## 3 Experimental Study
### Experimental Results and Discussion  
Figure 4 shows the thrust increment of the quadrotor compared with four isolated rotors with different rotor spacing.  
It can be seen that the thrust of the quadrotor with different spacing is obviously higher than that of the four isolated rotors. Additionally, it is can be found that the thrust increment is relatively small when the rotor spacing is too small or too large. Additionally, it can be speculated that the strong aerodynamic interference or almost no interference between rotors will cause a loss of thrust for the  
Figure 4: Thrust increment of the quadrotor vs. isolated rotor.  
quadrotor. Therefore, appropriate rotor spacing may reduce the aerodynamic interference between adjacent rotors, and also increase the thrust of the quadrotor. Actually, the quadrotor can obtain larger thrust at L/R = 3.2 and L/R = 3.6, where the largest thrust increment is 36.8% for L/R = 3.6 at 1500 RPM.  
Figure 5 shows the power increment of the quadrotor compared with four isolated rotors with different rotor spacing.  
As showed in Figure 5, the power of the quadrotor increases gradually with rotational speed. When the rotational speed is lower than 1700 RPM, the power consumption of the quadrotor is smaller than that of the four isolated rotors. Combined with the Figures 4 and 5, it can be found that although the quadrotor can obtain greater thrust at L/R = 3.2, it needs to consume more power compared with other rotor spacings. For the rotor spacing at L/R = 3.6, the quadrotor obtained a smaller power consumption with a larger thrust level.  
Figure 6 shows the thrust and power of the quadrotor and the isolated rotor.  
It can be found that the thrust increases obviously with power, and the thrust of the quadrotor is much higher than that of the four isolated rotors with the same power consumption, which shows that moderate aerodynamic interference between rotors can effectively improve the thrust and obtain a better aerodynamic performance for the quadrotor.  
In summary, based on the thrust and power distribution ranged from L/R = 2.2 to L/R = 4.0, the higher thrust with a lower power consumption is obtained at L/R = 3.6, where the quadrotor shows a  
Figure 5: Power increment of the quadrotor vs. isolated rotor.  
Figure 6: Thrust and power distribution for quadrotor vs. isolated rotor.  
better aerodynamic performance. In order to evaluate the aerodynamic characteristics of the quadrotor in the experiments, the Figure of Merit (FM) is showed in Figure 7.  
According to the definition of FM, it can be known that larger FM can be obtained with both large thrust and small power. From the Figure 7, it can be seen that the FM increased with rotor spacing until L/R = 3.6, where the performance decreased at L/R = 4.0. The main reason is the strong aerodynamic interference between adjacent rotors at a smaller rotor spacing, which not only increases the power, but also weakens the thrust of the quadrotor. Additionally, it is also interesting to note that the FM of L/R = 4.0 experienced a sudden drop at rotor speed ranged from 1800 to 1900 RPM. This may have resulted from the insufficient thrust from the collapse of the suction forces on the tip and the increase in vibration caused by the flow separation, which are relatively unsteady at this rotor spacing with a such long rotor arm, where the rotor is apt to have somewhat greater interaction with its own wake. This heightened interaction is reflected in the greater variability to decrease of FM. Additionally, L/R = 3.6 shared the largest FM, which is consistent with the analysis of thrust and power increment in the Figures 4 and 5. Therefore, the quadrotor at L/R = 3.6 shows excellent hovering efficiency and aerodynamic characteristics.