52 our batteries and range extender, and touch-and-go missions are possible, in which we land and take off with full power coming from the batteries. “An all-electric aircraft would land with reduced energy, because its batteries can’t replenish during flight. We don’t believe that’s been proven to be safe, so we recharge our batteries during flight to land with full power, do touch-and-go deliveries and still retain 30 minutes of reserve energy. And the FAA has recently announced that crewed eVTOLs are expected to have that level of performance too, maybe up to 45 minutes of reserve for night operations, which we could also offer.” The choice of Suter Industries’ TOA288 (a 288 cc, spark-ignited boxer twin weighing 8.9 kg and outputting 17.7 kW as peak power) as the core of the range extender came partly from it already being FAA-certified. Dufour also found it has the best power-to-weight ratio of any engine in the power range it wanted, and Suter’s location in Switzerland was helpful from communication, partnership and maintenance perspectives. “I think they’ve sold close to 250 of those units now, all as primary power sources,” Bendrey says. “As mentioned, we’ve coupled it with the Plettenberg motor/generator. “We needed something robust and industrial, and Plettenberg had some units that matched our requirements,” he adds. “We get 17.5 kW of shaft power from the engine, which is converted through the generator to 15 kW of continuous electric power. That more than meets our mission requirements.” Ten 14S battery packs, each with 7200 mAh of energy capacity, are installed. Each pack has its own BMS to regulate in-flight charging and discharging, with a local charge controller responsible for determining how much energy is needed by each lift-thrust system, balancing for differences between how much thrust is needed at each rotor for stable manoeuvring, and how much energy is needed to keep each battery working safely and stocked with reserve energy. “We run up to 58.8 V maximum, 52 V nominal, and the generator produces between 30 and 58 V depending on total power required,” Bendrey says. “The highest current draw takes place in the short distances between the batteries and ESCs, which keeps our total cabling small and lightweight.” Flight control The layout of the Aero2’s control surfaces has been developed through analysing data from simulations and real-world test flights, and reflects Dufour’s desire for complete redundancy in power, control and aerodynamics. This is not a strict requirement for entering commercial service, but the company is determined that safety must be maximised if customers are to entrust vital cargo and other missions to an all-new aircraft. Each of the Aero2’s two wings has two flaperons and two ailerons, while the H-tail structure has an elevator and two rudders, making for 11 control surfaces in all. That enables redundancy in the control of the UAV through all axes of freedom in hover, transition and cruise, so that safe flights and landings can be performed despite any system failures, October/November 2023 | Uncrewed Systems Technology Dufour chose the TOA-288 engine as its range extender for its FAA certification and its high power-to-weight ratio (Courtesy of Suter Industries) A total of 11 control surfaces are used across the Aero – four flaperons, four ailerons, an elevator and two rudders
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