thermal monitoring: there are around 50 thermistors installed in the wings, and then 80-100 in total if you include the sensors in the battery packs. Every single thermal sensor is tested for serviceability and range before being approved for launch.” Structural design and loading considerations Briggs estimates that there are around 250 separate carbon parts in the Z8. These flex and bend throughout flight, a necessary side-effect of AALTO using the lightest carbon composites it could find, although it helps to avoid structural stress and fatigue. “Modelling loads across the aircraft is straightforward, but the tricky bit is dynamic simulations of the HAPS,” Briggs adds. “Its flexibility means that at higher speeds, its wing will twist and change shape. Most modelling programs out there are designed for things like A320s, which are pretty stiff in their design and use of materials, but they don’t exactly come with a feature for a twisting wing.” AALTO compensates for that by using its flight data, combined with further data such as that published by Google Loon, to simulate in-house the shapes the Zephyr takes at different speeds and within different permutations of the stratospheric environment. These are refined with each flight to further optimise and validate the HAPS’ structural design. “Among the most important aspects of the primary structural design we look at is how to cope with torsional loads, which leads to the selection of materials and how they’re implemented in the design of the aircraft to counter those loads,” Briggs says. “The hardest time for the aircraft is during the ascent to and descent from the stratosphere. The weight, thermal protections and load distributions are optimised for the stratosphere, whereas in ascent and descent it’s dealing with heavier air densities and higher temperatures than usual.” In addition to the wing structures, the three carbon booms – the fuselage and the two motor booms – are sized to balance the CoG, with any further balance maintained by installing opencell foam pieces and subsystems. That though does not necessarily require any major symmetry in system placements or cablings. Flight management Most COTS avionics products are unsuitable owing to the temperature ranges, altitudes and weight budget the Zephyr’s engineers must design for. AALTO therefore assembles its flight controller and most other avionics in-house by taking delivery of COTS systems and stripping them down, often extracting individual chips and other PCB-level components and mounting them on its own weight and thermally optimised boards. “For instance, the INS comes in a box about the size of a fist, and we remake it into something the size of a penny. Our pitot tube is remanufactured to about a quarter of its usual size, and our satcom radios and everything else are all stripped right back,” Tyler says. “We use a proprietary master flight controller, to which most of the avionics are slaved. That controller is constructed with FPGAs around a primary CPU, with the FPGAs chosen to reduce overall power usage and processing power to just what our system architecture needs. Plus FPGAs can never get saturated, they just do their one job quickly and constantly, which is useful for avoiding faults.” The Z8 can manage up to 1255 waypoints, spread out over 200 days or more. These can be time-bound or action-bound, for instance setting the aircraft to loiter at a given waypoint, where it might activate a 5G modem for a specified duration, check ADS-B data for traffic, or call a new GCS to attempt to hand over control. Alternatively, a waypoint can be set to expire based on the passage of time for example. During lost comms, the aircraft can fly autonomously for up to 7 days without human input. This can be reduced depending on aviation regulators’ requirements, who may only allow for an hour or two of autonomous flight without a pilot in the loop. The data bus from the flight controller to the navigation sensors and servos, along with the power bus, are proprietary, with SWaP optimisation being their key design focus. 35 Uncrewed Systems Technology | December/January 2024 Microlink Devices’ triple-junction InGaAs solar cell enables the Zephyr 8 to charge at essentially any inclination and wavelength of sunlight
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