The operation of the internal combustion engine at a constant speed--quite likely at full throttle--and maximum power dramatically simplifies the electronic design but leads to quite a few negative aspects. When the aircraft does not need the maximum current to power its propulsion motors, the engine is still running at its highest speed, producing higher fuel consumption and leading to the heating and wear of mechanical components. If the lack of a refrigeration system is added into consideration, and the fact that the ventilation provided during the flight may be zero, depending on the type of operation, the only response must be to limit flight times and to carry out maintenance work more regularly. In fact, during our contacts with the company, we were informed that the generator was not designed--or even tested--for flights longer than 3 h, since no refrigeration system--neither a water nor an air cooling system--was available. Figure 3: Efficiency of an U812 II according to product specifications. Oscillations and a maximum efficiency of nearly 94% at high speed appear. The review of other products extended to commercially available systems continues with two H-UAS manufactured in Spain (Quaternium HYBRIX(tm) and Aerocamaras AEROHYB(tm) and the hybrid generator from the Canadian company Pegasus Aeronautics known as GE70(tm), whose design is relatively similar to our prototype. HYBRIX [16], was presented in 2016. Version 1.0 used a high-capacity 12S LiPo battery parallel-connected to a hybrid generator consisting of a small displacement two-stroke engine and a BLDC generator. It was probably the first H-UAS ever presented in Europe. However, the first units we could review used a very ineffective belt transmission between the engine and the generator, and propulsion motors were Hengli(tm) low-cost motors that could hardly fulfil the specifications of Quaternium(tm), (46510 Valencia, Spain). Version 1.2 seems to have been drastically changed, with the design of a more sophisticated and efficient hybrid generator; however, according to its specifications, its output power is only 2.6 kW [17]. In the same way that GAIA struggles due to presenting low thrust vs. weight ratio, this aircraft with only four rotors faces a difficult situation when flying with its MTOW. If the power generated is divided between four propulsion motors, the maximum available power per motor is 650 W. Although a cell having only four motors reduces weight and facilitates transport, it also increases power consumption and decreases stability. It is well known [18], that a BLDC's specific power (ratio of output power vs. weight) increases with power reduction. Therefore, splitting the propulsion system into a higher number of motors will reduce consumption and increase efficiency. Moreover, fault-tolerant autopilots cannot be used in a four-motor frame, as a motor fault cannot be compensated by the other three motors. No data are available about the voltage generated by this hybrid unit. Nevertheless, the analysis of its thrust vs. weight ratio can be easily conducted. For an MTOW of 25 kg and assuming a ratio of 1:1, every motor must produce a thrust of 6250 g with a maximum input power of 650 W. The selected propellers are from 30 to 32\" in order to use low-kV motors and thus reduce power consumption. If the g per watt (gr/W) efficiency that the motors must fulfil is calculated, the obtained value is rounded to 10 g/W. Although Quaternium(tm) does not identify the installed motors, they seem to be T-MOTORSTM. A complete review of the catalogue of the motor's manufacturer did not allow the authors to find any product capable of reaching such a high efficiency unless the  
Figure 4: Block diagram of the NOVA2400-FoxtechTM generator. motor were driven at an extremely low RPM.