Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

24 Dossier | Alti Transition controllers immediately take over. “We have designed the airframe with the motors tilting slightly forward, so as soon as they take over the airframe actually flares nose-high, which helps with slowing it down. Then the PIDs and the position control in the hover mode further flare the aircraft and it comes to a stop fairly quickly. The transition from fixed-wing flight to hover flight is smooth and quick,” he says. Weight and payload fraction are always at a premium in VTOL aircraft, and hybrids up the ante even further in this area. De Villiers therefore needed an airframe that was structurally and aerodynamically efficient, as well as one that was aesthetically pleasing. Airframe development Following a market study that revealed that no such airframe existed, the South African company turned to Australian advanced composites house Carbonix, working with it from mid-2015 to develop a bespoke design and building the first prototype towards the end of the year. Meanwhile, Alti also built and has been flying a sub-scale platform to test avionics and software at its base in Western Cape. The airframe is divided into four main sections: the main fuselage and wing roots, the outer wings and the twin-boom empennage with two vertical stabilisers joined by a single, high-mounted horizontal stabiliser and elevator. The sections are designed for quick assembly without tools, using a combination of standard screws and custom finger fasteners. The design ensures that parts only fit together one way, and intuitively. Experiments with magnets for some non-critical attachments were also carried out. Held on by screws, the three undercarriage legs can also be removed for packing into a transport case and to ease replacement if they are damaged. As all take-offs and landings are carried out vertically, no wheels are fitted. The wing has a high aspect ratio and is made from epoxy resin reinforced in most places with carbon fibres and in others with aramid (Kevlar) fibres. It is a closely coupled high wing with the bulk of the fuselage underneath and featuring a combination of dihedral, washout and winglets. Most of the airframe consists of a very thin carbon fibre shell, with Kevlar used for control surfaces, hinges and other flexible parts, and foam spars in the wings, more of which later. Aeroelastic engineering The wing design is an example of very sophisticated aeroelastic engineering, in which the flying, loaded shape was considered from the outset. “The blended root and washout have the effect of carefully defining span-wise lift distribution,” de Villiers explains. “The section and attitude of the inboard wing takes into account boundary-layer effects and flow management to the pusher propeller. Dihedral helps with stability, and washout prevents dropping a wing in case of a stall.” The winglets are in part a response to the wingspan constraint imposed on the original design, but come with further benefits. “They allow some energy June/July 2016 | Unmanned Systems Technology Alti’s owner Duran de Villiers poses with a Transition prototype, highlighting the high aspect ratio and winglets that contribute to the all-composite UAV’s endurance