Unmanned Systems Technology 028 | ecoSUB Robotics AUVs I ECUs focus I Space vehicles insight I AMZ Driverless gotthard I InterDrone 2019 report I ATI WAM 167-BB I Video systems focus I Aerdron HL4 Herculift

93 The chassis was designed to be structurally efficient and predictable in terms of operating loads, component attachments and the manufacturing methods used. Importantly, the team had to come up with a design concept, perform the required engineering, complete the manufacturing of the aircraft, and turn that into a flying prototype within four months. That led to what Marcos Fabian has described as a “very lean” development process. That process has resulted in a quadrotor aircraft weighing 13 kg (with batteries and equipment), that can carry up to 11.9 kg of payload capacity – a limit imposed by regulations requiring a MTOW of less than 25 kg for craft of this class; it measures about 130 x 130 cm (or 166 x 166 cm including propeller diameters) in size. At its maximum payload it can fly for up to 13 minutes, or up to 30 minutes with no payload. Airframe development The ‘H’ shape of the aircraft arose from the drive to obtain the highest possible structural strength-to-weight ratio. With that basic shape, payload size and estimated weight as a guide, the dimensions of the spars and other components could be adjusted in height, width, thickness, and other variables as needed to meet the operating load requirements. “It had to be a structure that was incredibly efficient, using structural elements such as C-beams, shear panels, and so on,” Marcos Fabian explains. “Everything that could be calculated ‘on paper’ had to be worked out in full, taking account of all the bending moments, shear forces and so on, so that it would be very predictable, calculable and statically determinate.” The HL4’s structure was worked out on paper using classical engineering calculations. V-M diagrams based on the operating requirements were used to determine the required dimensions, likely stresses, deflections and other features of the structural sections, long before going into FEA and other CAD tools. “In my opinion, any design engineer should be going through load calculations and running the gamut of hand-mathematical modelling before jumping onto computer modelling,” Marcos Fabian says. “If you want to get reliable results from FEA it’s much easier to know if they’re correct if you’ve done your classical engineering work ahead of time, and it’s great when they correlate to within 5%. “I wanted to use the carbon fibre legs as flat springs, so we looked into the energy absorption characteristics of the material and shape of the legs, and how to couple-out the reaction forces – landing, drag loads – at three attachment points, similar to what you’d do with an aircraft’s landing gear. As an engineer, you’re always after the most efficient and elegant load paths.” To design, build and fly a UAV that would fulfil SENER’s requirements in less than four months, Aerdron first looked at what components were available in the market, placing significant priority on finding the largest energy-storage batteries available, since that was a key component along with the motors. The UAV was then designed and built around the batteries, motors, propellers and payload bay. “We calculated the mass-moments of inertia in all axes – including the weight of the motors, batteries, payload – and input them into a simulator from UAV Aerdron HL4 Herculift | Digest If you want to get reliable results from FEA it’s easier to know if they’re correct if you’ve done the classical engineering work Unmanned Systems Technology | October/November 2019 Early design diagrams of the HL4’s structure