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64 use. The people we want to help won’t always have access to skilled engineers for maintaining the aircraft, so we have to do all we can to simplify its operation.” For data connectivity between the UAV and GCS, Bell can integrate (and has flown) a number of different data links, with differing frequencies, power outputs, and other variations between them. Fundamentally, the aircraft is designed to integrate smoothly with any ground station and data links. “Bell has quite a few FCC licences for different spectrums, and we tailor our comms links for different applications and locations as required,” Wittmaak notes. “On some missions, we might find that our usual spectrum is heavily populated, so we switch to a less congested RF band. Fortunately, the radios we use are modular, so you can take out a module and swap in a new one, and it’ll transmit over different frequencies and power settings without having to remove or disassemble the whole radio.” Ultimately, the APT is agnostic regarding which data links can be integrated. In other configurations, satcom or commercial LTE networks are integrated for long-distance comms and control. “The satcom link can leverage low-bandwidth and more affordable subscriptions to get a modicum of comms, in-the-loop control and health monitoring,” Wittmaak says. “Ultimately, the APT can leverage any data link that suits the operating environment, so long as it has good support, optimal connectivity, suitable packaging and power, as well as the appropriate level of encryption, be it AES-256 or some other standard for civil or military use.” Controlling tail-sitters Bell is well-known for having pioneered tiltrotor aircraft since the 1950s, from the original XV-3 demonstrator to the V-22 Osprey and the upcoming V-280 Valor. Although their manner of transition differs from that of tail-sitters, the company’s experience of developing tiltrotors has enabled a close familiarity with the physics of transitioning between vertical lift and wing-borne flight. “With that background, we’ve been able to program the APT to pilot through its transition corridor autonomously and seamlessly; other transitioning UASs often just switch, roughly, from one mode to the other,” Wittmaak observes. Typically, VTOL-transitioning algorithms often make use of airspeed in their calculations. However, measuring airspeed on tail-sitters is a perennial challenge, particularly if their designers hope to operate them in rainy conditions. Conventional pitot static systems can quickly fill with moisture while the tail-sitter is on the ground, ascending vertically or hovering. To that end, Bell’s team has flown with and without airspeed loops in their transition control algorithms, and has found ways to accurately estimate airspeed based on performance data through the APT’s digital thread, successfully calculating the necessary movements for transitioning between cruise and VTOL. Wittmaak adds that the flight control computer and vehicle management system (which includes the autopilot and navigation sensors) are largely internally developed systems, with third parties supplying the IMUs, video processors, back-up GNSS receiver and back-up GNSS antenna. Vectored-thrust modules Each of the four independent propulsion nacelles on the APT is identical from front to back, with a fixed-pitch propeller, electric motor, ESC and battery pack. That homogeneity is a deliberate decision made to reduce the aircraft’s parts count, simplifying maintenance procedures and reducing the associated costs. “For example, if an end-user gets a diagnostics message that one motor is degrading, they just unscrew four fasteners and one connector to remove the nacelle, and they can then swap in a fresh propulsion module in less than 5 minutes,” Wittmaak says. While the final designs of the propulsion units and their components are still being trialled and iterated, a few key innovations have been used to optimise their common manufacturing and operational aspects. For example, and as mentioned, their structures are entirely 3D-printed as monolithic parts. That ensures precise and lightweight construction, and the nylon plastics achieve the necessary ruggedness for absorbing impact loads from the struts during landing, without cracking or tearing. Unusually, the rotor on the front of each October/November 2020 | Unmanned Systems Technology Bell has flown the APT with and without airspeed loops in its VTOL transition, as pitot tubes can fill with rain or other moisture when tail-sitters are pointed nose-up