## 3 Results
### Internal Combustion Unit and Generator  
The ICU selected for the prototype was a DLE-111(tm) twin two-stroke gasoline engine. It was chosen because of its high specific power: 11.3 hp for a final weight slightly higher than 2.5 kg, including ignition and muffler exhaust silencers. Figure 9 shows the motor, and Table 4 shows its main specifications. The engine was modified to improve its reliability and efficiency. After a 15 h breaking-in period, bearings were replaced by high-quality SKF(tm) products; gaskets were also removed, and the crankcase was carefully sealed. Original sp \begin{table}
\begin{tabular}{c c} \hline \hline
**Engine Type** & **Two-Cylinder Two-Stroke** \\ \hline Performance & 11.3 hp at 7500 rpm \\ \hline Weight & 2.86 kg \\ \hline Ignition & Electronic capacitive (4.8–6 V) \\ \hline Displacement & 111.2 cc \\ \hline Compression ratio & 30:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: ICU specifications. Figure 9: DLE-111(tm) ICU. cooler Iridium NGK CM-6 sparks, thus providing the combustion chamber with a higher capacity for heat dissipation. Muffler exhaust silencers were installed both to reduce noise and increase output power. The propeller shaft was cut off, and an aluminium alloy 2024 coupler was CNC manufactured, together with a high-precision, high-torque elastic coupling, to link the ICU and the generator. The compression ratio in this engine was made very high in order to obtain 11.2 hp output power. For this reason, the start-up of the motor is harsh, and self-starting had to be discarded. The torque of this type of engine presents strong oscillations depending on the crankshaft position, which are damped in its use as a hybrid generator by the BLDC generator inertia. However, the maximum start-up torque was calculated as 115 Nm. Therefore, an external removable 12 V DC motor, in combination with a clutch and a one-direction bearing, was installed. A small two-stage gearbox was attached to the bench, and the final gear ratio was calculated to obtain a start-up that did not take more than 2 s. Figure 10 shows the clutch and one-way bearing. The DC motor torque is multiplied by the gearbox, and the clutch, together with the one-way bearing, easily moves at quite a high speed of the motor's shaft during start-up. Once the engine has started, its faster rotating motion releases the clutch, and the DC starter can be removed. Starting the engine in this machine was so difficult that, even with the largest starter for giant-scale RC planes acting on a cone installed on the motor shaft, it was impossible to produce a single rotation of the shaft. This type of engine, with an extremely high specific power, is intended to be manually started by using a glove and pulling on one propeller blade. Before the solution of the clutch and the one-way bearing was found, trapezoidal belts and even chains were unsuccessfully tested. Therefore, it is the authors' opinion that the mechanical start-up system may be useful for other researchers who aim to increase the power of their hybrid generators, or even for the use of this type of engine in fixed-wing UAS. The BLDC generator was a ROTOMAX 150 ccTM, which is so called because it was designed to produce an output power equivalent to an engine of 150 cc displacement. Figure 11 shows a photograph of the motor, and Table 5 shows its specifications. Regarding Table 5, some non-available data were obtained in our test bench of electric motors or measured with laboratory instrumentation. Figure 10: Clutch and one-way bearing. This motor was selected because its speed constant kV fitted best with the engine specifications. Its design is excellent, and it features slots in the stator with chamfered edges to improve ventilation. Bearings and their housings are designed to endure the axial force produced by the traction of a propeller; thus, they provide additional mechanical strength, since those forces does not exist in the prototype.