## 4 Results and Discussion
### Isolated Rotors in Forward  
When the helicopter flies at high speed, the tip velocity of the advancing blade reaches the maximum at \(90^{\circ}\) azimuth, which is easy to produce surge phenomenon, while the tip velocity of the retreating blade reaches the minimum at \(270^{\circ}\) azimuth, which is easy to produce stall phenomenon. As a result, the overlapping of the upper and lower blades exacerbates the complexity of the flow field and has a negative impact on the aerodynamic performance of the rotor.  
Figures 16-18 show the variations of thrust coefficients, torque coefficients, and power loads with time for isolated rotors only without fuselage in forward flight. It can be found from Figures 16-18 that there is a noticeable difference in the rotor thrust coefficients, torque coefficients, and power loads for state 1 and state 2 in forward.  
Figure 16: The isolated rotor \(C_{T}\) of the upper and lower rotors in forward flight.  
Figure 15: The isolated rotor \(q\) in hover.  
For state 1, the phases of the peaks of the thrust coefficients, torque coefficients, and power loads of the upper rotor and the lower rotor are coincident; the total thrust coefficients and torque coefficients of the coaxial double rotor are intensified. As can be seen from Figure 18, the total rotor power loads in state 1 is essentially identical with the power loads of the upper or lower rotor.  
However, for state 2, the thrust coefficients of the upper rotor reach a local maximum value and that of the lower rotor reaches a local minimum value; the thrust coefficients of the upper rotor reach a local minimum value and that of the lower rotor reaches a local maximum value. There is a complementary effect between the upper and lower rotor blades so that the fluctuation of the total thrust coefficients, torque coefficients, and power loads of the rotor blades is reduced, which is conducive to improving the flight stability of the coaxial helicopter in forward flight.  
In order to quantitatively analyze the aerodynamic performance of the isolated rotors in forward flight under the different rotor overlapping azimuths, the total thrust coefficients, total torque coefficients, and total power loads of the upper and lower rotors in state 1 and state 2 were processed with SMA, as shown in Figures 19-21.  
Figure 17: The isolated rotor \(C_{Q}\) of the upper and lower rotors in forward flight.  
Figure 18: The isolated rotor \(q\) of the upper and lower rotors in forward flight.  
Figure 21: The isolated rotor \(q\) in forward flight.  
Figure 19: The isolated rotor \(C_{T}\) in forward flight.  
Figure 20: The isolated rotor \(C_{Q}\) in forward flight.  
Compared with state 1, in state 2, the total thrust coefficients and the total torque coefficients are reduced by 0.15% and 0.90%, respectively, but the power loads are increased by 3.24%. It shows that although the conventional coaxial double-rotor working at state 1 provides slightly greater thrust than that working at state 2, the former requires greater torque and less power loads.  
Compared with state 1, the minimum value of thrust is 61.3% larger, the maximum value of thrust is 24.9% smaller, and the amplitude of thrust fluctuation is 76.3% smaller in state 2. It can be concluded that the overall aerodynamic performance of the coaxial double-rotor working at state 2 is better than that of the rotor working at state 1 in forward flight.