## 2 Materials and Methods
### Electric Propulsion  
A helpful rule of thumb is that one starts with the electric propulsion design first because it is easier to match the mechanical design of the fuselage to known components'  
Figure 4: An overview of popular UAV types.  
sizes and weights. That is especially important for the battery pack--usually the heaviest and largest element.  
At the time of writing, the eCalc [30] seems to be the best and most accurate tool, including, but not limited to finding the best motor-propeller-battery setup (see Figures 5 and 6). Another great tool, especially for predicting electric RC plane performance, is MotoCalc [31]. We used it for detailed pusher propulsion performance prediction, e.g., to estimate top speed in level flight and find the stall speed for various wing setups (see Figure 7). Considering the general assumptions mentioned in Section 1, we decided to look for high-D/P-ratio propellers (diameter-to-pitch ratio), a high-voltage setup, and lithium-ion batteries rather than lithium-polymer ones. A propeller with a high D/P ratio provides great static thrust (a lift force in the case of a drone), but the thrust drops suddenly when the airspeed increases. That phenomenon, however, may impact drone manoeuvrability. Typically, fast and agile drones (e.g., racing quadcopters) use the D/P ratio in the range \([1\ldots 2]\), where propellers with D/P close to 1 (e.g., 5 \(\times\) 4) allow high-speed flights, but consume much energy in hover. A high-voltage setup decreases the current needed (assuming a constant total output power)--lower current benefits in thinner electric wires, and smaller Electronic Speed Controllers (ESCs), thus a more lightweight setup. The Li-ion batteries usually offer greater capacity than Li-poly batteries of the same weight, but cannot work in high-current load conditions. The final setup of the quadcopter's electric propulsion is presented in Figure 5 and Table 1, and the predicted performance of the drone (and its range) can be found in Figure 6. Predicted stall speed, top speed, and the total flight time can be found in Figure 7. A summary of the predicted performance is presented in Table 2.  
\begin{table}
\begin{tabular}{l l} \hline Main Quadcopter Motors & 4\(\times\) T-Motor MN3110 KV470 \\ Main Quadcopter Propellers & 4\(\times\) T-Motor Carbon-Fibre (CF) 12 \(\times\) 4 \\ Main Quadcopter ESCs & 4\(\times\) Hobbywing Micro 35A 3-68 BLHeli \\ Battery Type & Li-Ion, 18\(\times\) Sony US18650 VTC5 cell, custom-made battery pack \\ Battery Setup & 653P \\ Battery Capacity & 7500 mAh \\ Battery Voltage & 21.6 V nominal, 18–25.2 V full range \\ Battery Weight & 936 g \\ Pusher Motor & 1\(\times\) T-Motor F60 PRO II KV1750 \\ Pusher Propeller & 1 \(\times\) APC-E 5 \(\times\) 5 (2-blade propeller) or 1\(\times\) HQProp Ethix S5 \\ Pusher Motor ESC & 1\(\times\) Beatles 50 A \\ Total Drive Weight & ca. 1800 g \\ \hline \end{tabular}
\end{table}
Table 1: The final electric propulsion setup.  
\begin{table}
\begin{tabular}{l l} \hline All-up Weight & 2900 g (2600 g + 300 g of payload) \\ Hover Flight Time & ca. 20.4 min \\
**Top Speed (plane mode, using pusher motor) 1** & **ca. 34–42 m/s (122–144 km/h)** \\ Top Speed (quadcopter mode only) & ca. 8.9–12.5 m/s (32–45 km/h) \\
**Max Range (VTOL, plane mode) 1** & **ca. 25 km** \\ Max Range (quadcopter mode only) & ca. 4.6 km \\ Stall Speed (plane mode) & ca. 11.1–13.9 m/s (40–50 km/h) \\ \hline \end{tabular}
\end{table}
Table 2: Predicted performance of the electric drive setup; assumed 90% max allowed discharge and 300 g payload (e.g., a video camera with a companion single-board computer).  
Figure 5: The eCalc [30] tool helped us find the best electric propulsion setup for the Elka1Q drone ; disclaimer from eCalc: * _The manufacturer limitation is NOT monitored_ (relates to motor revolutions).  
Figure 6: The eCalc [30] tool estimates the drone range and its general performance.