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72 affecting the power system design and its structure, as different wing sections can have different amounts of array on, and the sections can be interchanged for different missions. “We have been working on electric UAVs for more than 15 years, and batteries have always been the defining technology for performance. They have also shown perhaps the slowest progress in terms of specific energy [watt-hours per kilogramme] compared with early expectations. “To put that in context for a solar UAV, the solar panels can produce 1500 W of power from 1 kg of array. So with 12 hours of daylight, that is some 9000 Wh of energy per kilo of solar array – adjusting for the sinusoidal variations in elevation through the day, which means you actually take in only around 50% of the total sunlight. The best battery used in most EVs [18650 lithium-ion] provides 260 Wh for every kilo of battery, so batteries are some 36 times worse than the array on a sunny day.” As battery technologies and chemistries seem poised to advance beyond 300 Wh/kg, however, Prismatic is designing its electric UAVs to work with the same power structure regardless of the choice of battery type or chemistry being used. Design and testing The PHASE-8 is designed to undertake a small (and well-understood) range of tasks, manoeuvres and loads compared with most UASs. Because of that, the Prismatic engineers placed comparatively little emphasis on FEA, CFD and similar analytical approaches that are used for more complex designs. “This approach works particularly well for our ultra-light structures, which rely on thinwall hulls,” Dixon explains. “Such structures tend to be limited by buckling and material or assembly errors, rather than the basic material properties that computer models assume, so testing is the only viable way of demonstrating acceptable performance.” At time of writing, Prismatic reports having cycled its power systems for over 180 days continuously, and its flight controller for over 75 days, without failure or problems. The design team used CAD tools such as SolidWorks to enable smooth transfer to the CNC machining used for their moulds, as well as additive manufacturing for quick prototyping and scaling of parts and for the UAV’s more complex joints and fittings. The emphasis on test flights proved challenging for Prismatic’s team, however, primarily because of the low airspeeds and high energy efficiencies at which they sought to measure small variations in performance. “We are looking to measure sink rates of the aircraft of the order of just 25 cm/s. Compare that descent rate with a slow wind of 6 km per hour blowing down a slope of just 3 º , which most people would consider to be ‘level’. The speed of this ‘sinking air’ is 15 cm/s, which is far greater than the accuracy we need to measure the descent rate of the aircraft,” Dixon says. Therefore, when undertaking flight trials, the team uses digital elevation models of the test sites and continuously recorded data to achieve a close understanding of the air movement, and then correlates this with the long- endurance test results from the aircraft in flight. Most critical to worthwhile tests has been conducting the flights in environmental conditions that are as stable as possible, and flying the aircraft for hours at a time. By evaluating the observed sink rates as a function of DC power according to design expectations, the team has been able to validate the PHASE-8’s expected performance in the air, as well October/November 2018 | Unmanned Systems Technology Systems integration: in-house Composite structure elements: Piran Advanced Composites Tooling: Digital Fabrications Solar cells: MicroLink Devices Lithium-ion batteries: Kokam, Panasonic and GEB (among others) Charging system: in-house Flight controller: Options include PixHawk, Embention Servos: MKS Servos Motors and speed controllers: DJI Propellers: in-house Some key suppliers to the PHASE-8 The Prismatic team has built five PHASE-8 aircraft and conducted 13 flight tests, with the first dawn-to-dusk flight planned for early 2019