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39 Engine ancillary systems | Focus bearing, thus improving its reliability. This kind of integration can require a special tool (or jig), however, as the magnets on the rotor will pull it towards the stator, making it hard to install while maintaining the 0.8 mm or similar air gap between the two parts. Also, a metal coupling ring for attaching the rotor to the shaft should be used, which can be fastened to both the shaft and rotor, having been machined to the right profile as determined by CAD analysis. Future advances in materials science are also expected to bring further benefits to generator technology. In addition to new alloys offering weight-saving and thermal efficiency, new forms of high-temperature neodymium for example will enable motor starter-generators to withstand more heat with lower power losses, while improved stator lamination steels could reduce hysteresis losses, resulting in improved efficiency at maintaining magnetic fields, improving generator power output, efficiency and longevity. As the scale of UAV electrical power systems grows, the efficiencies and choices of power electronics are likely also to fall within the concerns of UAV engine and powertrain specialists. This will be vital to ensuring hybrid powertrains do not incur excessive switching losses when powering the (likely) multiple payloads, computers, sensors and other onboard electronics systems that UAVs will be expected to carry. Silicon MOSFETs remain widely used owing to their known reliability, as do silicon IGBTs. There is also some wider experimentation with newer types of power transistors such as silicon carbide (SiC) and gallium nitride (GaN). These trials are happening as a result of growing requests for powertrains to be lighter, more compact and to run cooler. Both SiC and GaN stand to save significant weight, volume and heat, not to mention improve power efficiency. However, wider availability and improved supply chains for these systems will be needed before they can be broadly integrated into UAV architectures. Lastly, future certifications will probably depend on safety in terms of redundancy. While the probability of generator failure is low among engine ancillary systems, different approaches can be taken to achieve redundancy. The most direct way is simply to install a second generator, potentially on a second engine, depending on the size of the UAV and the foresight of the manufacturer to include one. Multiple sets of windings that are isolated from each other can also be designed into a generator. This can mean making it somewhat larger and heavier for the same power output, by perhaps 10-40%, but this approach could become a regulatory requirement for UAVs flying over populated or restricted areas. Exhausts AM potentially represents a major leap forward for the design and manufacture of exhaust systems, for several reasons. While rapid prototyping has generally been the leading application for AM machinery among engine designers, a bigger area of interest now is that engine manufacturers can create geometries that would previously have been impossible or at least prohibitively expensive using CNC machining. For example, AM can enable the production of Helmholtz resonating chambers that wind in and throughout the exhaust muffler, to suppress noise and vibration to a previously unachievable extent, and packaged tightly to minimise volume. These cannot be created using traditional casting or cutting methods. There are various other kinds of internal structures for attenuating sound that also cannot be machined. Different geometries, structures and shapes for reflecting and cancelling noise that are internal to the exhaust housing will be able to reduce the volume of the engine’s output without reducing its power. Key to this is the ability to model the bulk pressure wave characteristics inside exhaust systems. While most CAD and CFD software typically executes this well with simple geometries, it can fall short when engineers are trying to better understand how different shapes (such as lattice structures) can help break up and attenuate sound. In the pursuit of better noise suppression in engines, it is therefore important for design engineers to develop models that can simulate for example how sound waves move through a lattice shape. In the future, more detailed combined models for Unmanned Systems Technology | April/May 2020 Advances in CFD and CAD are unlocking improved designs for thermal efficiency in high-power UAV generators (Courtesy of ePropelled)