## 3 Results
### Complete Prototype  
All the pieces of hardware described above were mounted on a manually built bench made with pre-formed aluminium profiles. Although it will be redesigned, profiles were used to determine the maximum dry weight. The result was promising, since even with all the limitations created by the use of commercial products and manual manufacturing, the obtained weight, including electronics, was 9.2 kg.  
The ICU and the BLDC generator were attached to the aluminium cage through the holders prepared in their enclosures for installation in an aircraft. The shaft of the ICU was cut off, and two aluminium alloy 2024 couplers were CNC manufactured to link both machines by means of a flexible coupling. The cage, carefully mounted, allowed an almost-perfect alignment to be made. The motor was fixed to four aluminium profiles, and it was slid until the shafts of the couplers were aligned and touched one another. The ICU was then firmly screwed with self-locking nuts and slid back again to install the flexible coupling.  
The aluminium coupler attached to the ICU passes through the clutch and one-way bearing. A small two-stage gearbox was installed in one of the sides of the bench. Thus, the DC detachable starter only needs to be inserted in a cylindrical hole to engage the start-up system.  
It was our goal for the starter to be connected to a car battery and to be removed as soon as the motor starts. It is not necessary to remove it for land tests, since the clutch and the one-way bearing decouple the entire start-up system. Figure 25 shows the bench fully mounted with the muffler exhausts.  
Special attention was paid to the coupling of the ICU and the BLDC generator for two reasons: (1) misalignment is one the main causes of vibration, and (2) the system is designed to be used with very strong throttle variations. Flexible aluminium with a perfectly balanced coupling capable of withstanding more than twice the maximum torque and a rotating speed of 10,000 RPM reduces vibration and damps the transient speed variations. For this reason, a Ruland(tm) high-torque coupling was chosen. Figure 26 shows the main features of the coupling.  
Figure 25: Global view of the prototype.  
Figure 27 presents a detailed view of the link between the ICU and the BLDC generator. Although the photograph does not show it, aluminium couplers were attached to both machines by means of six M5 screws. The diameter of the shaft inserted in the coupling and the dimensions of its internal bore were selected taking into account the mechanical properties of the aluminium alloy 2024. The transversal section of the shaft was calculated to endure a shear stress equivalent to a 200 Nm torsional torque. Such a high value was possible since aeronautical aluminium, such as that used in this application, presents an elasticity module equivalent to that of carbon steel.  
In Figure 25, other elements of the system can also be observed. Below the two gaskets' radiators, two electronically controlled fans were installed to provide refrigeration and maintain a suitable temperature on both cylinders. They turned at different RPMs depending on the measured temperature, which fluctuated with environmental and flight conditions as well the throttle level. The temperature was measured by two probes connected to two small LCD displays and sent to the FPGA, which measured temperature and controlled refrigeration.  
Although it is the authors' aim to replace the carburettor with an injection system in a further development, throttle control is achieved by acting on the carburettor's lever. To achieve this, a high-speed, high-torque titanium gear servo is used.  
Figure 27: Flexible coupling properties (detailed view).  
The movement of the servo is transmitted to the lever by means of a carbon fibre pushrod with ball joints; thus, no slack exists and the movement is very accurate. A 2.4 GHz-FHSS receiver was installed for land testing at different throttle levels and with sudden fluctuations sent from a RC transmitter. Figure 28 shows this system.