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61 saving supplies too, but manned aircraft platforms are a limited resource and may not be available in times of need. It looked pretty clear to us that the world needed some form of autonomous aerial resupply capability, so we started drawing up concepts that fill those needs.” Developing the APT Of all the ideas that Bell’s team came up with, in 2017, the APT in its present configuration – a battery-electric, biplane tail-sitter with four propulsion nacelles and a central payload ‘pod’ – headed the list. Development started with high-level design, calculations and analyses for which aspects and features of the aircraft were feasible. The team quickly moved into building demonstrators, the first of which were remotely operated systems intended mainly to explore the transition from vertical take-off to wing- borne horizontal flight, and to test how the configuration performed across its operating regime. “From there, we simplified our design towards essentially what we have now – a modular airframe that we call our H-frame – which is cost-effective to manufacture” Wittmaak says. “We then started working on the autonomous features such as its ability to accurately fly waypoints and transition smoothly in take-off and landing. “Once we had developed the basic autonomous mission systems, we delved into sensor development, software algorithms to improve flight performance and stability, and other features. It was an exploratory phase, but it has led us to the form of APT we have now.” At the time of writing, two main versions of the APT were being developed. Both have a largely identical shape and software, including flight control computers, sensor architectures and GCSs, with the main differences coming from scale. The first, and smaller, version, the APT-20, has a wingspan of 1.5 m and a height of 1.0 m, with a MTOW of about 25 kg. This is an important design target, as it makes it compliant with Part 107 regulations on UAV weight limits in the US. It also has a recommended payload capacity of about 8 kg (20 lb, hence its designation). Its optimal cruising speed is 90 knots, and it can fly for up to 29 km on its typical battery capacity if fully loaded with cargo to be dropped off halfway through its mission (as Bell envisions these UAVs being used in the future). Bell’s larger version, the APT-70, weighs up to 136 kg overall with a 31 kg (70 lb) payload limit. The wings measure 2.7 m across and the aircraft sits about 1.8 m tall on the ground. Once in the air, it can fly at up to 110 knots and offers up to 56 km of delivery range, with the same assumptions for it as the APT-20 in terms of the logistics missions it would fly. From the flight tests so far, the APT- 70 has a maximum endurance of 55 minutes if fully loaded with battery packs and without any payload. As another example of its capabilities, with a payload of roughly 16 kg and an additional 16 kg of extra batteries it has a 42-minute flight time, equating to a range of about 80 km. “The APT-20 can be certified across a wide range of airspaces thanks to its MTOW, but we feel the APT-70 is just as important because of how much extra carry capacity it offers,” Wittmaak adds. “As well as delivering more aid per trip, that also means you could install an EO/IR and Lidar sensor package in its pod for quick wide-area survey updates, which is important for disaster relief and defence operations where environments are unsafe and constantly changing.” The H-frame architecture The unique shape of the APT airframe – upper and lower wings, two pylons connecting the wings, four propulsion nacelles and the central fuselage pod – confers several structural, aerodynamic and (internal) environmental advantages. “A primary feature that made the biplane tail-sitter configuration appealing is that biplanes have always had a very good stall margin,” Wittmaak says. “They have a lot of lift capability in a relatively compact cross- section, permitting large payloads and allowing access to tight landing zones. Bell Autonomous Pod Transport UAV | Digest Unmanned Systems Technology | October/November 2020 The ‘boxed biplane’ shape was selected partly for its naturally high stall margin and high lifting capability in a compact form factor