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59 batteries, so we saw a great opportunity there to provide a solution that commercial users could adopt to satisfy their needs for longer endurance.” The CTO and founder of Quaternium, Jose Luis Cortes, had previously spent more than 20 years working with combustion engines, particularly on two- stroke motorcycle race engines. This familiarity motivated him to experiment with the idea of integrating a two-stroke engine into a multi-copter airframe to extend its range; that was in 2013. “I’d long thought that lithium battery technologies were rather too mature to experience any real breakthroughs in the near term, and that entirely different powertrains would be needed for a viable scaling-up of commercial UAV operations,” he says. “Neither VTOL planes nor hydrogen fuel cells seemed as viable back then, and in any case, my close experience with race and hybridised engines meant I could already envision how a series hybrid UAV would work. “For a small UAV with limited free volume for installing an engine, using a two-stroke was also best thanks to the size and reliability of these types of engines. Most four-strokes would be too large, and a gas micro-turbine at that size would not be fuel-efficient.” Drawing on his previous engineering experience, as well as an extensive range of calculations for weight, structure and power output, Cortes determined that a gasoline-electric powertrain would indeed be viable for multi- copter integration, and the first HYBRiX prototype was built in 2014. Fuentes says, “At the time, it was widely considered unfeasible that a hybrid- electric powertrain would work on a quadcopter – no-one really thought the power-to-weight ratio could be optimised. Cortes chose to open that Pandora’s box though, and his calculations in 2013 were validated by our first flight tests in 2015, which achieved a maximum endurance of 3 hours.” The craft has been iterated several times since then to optimise its performance, its cargo-carrying capability and its capacity to function smoothly in different climates, as well as to accommodate continual improvements in the UAV’s power-to-weight ratio. Every aspect of the system has been fine-tuned to add more time to its endurance limit. The HYBRiX 2.1 can lift a maximum of 5 kg of payload for 2 hours, for an overall MTOW of 20 kg. It is 509 mm tall and 1249 mm across without propellers (760-813 mm propellers are typically integrated), although its carbon tube arms can be removed to reduce its carrying size down to the 678 mm length of its fuselage. It is rated to a maximum endurance of 4 hours (when operating without a payload), a cruising speed of 50 kph and a top speed of 80 kph. These specifications (particularly regarding speed and endurance) mean Quaternium sees the HYBRiX 2.1 as particularly useful Quaternium HYBRiX 2.1 | Digest The HYBRiX 2.1 can carry up to 5 kg on a 20 kg MTOW for 2 hours (Images courtesy of Quaternium) Unmanned Systems Technology | April/May 2020 The UAV’s engine has been cycled hundreds of times to achieve the desired power output, reliability and a host of other critical project targets