The hybrid generator used in this aircraft is called NOVATTM, and it can generate a maximum continuous power of 2400 W. The maximum power is dependent on the flying altitude, [12; 13]. A brief analysis of GAIA's propulsion system reveals the most important drawback of present H-UAS: their extremely low thrust vs. weight ratio. Assuming flight takes place in a region with an altitude below 1000 m and that maximum power can be used, only 400 W is available for each rotor. This UAS is equipped with T-MOTORSTMUS Lite kV-85 (Nanchang, Jiangxi-China). According to the manufacturers' specification, the maximum thrust that can be obtained for an input power of 400 W is 4 kg [14]. Therefore, when it is intended that GAIA should be used with its Maximum Take-Off Weight (MTOW) of 23 kg, the thrust vs. weight ratio is nearly 1. The actual situation is even worse, as the data provided by T-Motors are inaccurate and the input power is higher than specified to obtain the desired thrust. This has been shown in laboratory tests and a simple analysis of the data provided by the company. Figure 1 shows a laboratory test carried out on an U8 II kV 85 motor. The test bench was a dynamometer with a load cell, a laboratory 2.5 kW power supply, an optical tachometer with a resolution of 1 RPM on a scale of 10,000, a digital oscilloscope and FlukeTM current probes. The highest efficiency propeller, according to T-MOTORS, was installed, and the obtained results can be reviewed in Figure 1. A lower thrust--in some cases, around 40%--and a similar increase in consumption were obtained. To confirm the accuracy of our tests, a second study was carried out only using data from the manufacturer to obtain the curve efficiency vs. RPM of this and other motors. The RPM and input power are provided in the motor's datasheets. The calculation of efficiency is straightforward. The quotient between the mechanical and electrical input power produces the results presented in Figure 2.  
This simple calculation was also conducted for other motors from the company, and the results presented the same or higher imprecision. To illustrate this situation, the same curve for their largest motor, U12 II-kV 60, is shown in Figure 3. This motor was selected as it seemed to be an ideal candidate for large H-UAS with high-weight payloads:  
Figure 1: Comparison between product specifications and measured data for a U8 II kV 85. Figure 2: Inaccuracy in the U8 II kV 85 specifications. Figures 1-3 are not only presented to illustrate that the problem of a low ratio vs. weight in H-UAS is even worse than one might expect. Our aim is also to show the lack of rigor shown by even the world's best manufacturers, with extremely high-quality materials and designs, regarding their products' specifications and the consequent need to test them under conditions as close as possible to those in which they will operate. In fact, even adding a conservative 25% margin of error, this lack of rigor almost prevented us from fulfilling the specifications in our last design and forced us to strongly reduce weight [15]. FoxtechFPV GAIA(tm), despite its limitations for operation with its MTOW, is a good and tested product that features one of the most widely used hybrid generators. In fact, it is the authors' opinion that GAIA 2400 TM is the best H-UAS than can be found on the market today. Different versions of the generator exist, and the latest, which produces a maximum power of 2.4 kW, is called NOVA2400 TM, [12; 13]. The generator consists of a two-stroke gasoline motor, directly coupled to a Brushless DC motor (BLDC) operating as a generator. The system includes a control unit--in essence, a three-phase diode rectifier--a battery backup in case of fault of the ICU and an optional 433 MHz monitoring system to check the fuel level during flight. Figure 4 shows that the design of NOVA is practical and valuable, with the aim of maximum simplicity, but it presents some drawbacks that we attempt to solve in our prototype and that are presented below.