## 3 EPU Parameters Identification Procedure
### Experimental Setup  
For the purposes of experimental measurements, it is necessary to select measuring equipment that has sufficient accuracy, resolution, and compatibility with the appropriate software package for data acquisition. There are several established setups [24] and commercially available measurement systems such as RCbenchmark 1580 [16] (see Figure 4) which was used in this research. Measurement of mechanical quantities, i.e., aerodynamic forces (\(f_{R_{i}}\)) and moments (\(\tau_{R_{i}}\)), takes place using a dynamometer consisting of three load cell sensors. The rotor RPM was measured electrically by a measuring probe that was connected to a single motor phase, and optically through an optical sensor that counts revolutions by detecting a marker mounted on the motor. From electrical quantities, battery voltage (\(u_{bat}\)), electric current (\(i_{bat}\)), and electric power (\(P_{EPU}\)) were measured. In addition to the mentioned mechanical and electrical quantities, the motor temperature can be monitored by utilizing an additional temperature sensor. Furthermore, the vibrations of propellers were measured by an accelerometer (embedded within test stand), and it was checked whether their intensity is below the limit that would significantly affect the results. In the case of unfavorable vibrations, or if measured values exceed the defined limit values, the software reports an error and stops the measurement process (safety cutoff) to protect equipment from damage. Additionally, in order to validate the setup, it is important to check if correct RPM is obtained (i.e., with optical rpm probe), and additionally check the validity of electrical measurements by a multifunction logger.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**BLDC Motor Designation** & \(K_{\text{p}}\) & \(d\) & \(u_{bat}\) & \(i_{maxEsc}\) \\ \hline SE1806 & 2700 & \(5^{\prime\prime}\) & 3S & 20 A \\ BE1806 & 2300 & \(5^{\prime\prime}\) & 3S & 15 A \\ MN1806 & 1400 & \(7^{\prime\prime}\) & 3S & 15 A \\ MN2214 & 920 & \(9\)–\(10^{\prime\prime}\) & 3S & 20 A \\ MN4014 & 400 & \(15\)–\(17^{\prime\prime}\) & \(4\)–\(6S\) & 40 A \\ \hline \hline \end{tabular}
\end{table}
Table 1: Considered EPU setups for selected brushless DC (BLDC) motors.  
Figure 4: RCbenchmark measurement system: (**a**) hardware; (**b**) wired measurement setup.  
Load cell sensors were used to measure thrust and drag from the propeller. The first sensor with a range of up to 5 kg, is located vertically on the setup and used to measure the thrust of the EPU (Figure 4a). The base with mounted EPU is attached to the left and right load cell sensors, that are used to measure the drag torque. The strain gauges of load cells are connected to signal amplifiers that are 24-bit analog-to-digital converters (ADCs) integrated into a data acquisition board. According to the diagram in Figure 4b, ESC, other sensors, and power supply were connected to the setup control board, which connects the PC via USB cable. Prior to the measurements, the dynamometer was calibrated according to the procedure described in the installation documentation [16]. To verify the measurement of electrical quantities and angular velocity of the rotor, a multifunctional logger PowerLog 6S [25] was used. Signal acquisition and data storage in.csv format was performed in the software package that comes with the setup.