## 4 Discussion
The overall efficiency in g/W which introduces the efficiency of the propeller to convert mechanical torque into thrust is calculated as the quotient between the measured thrust and the power consumed by the motor. The motor installed in the test stand is a 450 W, 14 poles BLDC motor with a speed constant of 1000 RPM/V. The propeller is an Aeronaut(tm) 12 \(\times\) 5 inches high-efficiency propeller and the motor is driven by an 80 A Electronic Speed Controller (ESC). All the tests were carried out powering the system with a 5600 mAh--40C, 3S lithium polymer battery (12.6 V fully charged, 11.1 V in rated conditions). Since the test stand incorporates three accelerometers, propeller was balanced, and all the tests were carried out with a maximum RMS vibration of 0.5 g. As the overall efficiency is minimum when motor delivers its maximum power, current was limited during the tests to 25 A, this way, the motor was never driven above the 70% of its rated power. Even with all these constraints maximum motor efficiency was 79% and overall efficiency 4.3 g/W. Figure 22 shows current consumption and overall efficiency. The first stage with current below 4 A  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**THROTTLE** & **THRUST** & **TORQUE** & **CURRENT** & **SPEED** & **POWER** & **EFFICIENCY** \\
**(\%)** & **(g)** & **(Nm)** & **(A)** & **(RPM)** & **(W)** & **(g/W)** \\ \hline
40 & 1465 & 0.45 & 1.8 & 1584 & 86 & 16.96 \\ \hline
42 & 1572 & 0.48 & 2 & 1631 & 96 & 1  
produces the maximum g/W values, by only increasing the current to nearly 14 A efficiency falls to 3.8 g/W. Its minimum value of 3.7 appears at 25 A. In view of the values obtained, despite its complexity and limitations, electrohydrodynamic propulsion is almost 10 times more efficient, in terms of energy consumption, than conventional electric propulsion, even when using motors with the highest specific power and performance. Its application to atmospheric flight is still far from being an alternative, although in this study it has been analyzed, to the best of the authors' knowledge, in a different way to that presented to date. Some of the problems it poses are not particularly difficult to solve. Although this research has used laboratory equipment as a high voltage source, flyback converters, whose topology is extremely simple might be used instead. This DC-DC converter is lightweight for limited current and using synchronous rectification it is possible to obtain efficiencies of over 90%. However, as indicated above, the absolute values of the thrust produced by the EHD thrusters are low. Attempting to scale them based on designs such as those used in this research is not feasible. The current consumption would be too high, and the power supply of the DC/DC converter would have to provide current values that are impossible to assume without reducing the energy efficiency to values well below those of conventional propulsion. Figure 21: Computer controlled test bench for the study of BLDC motors. Figure 22: Current and efficiency of a propulsion set (motor-propeller) for UAS. Since there have also been deep studies on the improvement of the drag coefficient (CD) of subsonic airfoils [16], extending the study to larger aircraft could perhaps reduce their fuel consumption. In this sense, civil aviation has devoted and continues to devote an enormous effort in aspects such as sophisticated designs for winglets to reduce the induced drag derived from the pressure gradient between the pressure side and convex side of the wing airfoil, thus reducing fuel consumption. An EHD thruster could be installed in both the leading edge or the wing tips to reduce drag even more, especially nowadays when fuel consumption, fuel prices and emissions are issues of great social and environmental impact. the study has also shown the variability of current consumption with length and distance, it is not known whether it is possible or not to redirect the thruster design to another architecture where this problem is minimized.