# Horizontal Wind Effect on the Aerodynamic Performance of Coaxial Tri-Rotor MAV
## 1 Introduction  
Compared with traditional Quad-rotor or Hex-rotor, the coaxial Tri-rotor the coaxial Tri-rotor has a much wider class including a compact structure without redundancy device since the vehicle mass is related to the rotor arm where the Quad-rotor or Hex-rotor is limited with more rotors to avoid rotor conflict. Also, it provides the unique capability of being able to resist any applied wrench or wind gust or failure tolerance with coaxial rotors (If one rotor, even three rotors, fails the system, it still has the freedom of movement). For a coaxial Tri-rotor MAV, the three coaxial rotors are evenly distributed along with the vehicle center. The aerodynamic interference is mainly including two parts: the rotor interference between the upper rotor and lower rotor and adjacent coaxial rotors [1; 2; 3; 4; 5; 6]. When the horizontal wind is introduced, the whole aerodynamic environment will be affected by the horizontal wind during flight. Therefore, the objective of the present work is to explore wind effect on the aerodynamic performance on a full-scale coaxial Tri-rotor MAV.  
Currently, the research on the multi-rotor MAV is mainly focused on the control strategies. [PERSON], [PERSON] and [PERSON] [7; 8; 9] designed the adaptive backstepping sliding mode controllers and realized attitude, velocity and position control of the Hex-rotor MAV. [PERSON] and [PERSON] [10; 11] adopted the dynamic model of a multi rotor MAV to analyze the attitude and translation and estimatethe aerodynamic forces and moments acting of a hexarotor MAV in flight. [PERSON] [12] presented an indoor path planning algorithm and overcome the drawbacks of Global Positioning System (GPS). So the shortest trajectory for the Hex-rotor MAV in the complex terrains is obtained. [PERSON] [13] designed a 4 channels PID controller to achieve the attitude control of a miniature Hex-rotor MAV. [PERSON] [14] proposed a controller with cascaded structure, which has the ability to maintain the flight state of MAV. [PERSON] [15] presented a novel Hex-Rotor MAV based on the unique configuration of its six driving rotors and overcame the effect of the under-actuation and strong coupling characteristics on the flight performance. [PERSON] [16] proposed the dynamical model of an original coaxial Tri-rotor MAV and applied a nested saturations control law to control the roll and the forward displacement, resulting in the better behavior of controller. [PERSON] [17] and [PERSON] [18] proposed the design and control of the single tilt tri-rotor and shown the effectiveness of the controllers design scheme through nonlinear simulation model. [PERSON], [PERSON] and [PERSON] [19; 20; 21] discussed a nonlinear robust control design procedure to micro air vehicle, resulting in the stable flight of MAV in the presence of perturbations.  
Therefore, only a few studies lay emphasis on the aerodynamic characteristics of a Multi-rotor MAV or even to consider the wind effect. [PERSON] et al. studied the hover performance of a Multi-rotor MAV by means of the combination of experiment and simulation [22; 23]. [PERSON] promoted a method to analyze the effects of airflow disturbance and rotor interference on the control scheme, which is based on the dynamic experiment of the Hex-rotor MAV [24]. [PERSON] reviewed the hover capability of MAV with varying solidity, collective, operating RPM and planform [25]. The results of the above studies are all conducive to analyze the aerodynamic characteristics of the coaxial Tri-rotor MAV. However, there are no aerodynamic studies of the coaxial Tri-rotor MAV with the Horizontal wind so far. Hence, this paper presents the aerodynamic characteristics of a coaxial Tri-rotor MAV with the effect of the horizontal wind.