## 4 Experimental Study  
In order to verify the effectiveness of the CFD simulations and study the aerodynamic characteristics of the quadrotor with different spacing and tilt angle, a test bench was constructed in Figure 15.  
The diameter of the rotor (NACA0012) is 400 mm with a chord length of 35 mm. The Reynolds number was approximately \(1.1\times 10^{5}\). The test bench mainly included: (1) power system that could be changed using DC power supply (model: ACE GESHI lithium polymer battery), DC brushless motor (model: EM2835) and PWM (Pulse-Width Modulation) adjustment mode. (2) Measurement system to measure the rotational speed, the thrust, the power, the rotor spacing and the tilt angle of the rotor. The rotational speed was read by the photoelectric tachometer (Model: UT371, accuracy \(\pm\) (0.04%\(\mathrm{\SIUnitSymbolMicro n}\) + 2d) r/min (1000-99,999)), the thrust was derived from a two-component balance, and the power was processed based on the recorded current and voltage values. The values of uncertainty that are presented in this study are all calculated for 95% confidence levels [30]. As a comparison, an isolated rotor is first analyzed to validate the effectiveness of the CFD methodology. The results have compared to experimental results as showed in Figure 16.  
The comparison of lift coefficient \(C_{l}\) and drag coefficient \(C_{d}\) in experiment and simulation showed that they are generally in good agreement which means that the current study is efficient to handle the unsteady flow in the low Re environment. Figure 16: Comparison of experiment and simulation. Figure 15: Experimental setup. Figure 17 shows the variation of the non-planar quadrotor thrust with different rotor spacing and tilt angle. It can be seen that the thrust of the test is always greater than that in the simulation. This is because the record in the test is a periodically changing peak, that is, the maximum value, while the simulated thrust is taken from the average value in the stable period. However, the overall thrust increases with increased tilt angle and decreases to constant with increased spacing, which means that the trust is more affected by the tilt angle than the spacing. When the spacing is increased to 1.3-1.4 \(d\), the thrust becomes stable. Moreover, the thrust of a non-planar quadrotor is always greater than the sum of four isolated rotors (up to 6.2%) which lead the conclusion that the non-planar quadrotor has good prospects in applications. Figure 18 shows that the PL variation of the non-planar quadrotor. As showed in the figure, the power loading of non-planar quadrotor decreases slightly with increased spacing. However, they are always lower than four isolated rotors. Since the tip vortex increases significantly with the increased tilt angle, the power consumption also increases to some  
Figure 17: Thrust variation of non-planar quadrotor. Figure 18: Power-loading of non-planar quadrotor. extent. Thus, the thrust is improved, but the power consumption is correspondingly increased, especially at a larger tilt angle and smaller spacing. Therefore, the influences of tilt angle and spacing on power are similar to that of thrust. This is because the power is highly dependent on the resistance and the increased tip vortices attached to the rotor surface. According to the power model of the non-planar quadrotor, the velocity \(V_{0}\) of the conventional planar quadrotor increases the slip velocity. It is caused by the moving wake and the tip vortex of each blade and reduced the inter-rotor interference. However, the non-planar quadrotor is in the tilting state, each rotor is in the flow field of the adjacent rotor, so there is a speed \(V_{0}\). For the face-to-face rotor pair, the speed \(V_{0}\) and the slip flow velocity is increased. Furthermore, the induced speed of the back-to-back rotor pair is also increased.