# Effect of Electric Ducted Fans Structural Arrangement on Their Performance Characteristics
## 1 Introduction  
Currently, there is a growing effort to expand the use of unmanned aircraft (UA) for the transport of people and goods, especially in urban areas [1], which impose completely new and more complex operational and safety requirements on the UA itself [2; 3]. Urban areas are characterized by a spatially limited environment with many obstacles, such as buildings, towers, power lines, and especially the occurrence of a large number of people [1]. The limited space for movement imposes on the UA an important requirement to save dimensions as much as possible, i.e., not to increase its size unnecessarily [4]. The movement of people, on the other hand, requires the highest possible safety level of the entire system. An example is the need to prevent injury of people from the rotating parts of the UA in the situation it gets close to them. The use of electric ducted fan (EDF) units has the potential to solve some of these issues and fulfill emerging requirements, in particular the requirements for the smallest possible dimensions and protection from rotating parts to prevent a possible collision with surrounding obstacles [5]. The outcome is that EDF provides a number of advantages compared to propeller drives, especially for vertical take-off and landing (VTOL) UA in urban areas. The research of EDF propulsion could thus contribute to the realization of new concepts of UA operation. The ducted fan concept has been used since the 1960s, but has not been widely used in conjunction with an electric motor. The ducted fan category includes almost any drivethat uses a rotor to generate thrust, which is enclosed around the perimeter in a cover that reduces the flow of air over the upper sides of the blades [6]. This area is already widely researched because one of the most widely used propulsion systems in manned aircraft is the ducted fan, known as the turbofan [7]. As for EDF, it is the same concept, only the rotor is driven by an electric motor instead of a jet engine [8]. Although the concept is well known, ducted fans equipped with an electric motor are very rarely used and they are also not fully investigated in the case of UA operation associated with low Reynolds numbers and thus different non-laminar flow characteristics. EDF propulsions main advantages include a high thrust-to-weight ratio, a high thrust-to-diameter ratio of the fan, physical protection of the fan and its surroundings, the possibility of thrust vectoring, and lower noise. The most important characteristic of a ducted fan in general, is the amount of thrust it is capable of generating for a given diameter of the fan. Compared to a propeller propulsion, a ducted fan is capable of generating up to several times more thrust with the same rotor diameter [9]. A natural feature of a ducted fan is its cover because the cover provides protection both for the fan itself and for the surroundings. For example, when using UA that would be used to operate in a populated area, EDF would provide protection to the surrounding environment. In connection with urban traffic, the ducted fan is precisely suitable due to its dimensions, as it reaches much smaller dimensions than a propeller, because it does not unnecessarily expand the dimensions of the entire UA [9]. If necessary, it allows for installation in the structure of the airframe of an UA, unlike the propeller, which must be in an open space outside the fuselage. The ducted fan also allows the use of thrust vectoring, which allows smooth maneuvering of VTOL aircraft while hovering [10]. This is particularly advantageous in the case of cargo transport, when unwanted tilting of the entire UA is eliminated. From a design point of view, EDFs offer high variability in adjusting the cover flow channel with different combinations of fan arrangements, for example: two-rotor specification, see [11]. Possible modifications are changes in the flow channel and changes in fans, such as changing the number of blades and diameter [12].