Unmanned Systems Technology 036

94 Digest | Zepher Flight Labs Z1 This proton exchange membrane fuel cell produces peaks of 1.4 kW from its maximum continuous output of 800 W at up to 25 V, and weighs 930 g (albeit with a 250 g pressure regulator), giving it a power-to-weight ratio that appealed to Coatney and his colleagues. Its energy efficiency of up to 55% and TBO of at least 1000 hours also stood out among the available options. “Neither Intelligent Energy nor anyone else has anything that’s perfect for our application, but the 800 W module comes close,” Coatney notes. “If development over the next several months goes well, it’s likely that we’ll be asking them for a customised installation.” The Z1’s powertrain uses two of these cells connected in series for a combined 50 V, 1600 W power supply; it also provides redundancy for the UAV to return home if one fails or has to be shut down owing to a safety-critical alert. The cells feed into a battery to regulate the voltage output, from which power is distributed across onboard systems. Hydrogen tanks are supplied by HyPerComp, with customisations for Zepher towards increased weight optimisation. They are based on the supplier’s experience of manufacturing tanks for the US Naval Research Laboratory’s Ion Tiger UAV and similar projects. “Our tank design is newer and lighter than the Ion Tiger’s,” Coatney says. “It wouldn’t achieve US Department of Transport certification but it easily meets SAE and ISO specs for hydrogen tanks used in motor vehicle propulsion. “While we can’t disclose an exact weight figure, it’ll be a Type IV tank, using a carbon fibre shell and polymer liner. Similar tanks built for various industries can hold hydrogen gas at up to 700 bar and have been proven over the past 10 years.” Variable pitch and VTOL A VPP helps the Z1 meet several of Zepher’s key engineering targets, the most obvious being the aerodynamic efficiency and therefore endurance through its ability to adjust and optimise the pitch of the propeller blades for any given airspeed or thrust output. As more thrust is derived for a given motor energy input, the VPP contributes significantly to the Z1’s 15-hour maximum endurance (or 13 hours with a 5 kg payload). Being able to run the blades more slowly during cruise also further reduces the Z1’s acoustic signature. There are also significant benefits regarding flight precision. Coatney explains, “If you look at any selection of US military helicopters, including modern ones like the Sikorsky S-97 and the Sikorsky-Boeing SB-1 Defiant, they all use VPPs to accelerate and decelerate while flat – without pitching the vehicle. “For us, that ability is even more important, as it allows us to descend quickly and precisely, even when it’s windy, and to climb aggressively when outbound. It might mean a slight trade- off in cruise efficiency at a few points of flight, but we’ll still gain more than if we’d gone with a fixed-pitch prop.” Zepher estimates that 3-4 seconds are needed for the Z1 to accelerate into horizontal flight after reaching a specified transition altitude, while it will take 6-8 seconds to slow down from wing-borne flight to a stationary hover before final descent. This fast-transition capability reduces the size of batteries required for hover flight, saving weight for fuel and payloads. “Forward-thinking groups among the February/March 2021 | Unmanned Systems Technology The Z1 has twin boom-mounted electric motors for VTOL and transitioning in and out of wing-borne flight, aided by a variable-pitch pusher propeller Two 800 W hydrogen fuel cells are connected in series to give the Z1 a 50 V, 1600 W dual-redundant power supply (Courtesy of Intelligent Energy)