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56 and GNSS antenna, as well as a lot of distance between the electric motors and all the antennas, to prevent any source of EMI from impacting on the transmission and reception of signals. And behind the wings we can have two different inverted- vee tails with elevons to achieve robust flight control.” Navigational lights from Aveo Engineering are installed about the wings, hull and tail. While not required by European legislation, some customers wanted to be able to tell the left wing from the right from a distance (and thus gauge the UAV’s heading), and proper aircraft navigational lights aid greatly with this at night or in foggy conditions. Structure and hull Designing the aerofoils and geometries of the Sierra VTOL’s parts was carried out using XFLR, an aero simulation tool developed by Professor Mark Drela at the Massachusetts Institute of Technology, to assess and optimise the aerodynamics. As mentioned, these were validated through flight testing before moulds were CNC- machined to reproduce the hulls in bulk. “XFLR isn’t the most user-friendly software out there, but it gets really good results for us,” Langus notes. “Normal aerodynamics modelling tools are actually aimed at simulating much larger aircraft with higher Reynolds numbers, but UAVs like ours operate at much lower regimes of Reynolds numbers. So XLFR is much more specialised in capturing the airflow scenarios we’ll be flying, hovering and transitioning in. “We iterated our models of most parts three times, specifically to optimise how we would do the composite laminations, to achieve the lowest weight possible while still hitting our structural performance targets.” The fuselage, tail and most of the wings are carbon fibre, although in the centre of each third of the wings are slots made from glass fibre. As this material is RF-transparent (unlike most carbons) it allows GNSS and other radio antenna signals to pass in and out with good reception. Inside the wings, there are spars and carbon reinforcements. “We wanted to use materials that are certified to aviation standards, which we selected from R&G Composites in Germany, so all our carbon sheets, glass fibre sheets and epoxy resins have aircraft certifications,” Langus says. “We do have to mix them in very precise quantities and processes though to keep to the regulatory standards, but that’s a key part of our routine production processes.” As mentioned, moulds for constructing the Sierra VTOL were produced in June 2019, although its engineers are now looking to change their approach in order to expand production of the airframes. “Currently, we use mainly hand-laid composite laminates, and have done so since the first delta-wing craft,” Langus comments. “That’s been good enough for our current production capacity and number of orders we have, but hand lay-ups take a bit more time than other composite construction methods. “Using precise measuring techniques makes it a highly repeatable process with good quality control. But to output UAVs at a higher rate while maintaining design precision, we’re looking at using prepregs. “We’re talking to specialist prepreg manufacturers here in Slovenia, and we’ll probably end up outsourcing the lamination process, but we’d cooperate closely with them so as to keep control over the production process and quality management.” Mission systems A few components on the Sierra VTOL stand out as particularly user-relevant. The most obvious are the payloads, which by and large have been EO, IR and similar systems fitting the UAV’s 3 kg carrying capacity, trading inversely with its endurance. “For instance, a 2 kg Lidar could be flown for around 90 minutes, which is more than enough for typical Lidar applications,” Langus says. “And more recently we have also been working with other companies in Slovenia, especially the postal service, which has used one of our Sierra VTOLs to deliver a package to a mountain cabin. “We’re also using the Sierra VTOL with hospitals across Slovenia to trial how we might carry out emergency medical deliveries, as well as with academic institutions such as the University of Tubingen in southwest Germany, which is making environmental February/March 2022 | Unmanned Systems Technology Digest | ElevonX Sierra VTOL Navigational lights from Aveo Engineering allow the Sierra’s location and direction to be discerned through fog or at night