# Lift-Thrust Integrated Ducted-Grid Fusion Configuration Design for a Ducted Fan Tail-Sitter UAV
## 2 Flight Characteristics of Ducted Fan Tail-Sitter UAVs  
Take the V-BAT UAV as an example, which serves as a compact and lightweight platform, deployed primarily in complex and mobile competitive scenarios, enabling rapid combat response and recovery by switching different payloads. As shown in Figure 1, there are typical flight phases for tail-sitter UAVs. In terms of flight characteristics, the UAV uses a tail-sitter ducted power plant which drives the vehicle up vertically after the propeller generates lift. After reaching a certain flight altitude, the deflection of the guide vanes at the tail end of the propeller creates a moment for attitude adjustment, allowing the vehicle to enter a transition state for takeoff-cruise flight. At this time, the thrust generated by the propeller partially translates into forward flight thrust, the vehicle begins to accelerate, and the wings generate lift until the aircraft gravity is fully balanced to begin cruise flight. The landing process is the opposite of the takeoff conversion, with the aircraft deflecting its attitude after deceleration, and control of this attitude is accomplished by the wings and propeller deflectors. The thrust part of the propeller begins to be converted into the lift of the aircraft. Until the attitude is fully adjusted to the vertical mode, the propeller bears full lift, and the aircraft completes the cruise to landing conversion flight and enters vertical landing mode to complete the landing process [19; 20].  
From the above process, it can be seen that there is a conversion of propeller lift to fixed-wing lift throughout the flight conversion process, except in the transition process, where propellers and fixed wings work separately. In fact, the shape of the bypass itself has a certain projected area. If the configuration can be reasonably improved and designed, and the lift potential of the duct in the horizontal direction is utilized, it will help improve the overall flight performance.  
Figure 1: Typical flight phases of a ducted fan tail-sitter UAV.  
The Duct-Grid Integrated Shape Design That Takes into Account Both Takeoff and Landing as Well as Cruise Flight  
The purpose of the transition flight of the ducted fan tail-sitter UAV is to reduce the requirements for takeoff and landing sites, so that the aircraft can be flexibly applied in scenarios such as ships and vehicles. However, we can also observe that since the lift during the cruise phase is entirely borne by the fixed wings, the size of the wings is actually not very small. For this aircraft configuration, the power unit uses ducted propellers, whose essential purpose is to improve the efficiency and noise level of the propellers through the enveloping effect of the ducts [21, 22]. During the cruising process, the effective lift area formed by the duct can also assist in increasing lift to a certain extent. Therefore, this article increases the lift contribution of the duct-grid fusion design during the cruising phase, partially unloading the lift of the fixed wing and reducing its external dimensions, further improving the site adaptability of this configuration.