## 1 Introduction  
The quadrotor has a wide range of applications in both military and civilian applications due to its ability to adapt to all types of flight conditions. For a traditional planar quadrotor UAV, rotors are rotating in the same plane with four degrees of freedom. When rotors are tilting an angle around the axis of the rotor arm, this kind of non-planar quadrotor UAV obtains the six degrees of freedom. Additionally, the rotation speeds of four rotors are the only four independent control inputs, which do not allow independent control of the position and attitude. For example, the horizontal movement needs the change in the attitude [1; 2; 3; 4; 5; 6; 7; 8]. Therefore, inspired by the tilt angle of the rotor, a non-planar quadrotor structure with a more controlled advantage has been developed in last 5 years [2]. The non-planar quadrotor has an additional actuation freedom that provides complete control of the position and the orientation of the quadrotor, making it virtually a fully actuated aircraft. Considering that the non-planar design provides a total of 6 degrees of freedom of manoeuvrability, the manoeuvrability will be an important feature of many future applications of UAVs, especially those involving interaction/manipulation in a complex environment. However, most of the research conducted for quadrotor has focused on attitude control and trajectory tracking. [PERSON] and [PERSON], et al., used a PID controller-stabilized quadrotor in simulation and actual flight experiments [9; 10; 11]. Also, in [12; 13], the reverse thrust and sliding mode methods have been utilized to control the quadrotor tracking trajectory. [PERSON] and [PERSON], et al., established the dynamic model of additional aerodynamic effects for non-planar quadrotor aircraft and proposed a non-linear trajectory tracking controller based on dynamic feedback linearization technology [2; 6]. The literature shows that the aerodynamic model of quadrotor has been oversimplified in most studies. Therefore, the accuracy of the aerodynamic model needs to be improved [14]. [PERSON], et al., observed that the error of the quadrotor model increased the error of the desired trajectory [15]. The STARMAC project team from Stanford University applied the blade flapping and thrust changes to model at high-speed forward flight [16]. The control effect has been significantly improved but the aerodynamic model is still simplified. Currently, only few studies have been conducted on the distribution of the flow field for quadrotor. [PERSON] proposed a coupled flow simulation method, which is leading a fundamental understanding of the flight behavior of the quadrotor [17]. [PERSON], et al., proposed a mathematical model at forward flight considering the interference of the rotor in hovering [18]. [PERSON] discussed the modeling of the quadrotor under different flight conditions, including lower wind speeds considering the ground effect [19]. [PERSON] and [PERSON] studied the aerodynamic characteristics of a quadrotor helicopter in a uniform flow field through wind tunnel tests and the aerodynamic interference problem of the twin-rotor structure [20]. [PERSON] derived the force and moment of the quadrotor using the blade element momentum theory [21]. [PERSON] studied the aerodynamic design of the propeller with the shear stress transport K-Omega (SST k-\(\omega\)) turbulence model and optimized the aerodynamic performance of the UAV [22]. [PERSON], et al., demonstrated that the aerodynamic interference is much more complex than the isolated single-rotor flow field, such as thrust fluctuations [23]. These preliminary results are advantageous to understand the flow field of the quadrotor. However, there is no research focusing on the flow field around the non-planar quadrotor and its aerodynamic characteristics. Besides, an oversimplified model is not suitable for describing the unique aerodynamic characteristics. Above all, it is necessary to study the aerodynamic characteristics of the non-planar rotors with different spacing and tilt angle.