Ducted Fans have been used in a wide range of vertical take-off and landing (VTOL) aerospace vehicles, including manned and unmanned systems. This paper focuses on performance improvement of ducted fans in forward flight applied to VTOL UAV and Tiltrotor aerial vehicles.

The concept proposed is called Ring Wing Ducted Fan (RW-DF). It is the combination of two concepts, the ducted fan and the ring wing technology.

THE CHALLENGE
Ducted fans have several advantages compared to open rotors and fixed wing airplanes. They provide higher thrust to power ratio, improved safety and lower noise levels for a given diameter when compared to open rotors. Although ducted fans provide good performance in many vertical take-off and landing (VTOL) applications, there are still unresolved problems. The main problems are associated with forward flight, for example flight endurance compared with fixed wing airplanes and flight stability when passing from hover to forward flight.

THE SOLUTION
In order to solve the above problems, the novel concept proposed combines a ducted fan with a ring wing. The RW-DF concept has a potential to significantly improve the performance and stability of VTOL UAVs and Tilrotor based vertical lift systems in forward flight.

HOW IT WORKS

As shown in Figure 1, the upper section of the duct is not equal to the lower section. However, in hover the different sections do not affect the performance, but in forward flight the different wing sections produce lift, increasing flight endurance and the stability.
The duct is composed by two semi-ducts (upper half and lower half) each one with different airfoil section and divided for two ring fences (see Fig. 2) in order to increase the aerodynamic efficiency. The internal duct section is symmetric for not reducing the performance of the fan, but the external duct section change with the goal of creating two different wing sections (as a ring wing). When the vehicle reach a cruise velocity the lift force created by the Ring Wing Ducted Fan (RW-DF) increase the flight endurance.

This new concept has applicability on the market quite easily, because it requires a similar manufacturing process and the same material as existing product models, only changes the form.