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38 technologies are enabling improvements in generator design and operation. For example, dissipating heat is arguably the biggest challenge to improving generator efficiency beyond 91-92% (the current limit of most systems). One approach to better cooling is to integrate the engine’s water cooling system into the generator. This approach could conflict with generators being made smaller, however, as smaller systems will concentrate more heat into less volume, with less space inside for cooling water jackets. Also, by removing the iron core present in most designs, advances in (and greater commercialisation of) technologies such as coreless alternators and Halbach array generators could cut a lot of size and weight. But the viability of such systems will rest on whether they can dissipate heat effectively. Without this capability, they will not produce as much continuous power as conventional axial or radial flux electric starter-generators, as they will not eliminate their own heat during sustained operation. As a result of these challenges, many of the more successful generator improvements are in airflow enhancements. Generator rotor hubs can now be profiled using CAD and CFD software, enabling them to be designed to draw in air through the stator windings, meaning they essentially act as fans. At the same time, the stator teeth can be designed with more open channels to draw in cooling air at their bases. This is important, as while air is always able to flow freely over the tops of the stator coils (through the air gap between the stator and rotor), the windings lower down are often forgotten, meaning most generators will cool the top of their coils while ignoring the bottom of them. Improvements in CAD software such as Maxwell, from Ansys, have been key to improving generator designs, while AM is vital for iteratively printing rotor and stator profiles to optimise for airflows, as CNC machining is cost-inefficient for prototyping. Certain parts of such generators will probably also rely on AM for bulk manufacturing, with their geometric complexity and small size being unsuited to CNC production techniques. The associated software and hardware can also be used increasingly to determine how to make generators more efficient in a number of key ways. For example, cutting material away from where it is not needed could reduce weight while keeping the power output the same, improving power density. In addition to generator design improvements, it is becoming more common to integrate generators into engines without using bearings (which are typically the most common point of failure in electric starter/generators). This is most easily accomplished through a direct-drive installation. Here, the rotor is mounted on the engine shaft and the stator is on the engine block, with the shaft running through its core, which means the generator does not need a April/May 2020 | Unmanned Systems Technology Using a positive-displacement piston pump can avoid key problems associated with gerotor pumps (Courtesy of Power4Flight) It is increasingly common for UAV engines to mount alternators directly on the driveshaft for producing upwards of 600 W (Courtesy of Acutronic)