**4.1 Propulsion subsystem design considerations**

The propulsion subsystem is defined by the parameters of the geometric arrangement and characteristics of the EPUs. A suitable fixed-pitch propeller is mounted on the rotor of the outrunner BLDC motor (**Figure 8**). The basic parameter of a propeller is its diameter. As the diameter of the propeller increases, the angular velocity of the

*Framework for Design and Additive Manufacturing of Specialised Multirotor UAV Parts DOI: http://dx.doi.org/10.5772/intechopen.102781*

#### **Figure 8.** *Electric propulsion unit of multirotor type of UAV [31].*

motor rotor decreases. The motor is defined by a motor velocity constant kV. Motors with a lower motor constant are used in combination with larger diameter propellers and are driven at higher voltages. The ESC is responsible for starting the motor and, depending on the control signal, controls the motor speed. The EPUs are connected to one or more LiPo batteries of the appropriate number of cells and capacity.

The motor stator must be connected to the aircraft assembly which consists of a central part and the rotor arms. Propulsion assembly design is the most complex part of the overall design in terms of the mechanical properties that assembly parts should possess. The aircraft can be used in a wide range of powers, from a few tens of watts to several tens of kilowatts. It is necessary to choose materials and technologies concerning the selected propulsion components. **Figure 9a** shows the stator geometry which is important from the aspect of mounting the motor to the aircraft assembly. **Figure 9b** shows the characteristics of the propulsion unit considered in the case of a heavy-lift aircraft.

**Figure 9.**

*Electric propulsion unit: (a) BLDC motor geometry [31]; (b) characteristics.*

**Figure 10.** *Fully-actuated multirotor configurations with passively tilted rotors: (a) PTX6; (b) PTX8.*

The configuration of the multirotor UAV is defined by the geometric arrangement of the rotors. Mostly conventional configurations with a planar rotor layout are commercially available. It is possible to select configuration parameters that will result in an increased degree of actuation, which potentially allows the performance of complex tasks in the field of aerial robotics. A fully-actuated aircraft with passively tilted rotor arms are considered in this research (**Figure 10**).
