36 “The aircraft is designed to be as aerodynamically stable as possible, which reduces the amount of on-the-fly work the flight controller needs to do,” Briggs adds. “That enables us to strip it and its network back to make them as light and low-power as possible.” Although that right-sizing means the Z8 currently has limited processing capabilities, Tyler says he is interested in using AI at the edge in future, and forming collaborations to that end. “We’re discovering entirely new phenomena in the stratosphere, like the behavioural patterns of gravity waves, and if onboard AI can use data from sensors capable of identifying those waves, the HAPS could intelligently avoid, study or take advantage of them depending on the mission requirements, just as we aim to in our pre-flight route planning for wind patterns.” MEMS IMUs are taken from Advanced Navigation’s Spatial 37 g GNSS-INS and installed in the Z8’s avionics for inertial data. Briggs notes that the Zephyr’s movements and dynamics are quite slow during flight, so 1 Hz of GNSS updates is fast enough for the Zephyr’s safe flight calculations, let alone the 10 Hz typical of the Spatial product. Different civilian and military customers are likely to have specific GNSS requirements, such as those for accuracy, robustness or security, in which case they will be integrated and redesigned where necessary. Other redesigns or supplier changes could be needed in the future to better enable certification of the Zephyr, which is challenging with the present, heavily bespoke engineering approach. Servos The Zephyr uses fewer control surfaces than most large, fixed-wing platforms – a rudder for yaw, an elevator for pitch and the variable thrust from the two propulsion motors for additional yaw. Briggs and Tyler nickname the elevator a ‘stabilator’ (not to be confused with Piper Aircraft’s use of the ‘stabilator’ to describe ‘all-flying tailplane’ configurations in which the entire rear wing rotates around a shaft). While an elevator would only have one or two flaps moving within a horizontal tailplane, the Z8’s entire tailplane moves up and down. “That means using two rotary servos, one in the rudder, the other in the stabilator, both above the tail fin,” Briggs says. “As well as keeping weight and power needs low, fewer servos means fewer points of mechanical failure. The stratosphere isn’t a kind environment, there are temperature extremes, radiation, ozone and other conditions that aren’t conducive to complex devices.” While the servos’ power, size and weight requirements were met in a straightforward manner, the HAPS’ lengthy endurance requirements were more challenging; the Z8’s last flight exceeded 1500 hours, and future flights are aiming for 4800. The actuators are therefore tested in-house to multiple times their stated lifespan using a combination of accelerated and non-accelerated testing regimes before using them in flight. Solar power The Zephyr’s solar panels are assembled from strings of Microlink Devices’ InGaAs triple-junction solar cell, partly for the high solar conversion efficiencies it provides at varying illumination levels and angles, which at the moment are up to 31% efficiency under AM1.5 illumination. They can absorb energy from UV, IR as well as visible light, and are just under 40 µm thick, with a mass density of below 250 g/m2. Potentially that gives just over 250 W/m2 and 1000 W/kg. “It’s an expensive solar cell, but the panels we make from them consistently provide more energy than we need, so we don’t cover the whole wingspan with them – you can see from above that it’s only on part of the wing, shaped in strips to align with where the batteries and chargers are, and we’ll increase or decrease the solar panel area depending on where the aircraft is flying and what time of year it is,” Tyler says. While the main wings provide the majority of power generation during the day, the dihedrals typically receive better solar coverage and hence generate more energy in the hours just after sunrise and just before sunset. The flight plan will often include specific patterns to ‘catch’ additional sunlight as the sun rises and sets. “Flying specific, planned patterns captures more sunlight, and creates December/January 2024 | Uncrewed Systems Technology Dossier | AALTO Zephyr 8 Energy storage relies on Amprius’ prismatic silicon nanowire anode cells, with AALTO set to start integrating the company’s 500 Wh/kg cells in the future (Courtesy of Amprius)
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