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62 Digest | Bell Autonomous Pod Transport UAV “The tail-sitter cross-section also needs to be really stable during landing. If it is too long in one direction and too narrow in the other, the aircraft will have tip-over challenges when landing in crosswinds or on uneven surfaces. If you’re flying disaster relief packages or military resupply missions, you’re likely to be constrained on landing space and ingress area, and you’ll almost always be descending onto uneven terrain. “Our H-frame gives the APT four struts that ensure it lands stably and is comfortably propped up, with its weight distributed across a balanced area, including on quite uneven or soft ground.” The aircraft’s structure and wiring are modular, allowing it to be disassembled and stowed in a case or reassembled and made ready for launch within 15 minutes. Almost all the avionics, processors and comms equipment are installed in the wings, while all the high-power propulsion electronics and electrical systems are stored in the four propulsion nacelles, to minimise interference and improve sensor accuracy. “That’s especially important for our antennas, because their coax cables are sensitive to interference,” Wittmaak adds. “And packing all the electronics in the wings means they sit next to a constant rush of cooling airflow, which makes for better thermal management.” By contrast, the central pod does not carry any mission systems or equipment. The aircraft can be flown with or without the pod, and it can be changed to carry whatever payload the end-user wants, including different types of cargo carriers. That makes it and the overall space between the wings and pylons the foremost point of customisation for different applications and use-cases. “The National Guard, for example, might need to fly tools, parts and other run-of- the-mill supplies between bases most days, but then get called in to do FEMA emergency supply runs,” Wittmaak says. “FEMA could have its own pods that are just fastened in place – any shape, size or weight so long as it fits on the aircraft. “In future, we may add electrical interfaces to the pylons for pods, so if users wanted to plug in payload sensors or other devices, the necessary I/Os would be there.” Bell has used a range of materials to construct the airframe, including carbon composite (which Bell has worked with extensively since the early days of the V-22 Osprey tiltrotor) in the wings and pylons and additively manufactured nylon plastics for the propulsion modules. Interfaces and analytics All of the APT’s electrical components are what Wittmaak refers to as “smart”. Each one internally monitors its own health, total usage, performance and configuration, and reports that information back to the vehicle management computer (VMC). These components include the ESCs, servos, the FCC, the VMC itself, batteries, data links, INS, GNSS receiver and sensors. The VMC is able to report any real-time errors in onboard systems to the operator and logs key parameters from each component, as well as flight plans, aircraft telemetry and environmental conditions. This information is fed to the VMC and downloaded by the GCS upon landing. Bell’s prognostic and diagnostic database, MissionLink, is subsequently populated with that data, and the company continuously develops algorithms to have the ground systems autonomously analyse and predict maintenance requirements for each APT and its systems. “Having the ability to transfer large amounts and different types of component data over our internal aircraft comms channels is key to developing our ‘digital thread’ – the path across which our UAV data flows,” Wittmaak says. “The thread feeds into our digital twin – the data we collect on the ground that represents cumulative usage, performance measurements, environmental data, configuration and flight plans. We use the digital twin to monitor the configuration and performance of the APT, and look for any component degradation over time. “That level of connectivity and smart analytics is essential for a truly autonomous system that anyone can October/November 2020 | Unmanned Systems Technology As all avionics are packaged in the wings, and all power systems in the nacelles, the APTs can be flown with any cargo pod (or none at all) installed in the centre