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65 Bell Autonomous Pod Transport UAV | Digest nacelle is also gimballed, an innovation designed to help with roll moments as well as improving the craft’s landing stability in windy conditions. Accordingly, Bell also refers to the nacelle design as its “vectored-thrust module”. “As tail-sitters hover in winds, they have to lean into the wind in order to hold their position,” Wittmaak says. “However, the APT’s vectored thrust can be used to compensate without changing aircraft attitude. “Also, multi-rotors use differential torque for yaw control, but it isn’t as crisp as is required for windy operations. The APT uses its vectored thrust though to command precise, high-authority yaw commands. “So, when we lean into that wind we can pan the electric motors up or down until we’ve turned the APT so that it’s at the most stable angle necessary for it to touch down upright and stay like that. We have comfortably demonstrated fully autonomous landings in 20-plus mph winds, including one instance that used a cargo pod with its own retractable landing gear.” For principal roll control, the APT is designed with active elevons – two on the end of each nacelle – without any ailerons, so the elevons roll the UAVs as they corner. On take-off, the elevons and their servos also assist with yaw control, providing additional wind compensation in a similar way as when landing. The aircraft is designed to fly at up to 7000 ft (2134 m), with further trials planned to prove-out its expected flight ceiling. “Being an all-electric vehicle, there are none of the problems with aspiration that you might get with a fuel engine at high altitudes,” Wittmaak says. “If anything, it’s cooler at 6000-7000 ft, so the motors, batteries and ESCs perform more efficiently up there. In any case, they’re all cooled sufficiently by the air wash from the propellers in normal operations. “We are especially happy with how 3D printing has enabled this system, because there’s a huge amount of integration going into that nacelle. Each propulsion module has a rear servo, two elevons, two 22 Ah battery modules, a forward servo for the thrust vectoring, a motor, a gimballed motor bucket, an ESC and signal lights. A hybrid structure of carbon and something else would be much heavier and more expensive to manufacture.” The future Bell’s trials of the APT are continuing, with its team keeping their eyes on the best modifications for optimising its performance, cost and useability. Wittmaak anticipates upcoming changes will include environmental sealing and embedded ESCs on the propulsion modules to improve their efficiency, thermal management and overall thrust- to-weight ratio. “We believe we can mitigate the loss of a propulsion unit by how we’ve configured our vectored-thrust modules to compensate, in all modes of flight, to maintain stable control for a practical period of time,” he adds. Further in the future, Bell plans to develop a larger, hybrid-electric version of the APT-20 and APT-70, given that several hundred kilos of batteries would be needed to enable an all-electric craft to carry heavier cargo. The design of that craft will be closely informed by the many lessons Bell has learned from its first generation of APTs. Unmanned Systems Technology | October/November 2020 APT-20 Wingspan: 1.5 m Height: 1.0 m MTOW: 25 kg Payload capacity: 8 kg Cruising speed: 90 knots Range: 29 km APT-70 Wingspan: 2.7 m Height: 1.8 m MTOW: 136 kg Payload capacity: 31 kg Cruising speed: 110 knots Range: 56 km Specifications The APT’s propulsion nacelles are designed with gimballed rotors to help with roll moments and landing stability in wind