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40 Focus | Engine ancillary systems fluid and sound dynamics will be vital for examining how to attenuate exhaust noise without disrupting exhaust gas flow. Fortunately, AM also enables engineers to produce, test and analyse different exhaust structures quickly, to amass the necessary data for better CAE and CAD simulation. Materials such as aluminium or titanium can be worked with in this way, to examine their advantages in terms of weight or fatigue life. In addition to AM, some developers are investing r&d into variable exhaust systems, which use control elements inside the exhaust housing to balance the trade-off between sound attenuation and power output during a UAV’s flight. A UAV’s flight could be thought of as having two modes: a high-power mode for take-off, climb, and sprint; and a less powerful one for cruising and loitering. During the former, high noise output is largely accepted as inevitable; however the latter lasts far longer, and requires better fuel economy, minimal vibration from sound output, and often a reduced noise signature for users, especially those in the defence or intelligence communities. A variable system would therefore include one that opens for take-off and allows 20-25% more power over cruise, and another that would open for cruising (while the former closes) to enable quieter, more fuel-efficient operation. There is also considerable potential for this technology to enable the use of smaller engines for the same power output, as a result of the larger exhaust structure for take-off power being coupled with the smaller, operating one. Cooling Combinations of AM and design software advances also stand to make major improvements in cooling systems. The ability to model (and rapidly test) how heat moves and dissipates across different metal structures could lead to entirely new architectures becoming standard in engine designs. For example, one approach to cooling an engine cylinder has used an interior lattice structure rather than the tried-and- tested system of external cooling fins, with test results showing that less cooling air was needed to maintain the desired engine temperature. This could mean smaller inlets for cooling ducts, and thus less drag imposed by them on the hulls of UAVs. For engine designs that use liquid cooling systems, the coolant jackets can be additively manufactured integrally to the cylinders (and other key areas of thermal concern across the engine block) in ways that concentrate liquid flow at known hotspots. The geometries of the jackets required for this could not have been simulated a few years ago, and to date cannot be machined or cast, so they could unlock new forms of operation. For example, a dual-path water jacket could separate the cooling of the cylinder head from that of the cylinder and exhaust port, enabling engines to run their cylinder heads at much higher temperatures than their cylinder walls. That will preserve the walls’ durability while allowing higher peak power outputs (this could be particularly advantageous for heavy-fuel engines). AM would also make it easier to customise individual cylinder jackets. One option here would be to design for more concentrated cooling of middle cylinders – which typically run hotter than those at the edges of the engine block – helping to balance the temperature across the engine block and reduce points of stress related to thermal expansion from building up. Conclusion With further advances reducing the purchasing and operating costs of AM, its ability to reduce material waste and adjust design parameters in short-run production lines is likely to make it a critical tool in full-scale production of many engine parts. Furthermore, the additional data gained through trials and analytics of such parts (using CAN bus-enabled sensors and machine-learning technologies) should be expected to feed back into ever- greater improvements in design and simulation software, potentially making UAV engine systems world leaders in reliability, efficiency and longevity. Acknowledgements The author would like to thank Sean Hilbert of Cobra Aero, Gavin Brett of Currawong Engineering, Bill Vaglienti of Power4Flight, Nabeel Shirazee of ePropelled, Sergio Moscat of Moscat Ingenieria, John Landells and Jack Birmingham of Reventec, and Kevin Peryea and Peter Perrine of Acutronic Power Systems for their help with researching this article. April/May 2020 | Unmanned Systems Technology Additive manufacturing is enabling new approaches to designing exhaust and cooling systems (Courtesy of Cobra Aero)