtip, is the representative and first practical application of the Dual-TRUAV [2]. Adopting the same configuration, Korea Aerospace Research Institute (KARI) and Korean Air (KA) have developed a tilt-rotor UAV named Smart UAV [3, 4]. The flight tests of Smart UAV are completed, but until now, this type of UAV has not been applied in practice. The Dual-TRUAV has the disadvantages of a complex tilt mechanism and serious interference between the rotors and wings, which means that its design, manufacturing, and flight control have great technical challenges and high cost. In view of that, many countries have begun to explore and research the Multi-TRUAVs, which provides a new solution for the development and application of the tilt rotor UAVs, such as the Panther of Israel Aircraft Industries [5], TURAC of Istanbul University [6], etc. The Multi-TRUAVs have a simplified mechanical structure and more symmetrical aircraft layout in the longitudinal direction. However, due to the increase of the rotors and the existence of multiple flight modes, there are still some problems that need to be solved in the design and flight control of Multi-TRUAVs, thus it has attracted the attention of many researchers [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. [PERSON] designs a flying-wing UAV named TURAC which has two rotors and one main coaxial fan [8]. The mathematical model of TURAC was derived and the aerodynamic coefficients were calculated by CFD simulations. Based on these, the transition strategy was also proposed to achieve mode switching [9]. [PERSON] designed a tilt tri-rotor UAV, all of whose rotors can tilt. Based on this prototype, a series of research work was carried out [10; 11]. It adopted an explicit model predictive control scheme relying on constrained multiparametric optimization to achieve flight control. [PERSON] developed a quad tilt-rotor UAV; only the two front rotors can tilt. The model of the vehicle is constructed based on experiments and numerical analysis, and a control scheme composed of robust servo linear quadratic regulator and extended state observer is designed [12]. [PERSON] and [PERSON] developed a unique tilt-rotor UAV which was controlled by a proportional integral differential (PID) controller; the simulation and flight test were also carried out [13; 14]. As for a tilt tri-rotor UAV with the rear servo's stuck fault, [PERSON] designed a robust integral of the signum of the error based controller to maintain the tilt tri-rotor UAV's attitude stability [15]. In the actual flight process of the Multi-TRUAVs, it commonly suffers from many control difficulties. A PID control method commonly was used in the flight control of the tilt rotor UAVs [16; 17]. [PERSON] adopted the conventional PID control to generate the control inputs of a tilt rotor quadplane, and the control performances of the tilt-rotor configuration and the pusher quadplane configuration were also compared. The simulation results indicated that the conventional PID control is still more challenging in the control of the tilt-rotor configuration [17]. [PERSON] reviewed some linear control methods such as state feedback, LQR, and robust control which were often used in the controller design of the tilt rotor UAVs [1]. However, these methods struggle to deal with the strong nonlinearity and external disturbances. The dynamic characteristics of the tilt rotor UAVs are inherently strong nonlinear. Meanwhile, the model is usually subject to parametric uncertainties and unmodeled dynamics.