Electric Propulsion System Analysis and Optimization for Multi-Rotor Drones

Efficiency contour-based analysis and optimization methodology has been developed in the past, mainly for fixed-wing aircraft applications. In this study, a technique using static thrust measurements of multiple propellers is adapted to obtain a combined Electronic Speed Controller (ESC)-motor efficiency map. With the more realistic efficiency map, a methodology is proposed to analyze the electric propulsion system of multi-rotor drones. It is found that even when a drone is in forward flight, the axial velocity of the freestream perpendicular to the propeller disk is very small, which causes the propeller to operate close to the static condition. As a result, a single propeller represents only a single curve on the performance map. To overcome this problem, performance maps are plotted using multiple propellers, allowing comparisons between multiple propellers in a single plot. Another important point to note is that when comparing different propellers, it is the power consumption at the battery that must be compared, not the combined efficiency of the ESC and motor. For the same forward speed, multiple propellers are identified that consume the same amount of power at the battery. However, the smaller diameter propellers tend to operate in the region where the combined efficiency of the ESC and motor is higher, generating less heat. The final choice of propeller may depend on other considerations such as size, weight, heat, or noise. Finally, the battery is analyzed using chemistry simulations. For a constant power output, voltage and current are obtained as functions of time, and the result show that the usable energy is about 12% less than the nominal energy. In addition, the decreasing voltage and increasing current lead to increasing ESC efficiency during flight. Thus, a conservative estimate of the flight time or distance can be made with this constant power output assumption.