# Lift-Thrust Integrated Ducted-Grid Fusion Configuration Design for a Ducted Fan Tail-Sitter UAV
## 2 Flight Characteristics of Ducted Fan Tail-Sitter UAVs
### Outline Design of the Duct-Grid Fusion Configuration  
The simulation results of the aerodynamic characteristics of the duct show that the main source of thrusts generated by the duct is the suction area of the lip. The ducted fan attracts and accelerates the external airflow. After passing through the arc-shaped lip area, the airflow velocity in that area increases and the static pressure decreases, thereby forming additional thrust. However, the shape of the duct itself is an axisymmetric rotating body, which means that its lift surface characteristics have not been effectively utilized in cruising flight.  
This article proposes an improved design for the local duct shape based on the unique characteristics of grid channels [25; 26]. On the one hand, by embedding grids into the duct through asymmetric design, the axisymmetric characteristics of its shape or flow field can be broken, providing a basic environment for the formation of the lift zone on the duct surface; on the other hand, by utilizing the effect of multiple grid channels, the pressure difference inside and outside the duct is balanced, resulting in minimal mixing of internal and external airflow, thereby reducing interference with thrust effects, as shown in Figure 6.  
Figure 5: Additional thrust source of the ducted fan.  
Figure 6: Outline design of the duct-grid fusion configuration.  
According to the flow characteristics of the duct section without grids, the lifting grid design is carried out as follows:  
Step 1: Based on the concept of asymmetric geometry/aerodynamic design, the opening position is determined on the inner wall of the duct to form the conditions for generating lift on the duct wall;  
Step 2: Determine the grid diversion angle;  
Step 3: Determine the grid channel width;  
Step 4: Design the grid channel shape based on the curvature of the guiding string as a measurement standard;  
Step 5: Design the range of grid circumferential angles and the number of grids;  
Step 6: Complete the design of grid duct fusion configuration.  
It is inevitable that due to the asymmetric grid duct design scheme, lateral forces will be generated during vertical takeoff and landing, but they can be balanced by the deflection of the rudder surface on the bottom of the duct. Despite the cost of controlling the takeoff and landing phase, the cruise phase still occupies most of the entire flight envelope. Therefore, the exploration and utilization of the lift of the duct have significant benefits.  
The basic external parameters of the initially designed grid channel are as follows: the distance from the front end to the duct lip is 0.36 m, the channel width is 0.07 m, the grid inclination angle is 60\({}^{\circ}\), and the grid circumferential distribution angle is 30\({}^{\circ}\). The number of grid distributions in the circumferential and flow directions of the initial configuration is both 2, forming a 2 \(\times\) 2 distribution form as shown in Figure 7. The parameters such as the inclination angle inside the culvert are not considered here. The main reason is that the grid channels here induce changes in the front flow field and balance the differences inside and outside the duct. Therefore, the internal structure has a relatively small impact on the overall design concept.