## 4 Results and Discussion
### Calculation Results  
In Figure 6(a), declination angle-speed envelope of the UAV ducted lift-increasing system is calculated based on the case of the stall angle of attack. It can be seen that in the hover phase, the force deflection angle of the ducted lift-increasing system is 75 degrees, and the minimum flight speed when deflected to the fixed-wing flight mode is 43 m/s. Figure 6(b) shows the curve of lift fan thrust, the thrust of the ducted lift-increasing system, and body lift and drag in the transitional flight envelope. It can be seen that when hovering, the gravity of the UAV is balanced by the pull force of the lift fan and the ducted lift-increasing system. With the deflection of the ducted lift-increasing system, the lift of the UAV is provided by the power components and gradually transitions to the wings. As speed increases, the lift and drag of the UAV also increase.  
Figure 8a is given based on a power limit of the deflection angle-speed envelope of the ducted lift-increasing system. The results in this figure differ from those shown in Figure 7a in that in the high-speed range, the perpendicular angle of the ducted force may be maintained until the trigger power limit. Figure 8b shows the curve of the power of the UAV's power components with the flight speed during the transition process under different attitude angles. Under the condition of power limitation, if the required power and the maximum available power are equal, a power-based deflection angle-speed envelope of the vertical take-off and landing fixed-wing UAV's ducted lift-increasing system can be obtained. It can be seen that with the increase in speed at the same attitude angle, the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(i_{df}\)\({}^{\circ}\) & 0–30 & \(P_{f}\)/kW & 26 & \(\eta_{p}\) & 0.95 \\ \(i_{df}\)\({}^{\circ}\) & 0–95 & \(P_{d}^{\prime}\)/kW & 14 & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Calculation parameters and empirical coefficients.  
Figure 6: UAV lift and drag coefficient schematic diagram of force during transition flight.  
Figure 7: The calculation result of the transition corridor based on the maximum lift coefficient. (**a**) Deflection angle of ducted lift-increasing system force. (**b**) Power and lift and drag curve.  
required power first decreases and then increases, which is closely related to the inflow characteristics of the duct. At the same time, it can be seen from the three curves with different attitude angles that the transition power at a large attitude angle is greater than that at a low angle of attack. The large attitude angle transition will trigger the power limit at low speed, because the higher the attitude angle is, the greater the resistance during the transition and the greater the required power.  
Combining the flight envelope calculated based on the stall angle of attack and the flight envelope calculated based on the power limit, the transition flight corridor of the vertical take-off and landing fixed-wing UAV can be obtained (see in Figure 9). It can be seen that the left boundary of the transition corridor in the low-speed section is the deflection angle-speed envelope of the ducted lift-increasing system based on the stall angle of attack, and the right boundary of the high-speed section is the power-based deflection angle-speed envelope of the ducted lift-increasing system. The UAV can complete the transition flight in the transition corridor between the two envelopes.  
Figure 8: Results of the calculation of the transition corridor calculation based on power limitation. (**a**) Deflection angle of ducted lift-increasing system’s force. (**b**) Power curves of different attitude angle.  
Figure 9: UAV transition corridor.