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10 Platform one December/January 2019 | Unmanned Systems Technology Lange Research Aircraft has unveiled a model of its Antares E2 OPV, a 23 m wingspan fuel cell-powered aircraft capable of up to 40 hours of flight, and at up to 20,000 ft (writes Rory Jackson). Originally based on the glider-type aircraft built by Lange Aviation, the Antares E2 has six electric motors mounted above its wings, with fuel tanks, six fuel cells and other key power components held in pods beneath them. The craft can carry up to 200 kg of payload in the fuselage or cockpit, with 4 kW of power over a 28 V DC input made available for sensors. “As far back as the 1990s, we saw advantages in the reliability, low vibration, picture quality, noise and maintenance of electric powertrains over fuel aircraft engines,” said Lange Research development engineer, Andor Holtsmark. “The problem of course remains the low energy density of batteries, which cannot power an aircraft over hundreds of kilometres. Today’s best COTS batteries are still a factor of 15 worse than ICEs, and relying on solar cells means you have to fly very high, usually above the clouds, which left us with hydrogen fuel cells as the best source of electric power. “Diesel-hybrid systems aren’t as clean or efficient as we want, and solar power tends to shoehorn your design towards a thin, spindly airframe with limited payload capacity. “A fuel cell hybrid system can deliver 50-60% energy efficiency with closely controlled and monitored flight, compared to the 35-45% efficiency of diesel-hybrid powertrains. The problem lies with fuel cell weight versus kilowatts generated, so we still needed to design the Antares E2 to fly with as little energy consumption as possible. “Also, we couldn’t use pressurised hydrogen, as carrying 5 kg of it at 350 bar would require a 95 kg carbon fibre pressure vessel. And liquid hydrogen comes with severe energy balance issues as you need significant energy for the cryogenic fuel storage. That also meant emissions in excess of our ecological objectives for this aircraft.” These concerns and targets spurred Lange to equip the Antares E2 with reformed-methanol fuel cells, from Serenergy. The E2 generates power by first drawing in oxygen, water and methanol from tanks in the two outermost wing pods. These are pumped through the wings into the innermost pods, where the oxygen is fed into the fuel cells, and the methanol and water are fed into catalytic steam reformers. The reformers output hydrogen into the fuel cell and CO 2 into the air (but no more CO 2 than is extracted during green methanol production, the company says). In addition to the 6.7 kW of power produced by each fuel cell stack, the water generated is channelled back to the tanks in the outer pods and re-used in the reformers. Excess heat and purge gas generated by the fuel cell are also recycled by helping to heat the reformers. Each fuel cell has a DC-DC converter to control the individual electrical loads required for the propeller drives, servo actuators, avionics, payload sensors and other onboard systems. At least 50% of the energy produced is used by the motors, which have a peak consumption rate of 15 kW, giving the Antares E2 a maximum airspeed of 135 kph (with a minimum/stall speed of 62 kph). “There will always be a bit of unburnt hydrogen gas and excess heat from a fuel cell; by using integrated reformers, we can use this for a more sustainable UAS overall than other current fuel cell aircraft,” Holtsmark said. “We’ve also opted for solid-state electronics with built- in diagnostics protocols as far as possible to minimise the difficulty of maintenance.” Taking flight on fuel cells Airborne vehicles The Antares E2 OPV is capable of 40-hour flights and can carry up to 200 kg