## 4 Discussion  
In Sections 3.2 and 3.3, experimental results have been presented that demonstrate how a dual configuration of two asymmetric capacitors in parallel produces higher thrust values while optimizes energy efficiency. Although the absolute values of thrust are low, in the order of 6 g, and the possibility of scaling them is, at least, complex, its energy efficiency, measured in g/W, will be compared with the maximum provided by a conventional electric propulsion system, and the potential use of electrohydrodynamic propulsion in manned or unmanned aircrafts will be evaluated. In fact, this comparison is aimed to verify how, despite the above limitations, in terms of power consumed per g of thrust generated, EHD thrusters outperform electric propulsion. Manned aircrafts with electric propulsion use axial flux synchronous motors that directly drive the propeller shaft, either individually mounted or several in a tandem configuration. This type of machine provides high torque with a mechanical efficiency of the motor around 90%. Despite these good performances, its current consumption is very high, limiting the flight autonomy to values not exceeding 30 min, and its specific power (ratio mechanical power of the propulsor group to weight) is not as high as in other types of motor. BLDC motors are the electric machines with the highest specific power [18], reaching values of 350 g/kW, therefore, the comparison will be made with this type of motor, which will be the one with the highest thrust to power ratio as well. To verify the higher performance of the EHD thrusters, two sources will be used; on the one hand, the values supplied by the most known manufacturer of this type of motor, and on the other hand, measurements collected in laboratory with instrumentation specifically designed for this purpose. Table 3 shows the specifications of the BLDC motor with the highest performance in the current Unmanned Aerial Systems (UAS) market. It is a product manufactured by T-Motors(tm) in which high-quality materials have been used to achieve the lowest levels of losses: extremely thin and permeable laminations, high flux density NdFeBo permanent magnets, and silver plated conductors. Although the quality of the motor and its performance are excellent, it has already demonstrated in [19] that the data supplied by the manufacturer are inaccurate or overly optimistic, even although the propeller that produces the maximum efficiency was used in their measurements. Therefore, it must be considered that the column of values showing the overall performance in g/W provides higher values than the real ones. By reviewing the table, it can also be appreciated that the maximum efficiency is produced at very low rotational speeds, where only a 11% of the maximum power that the motor can provide is being used, and drops to very low values, in the order of 10 g/W, as soon as the 50% of the available power is reached. If these values are compared with those presented for the electrohydrodynamic propulsion in previous sections (30 g/W), the overall efficiency is three times higher. If the results are obtained from experimental  
Figure 20: Dual thruster 3D plot, efficiency as thrust/ power ratio, voltage, and distance between electrodes. measurements on a standard UAS motor, the resulting performance for electric propulsion is even lower. Figure 21 shows a test bench for the study of BLDC motors. This instrument includes two probes for measuring speed, three temperature probes, and load cells to measure the thrust produced by the propeller-motor assembly and the mechanical torque developed by the motor. It is a Tytorobotics(tm) Test Stand 1585. The software available for the instrument allows 18 variables of the motor to be recorded and exported to a data file. As the mechanical torque can be measured, the efficiency of the motor is calculated as the quotient between the mechanical power delivered to the shaft and the electrical power consumed.