34 limits for all-up-weight and g-forces. CFD and FEA simulations are also regularly run to support optimisations for bonding and structural design using newly gathered flight data. “The load-testing measures not just the amount of deflection, but also checks that there are no sub-element failures resulting from the loads applied,” Briggs explains. After passing those tests, the airframe moves down the production line into avionics integration. Most of the wire harnesses and principal components for flight control, navigation and comms are installed here, after which they are also test-cycled to verify their functionality. The same then follows for the payloads as well as the batteries, power distribution, motor control and propulsion systems. “Before that though, we’ll have tested all those parts in our combined thermal and vacuum chamber, in which we can simulate and exceed the range of pressure and temperature conditions the stratosphere can expose our electrics and electronics to,” Tyler says. Lastly, Mylar coverings are placed and glued around the mechanical, electrical and electronic systems to close them off from the environment. The solar array is glued on top of the Mylar on the wings, although future Zephyr designs could save weight by using the PV panels as housing members, forgoing some Mylar underneath. “We want our final assembly lines formalised into something we can export and set up in different countries, so we’re not always building the whole aircraft here, or dealing with the costs or damage risks from shipping worldwide from Britain,” Tyler notes. “We could have components turn up and put together into a launch-ready aircraft anywhere.” Airframe materials The carbon fibre aerostructure is designed around a span-loaded wing with a single central spar and lattice ribs. Fairings and trailing edges are a lightweight open-cell foam, while as mentioned the skin is Mylar. “Mylar is commonly used in this type of aircraft,” Briggs says. “It’s very lightweight and adds a lot of stiffness to the HAPS, and it was used on previous lightweight aircraft like the Gossamer Condor. It’s been improved since then to make it thinner and lighter.” The open-cell foam provides similar weight and structural strength advantages, along with good thermal insulation properties for the batteries and electronics. It is also easily cut, sanded and shaped as needed for mounting avionics, batteries and other subsystems inside, as well as conforming to the shapes needed for the trailing edges. In addition, a silver foil based on another type of Mylar is used throughout the wings for key subsystems, to provide extra thermal insulation against the extremes of cold during the night and heat during daytime. Briggs adds, “Getting the insulation right so you can self-heat your batteries and electronics at night but not cook them during the day is a key balancing act. “Onboard equipment needs constant December/January 2024 | Uncrewed Systems Technology Dossier | AALTO Zephyr 8 Above: Key navigation components are extracted from Advanced Navigation’s Spatial GNSS-INS (Courtesy of Advanced Navigation) Left:AALTO’s huge store of flight data is used to simulate how the Zephyr might flex at different speeds and conditions, enabling it continually optimise its structural design
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