71 ground-based applications, but Slater and his colleagues were drawn to its mechanical simplicity, low weight and material selections, seeing potential in it for aircraft propulsion. “Efficiency was also critical, because we wanted to fly 1000 km per flight. That had already honed our selection of engines down to a single-cylinder fourstroke, known to be the most miserly type of engine at sipping fuel,” Slater muses. “The efficiency requirement meant we couldn’t have a Wankel or two-stroke. It was either a single- or two-cylinder four-stroke, and anyone who knows motorcycle endurance rallying knows that a single cylinder is also an optimally lowrpm, high-torque, four-stroke engine. “Anyone with an understanding of propeller design knows a propeller should have lower rather than higher rpm: the higher the prop rpm, the more each blade catches up with the airflow disturbed by the advance prop blade during rotation. “And we didn’t want the added weight of a gearbox or liquid-cooling circuit, so we needed an air-cooled engine, with max 5000 rpm at its output shaft, and a preferential operating range of 3600-4000 rpm to match our concept propeller sizing.” Slater and his team opted to develop Short’s engine for its new aircraft, which they named Prion Mk.3, after MagSurvey’s Prion Mk.2, which flew for the first time in July 2007 (in an airworthiness inspection flight for the UK CAA) after 1.5 years of development. “Short had a lifetime of engineering experience and he fed that into the engine. Recognising that, we acquired the IP for it from him in 2013, when we did the management buyout – why it carries his initials as the DS120 to this day,” he says. That buyout occurred after TGSNOPEC chose to scale back its usage of UAVs, but not before the Norwegian parent company tasked the teams behind the Prion Mk.3 (and the DGS120) with an Arctic-based aeromagnetic mineral survey, successfully locating kimberlite pipes – the most important source of mined diamonds today – to attract potential buyers. “But, with regulators having made it impossible to repeat such mineral surveys outside the Arctic and similarly remote locations, we engineers bought the company and began diversifying,” Slater recounts. “However, we kept developing the engine on an unchanged track, because it had proven itself to be extremely capable, including incredibly consistent and reliable performance in long BVLOS flights lasting many hours. We’d also accrued a huge library of data from active flight exercises in the Arctic, and elsewhere, to inform future optimisations in areas like fuel efficiency and lightweighting.” Today, the DS120 is a 120 cc air-cooled engine, weighing 4.6 kg and producing 8 hp at a 4600 rpm maximum continuous operating speed, with a redline speed of 5000 rpm. Naturally aspirated, it runs on RON90 petrol with 2.5% oil mix, although the company has previously developed a heavy fuel version and is open to doing so again. To date, it has operated up to 10,000 ft (3.04 km) AMSL and has a TBO of 1000 hours, with servicing intervals every 50 and 200 hours. It typically sits upside down, with its cylinder at the bottom and the crankshaft running at the top, to keep propeller vibrations away from any payloads, and the steel crankshaft away from mining survey magnetometers to prevent ferromagnetic interference. A camshaft sits near (and is geardriven by) the crankshaft, and actuates the cylinder’s two conventional overhead poppet valves via a conventional valvetrain of pushrods and rockers, with coil return springs. UAVE 120 cc four-stroke | Engine dossier Anyone who knows motorcycle endurance rallying knows that a single cylinder is also an optimally low-rpm, high-torque, four-stroke engine Uncrewed Systems Technology | December/January 2025 The DS120 powers UAVE’s Prion Mk.3 UAV (AKA Dragon, for defence applications), a 30 kg, 3.8 m wingspan aircraft
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