## 3 Experiments
### Experimental Results  
As the key parameters to analyze the hover efficiency of the Hex-rotor UAV, the thrust and power consumption are obtained with different wind speeds in Figure 4.  
According to Figure 4, it is interesting to note that the thrust increased with wind speed, especially with a higher rotor speed. However, the power presented an opposite trend. This is because the interference between the rotors is much stronger when the horizontal airflow is introduced, which may improve the aerodynamic performance by increasing thrust and decreasing power.  
Thrust and power increments of the Hex-rotor UAV in the low-speed wind tunnel experiments are shown in Figure 5.  
Figure 3: Sketch of the signal process.  
Figure 2: Experimental setup.  
As shown in Figure 5, the Hex-rotor UAV showed a good aerodynamic performance at a lower rotor speed arranged from 1600 to 1900 RPM. Especially at 1700 RPM, the UAV reached the best performance with the highest thrust increment and lowest power increment. As the rotor speed increased (1900 RPM-2300 RPM), the hover efficiency decreased with the wind speed since the thrust generated by the same speed gradually decreased, and the power consumption increased instead. Compared to thrust at 0 m/s, at the horizontal airflows of 2.5 m/s and 4 m/s, the thrust decreased by a maximum of 2.92% and 3.82%, respectively. Also, the thrust of the Hex-rotor UAV was reduced by 4.31% and the power consumption was increased by 4.13% when the horizontal airflow was 4 m/s at 2200 RPM (work mode). The main reasons are: (1) at a lower rotor speed, the interference between the rotors domain the whole aerodynamic environment where the wind effect was relatively small at this stage, (2) at a higher rotor speed, the downwash began to couple with the incoming flow. In this case, the shape of the downwash deformed which may have increased the power consumption.  
Power loading at different horizontal airflows and RPM is shown in Figure 6.  
Figure 4: Thrust and power consumption with different wind speeds.  
Figure 5: Thrust and power increment at different speeds.  
According to Figure 6, _PL_ decreased when the horizontal airflow was introduced, and the _PL_ at 4 m/s achieved minimum. Also, as the speed increased, the Hex-rotor UAV achieved the maximum power loading value at 1800 RPM. In this case, the increase of the horizontal airflow enhanced the aerodynamic interference of the Hex-rotor UAV, which led to the decrement of the power loading. Therefore, it can be inferred that the smaller airflow and low speed (2.5 m/s and 4 m/s, 1500 RPM) are beneficial to improve the aerodynamic characteristics of the Hex-rotor UAV.  
During the experiments, the rotation speed and thrust of the Hex-rotor UAV were measured by the optical tachometer and the thrust sensor, respectively. According to the Kline-McClintock equations, uncertainty of the thrust coefficient is [21; 22; 23]:  
\[\Delta C_{T}=\sqrt{\left(\frac{C_{T}}{T}.\Delta T\right)^{2}+\left(\frac{-2C_ {T}}{\Omega}.\Delta\Omega\right)^{2}} \tag{5}\]  
Therefore,  
\[\frac{\Delta C_{T}}{C_{T}}=\sqrt{\left(\frac{\Delta T}{T}\right)^{2}+4\left( \frac{\Delta\Omega}{\Omega}\right)^{2}} \tag{6}\]  
Similarly, the uncertainty of the power coefficient \(C_{P}\) and power loading \(PL\) is expressed as:  
\[\frac{\Delta C_{P}}{C_{P}}=\sqrt{\left(\frac{\Delta Q}{Q}\right)^{2}+4\left( \frac{\Delta\Omega}{\Omega}\right)^{2}} \tag{7}\]  
\[\frac{\Delta PL}{PL}=\sqrt{\left(\frac{\Delta T}{T}\right)^{2}+\left(\frac{ \Delta Q}{Q}\right)^{2}+\left(\frac{\Delta\Omega}{\Omega}\right)^{2}} \tag{8}\]  
Therefore, the uncertainties in \(C_{T}\), \(C_{P}\), and \(PL\) are 1.2%, 1.1% and 1.5%, respectively.