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74 mechanical design and drawing work in- house, and then had them built-to-print. “Developing the generator was within my remit, and its specifications define a lot of the aspects of the overall turbo- generator engine. A change in the generator’s parameters could mean that the drive shaft needs to be longer, so you might need to recalculate all the dynamic behaviours and so on. “All APUs use a reduction gearbox to lower the rotor to an acceptable speed, and developing the TG-R55 using a direct-drive approach brought some major technical challenges, both in mechanical and electromagnetic terms. Guimbard and I had to collaborate very closely on that, because the interactions, dynamics and different requirements from the turbine and the generator took a lot of time and effort to balance. “Significant safety issues had to be tested and iterated as well, as permanent magnets are constantly exerting a magnetic field, so a rotating permanent magnet machine will always have a certain voltage at its output. We’ve had to work on the failure modes and develop our own solutions to mitigate those issues,” Nguyen says. The generator is entirely air-cooled, which is consistent with the theme of mechanical simplicity. A fan inside the generator is also driven directly by the turbine shaft, turning at 90,000 rpm to draw air into the aluminium generator housing. The air’s path is designed to push it through a guide vane, cooling the stator and rotor, before it exits through the gaps between the enclosure’s fins. Future tests and development Nguyen has led Turbotech’s engineers in constructing their own testing rigs in order to trial, analyse, and optimise their systems’ operation and performance. Key components for analogue front ends and data acquisition were selected to ensure a dense and accurate set of data feeds, with LabView used as their control and tracking software. “Given the unusual shape of these turbine engines and the considerable electronics, a huge amount of cabling, hydraulics and frame manufacturing was needed to build these rigs precisely as we wanted them,” he says.  “Rather than outsourcing, we actually manufactured these aspects in-house with on-site machinery we occasionally use for prototyping. That decision was taken partially to avoid slowing down development, but also because when we’re testing, if something with the April/May 2020 | Unmanned Systems Technology At the base of the TG-R55’s generator is a cooling fan for drawing in air (Author’s image) Turbotech has developed its own testing rigs for trialling and optimising its engine designs (Author’s image)