Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

28 “By doing that the Tron can withstand high crosswinds, holding steady against the weather-cocking tendency of the fuselage and tail, something that is impossible with tail-sitter configurations like the ones recently shown by Google and others. Precise landings are a key capability for fully automatic missions.” Structurally, the Tron is an all- composite construction with the airframe made from thermosetting resin reinforced primarily with carbon fibre, but with glass and Kevlar fibres where required for radio transparency or extra toughness. In some areas the composite skins are separated by lightweight core materials to form sandwich structures. The most highly stressed components are autoclave-cured at 60 C, and Quantum is looking at investing further in autoclave- based manufacturing for next year to save more weight, Seibel says. The wing is a sandwich structure with two carbon fibre main spars, and is connected to the load-bearing ‘hull’ – the central bulbous portion of the fuselage – with a carbon fibre joiner, while the thin tail boom is also a spar-and-sandwich structure. Snap-together ‘automatic’ connection of the parts enables assembly in a matter of minutes. Subsystems are secured to the airframe structure via bonded-in metal inserts, into which they are screwed and glued with Loctite thread- locking compound, although it is a formulation that allows easy exchange of components on a modular basis. The motors, for example, are screwed to the motor spars in this way. With UAVs for professional use, and those for which certification is sought, design for redundancy is an issue that always arises, and one to which Seibel and his colleagues have given much thought. “For me the question is where do you start and where do you end?” he says. Quantum’s philosophy is to incorporate redundancy where it makes sense. “On a UAV with a minimum take-off weight of 14 kg it is impossible to make everything redundant – if you did that you would have eight motors, not four. “Every system that has four motors will crash if you lose one motor in the hover. So if you accept that while you are hovering or in a transition it is not fully redundant then that makes things a lot easier.” Seibel says that in wing-borne flight a lot of redundancy can be achieved with relatively little effort. The single aileron on each wing is enough to provide adequate control in roll so long as its opposite number does not fail at an extreme angle of deflection. The same applies to the ruddervators on the V-tail, each of which is moved by a single servo. In any case, none of the control surfaces spend very much time at extreme deflection angles during cruise flight because the autopilot uses constant, rapid and tiny corrections to keep the aircraft stable. That means it is very unlikely that any of the servos will fail at an extreme angle, with most failures simply involving a control surface stuck at a neutral angle. Because the two battery packs are separate but cross-connected, if one fails then the Tron can still fly safely on power from the other. Three IMUs are used so that two good ones can out-vote a bad one and ignore its signals if it disagrees with theirs. The much smaller risk that two ‘bad’ IMUs might gang up on the ‘good’ one and cause a crash is simply accepted. Weight issue Seibel admits that the Tron is larger than would be ideal for precision farming. Quantum wanted to build a 5 kg vehicle – the regulations governing which are relatively light – but the principal of one of the early partners that sponsored its development wanted a UAV with a payload of 2-3 kg, which dictated the higher maximum take-off weight. As he was providing the money he won the argument. Seibel emphasises that complexity, cost and risk rise in a non-linear manner with weight, something Quantum learnt with the development of an earlier prototype vehicle, the 25 kg VRT (Vertical), which flew and transitioned successfully but has been abandoned for technical and monetary as well as safety- related reasons. With UAVs weighing 5, 8 or 10 kg there is a good choice of off-the-shelf key components such as servos, motors and electronic speed controllers (ESCs), but June/July 2017 | Unmanned Systems Technology Dossier | Quantum Tron Precision hovering capability is essential to a fully autonomous hybrid UAV, and transitioning to and from the hover was the area that absorbed most of Quantum’s development effort