# Lift-Thrust Integrated Ducted-Grid Fusion Configuration Design for a Ducted Fan Tail-Sitter UAV
## 2 Flight Characteristics of Ducted Fan Tail-Sitter UAVs
### Aerodynamic Characteristics Analysis of Duct-Grid Fusion Configuration  
The aerodynamic characteristics of the duct-grid fusion configuration were analyzed using the same simulation method as the original duct fan, with the simulation conditions remaining consistent. The sliding grid technology was also adopted to simulate the motion phenomenon of the propeller relative to the duct. The comparison of the obtained aerodynamic results is shown in Table 1. It can be seen that after embedding the grid in the duct, the thrust remains basically unchanged, reflecting that even if multiple sets of grid channels are embedded on the duct wall, there is no significant increase in resistance. Figure 7: Grid design parameters embedded in ducts. The overall torque slightly increases since the main component contributing torque is the propeller. When the axial symmetry of the duct is broken by the grid channel, the local flow field of the propeller is disturbed, causing leakage resistance when the blade rotates through the grid channel mouth, resulting in a slight increase in torque and corresponding power. The results in Table 1 also indicate that considerable lift is generated on the duct wall due to the embedding of grid channels. Taking the takeoff weight of the V-BAT at 57 kg as an example, if the power plant configuration described in this article is adopted, the 30 kg lift generated on it can share 52% of the total lift demand of the aircraft, which can unload the wing lift. With the same wing load, the wing area can be reduced by 52%. If a rectangular wing is used, the span can be reduced by half. This design concept brings great benefits, as it can be used in a narrower space or more drones of this type can be deployed in the same space. Figures 8 and 9 show the flow field information related to the duct-grid fusion configuration. From the cross-sectional streamline diagram, although grid channels are embedded on the duct wall, a local airflow stagnation zone is actually formed inside due to the small pressure difference between the inside and outside of the channel. The airflow inside the duct does not pass through the grid channels in large quantities and affect the outside of the duct. The advantage of this phenomenon is that the air inside the grid fills the channels, so that the internal and external streamlines are not heavily mixed, thus not having a significant impact on the thrust. The greatest advantage of the existence of the grid channel is that it induces further expansion of the low-pressure area near the lip, resulting in an increase in the resultant force vector \(F_{1}\) near the lip. However, the thrust component \(T_{1}\) is equivalent to the thrust component \(T_{2}\) on the side without the grid. The component \(L_{1}\) in the lift direction is significantly increased compared to \(L_{2}\) on the side without the grid. Therefore, the overall effect is that the total thrust \(T=T_{1}+T_{2}\) remains almost unchanged, while the total lift \(L=L_{1}-L_{2}\) increases significantly. In other words, after embedding the grid channel into the duct, the basic performance of the thrust direction of the culvert is maintained as much as possible, but it induces changes in the low-pressure area near the lip, resulting in a lift difference. The main thrust of the ducted fan originates from the propeller momentum and the suction zone induced by the propeller at the duct lip. A smooth airflow transition from the lip to the duct expansion section ensures the additional thrust of the duct. As shown in Figure 7, the design of a grid channel in the straight duct section creates a stable \"reverse flow cushion\" inside the flow field around the duct, which does not significantly interfere with the original flow direction or flow rate of the transitional airflow (Figure 10), thus maintaining the basic thrust characteristics. However, due to the presence of the grid channel, it induces changes in the suction force difference on both sides of the duct, thereby generating additional lift.