Unmanned Systems Technology 020 | Alpha 800 I Additive Manufacturing focus I USVs insight I Pegasus GE70 I GuardBot I AUVSI Xponential 2018 show report I Solar Power focus I CUAV Expo Europe 2018 show report

24 Dossier | Alpha Unmanned Systems Alpha 800 NACA profile and provide a lift-to-drag ratio of 14:1 in a hover. Escarpenter stresses that Alpha is very conservative in its performance claims to allow for the effect of variations in environmental conditions. Under standard conditions such as 15 C at sea level, for example, it is possible to lift heavier payloads, but as soon as those conditions change the performance is reduced. “For instance, the typical winter temperature in Madrid is 5 C, but in summer it can be 35 C or more,” he says. “That means a 10% reduction in air density and thus the same in the maximum lift capacity. In our case that means a reduction of 1.4 kg of maximum lift.” The rotor head itself is also very simple, he says. The major movement is the change in pitch as the blade rotates about the spindle under the control of the pitch links, with the flapping and lead-lag motions accommodated by the elastomeric dampers in the blade grips. “Also, the autopilot corrects part of the effect of the asymmetric lift generated in forward flight using a trim gain. That reduces the angle of attack of the half of the disc that is producing greater lift, and its effect is proportional to the indicated air speed – the higher that is, the larger the correction made by the autopilot.” An important design consideration with any helicopter is the angle between the rotor mast and the aircraft’s long axis. A forward tilt of the mast, and the thrust vector as a result, will tend to bring the fuselage to the horizontal in forward flight at cruising speed, which is more aerodynamically efficient, and a nose-up attitude in a hover and at low speed. A vertically positioned mast, on the other hand, will result in a level fuselage in a hover and at low speeds but a nose- down attitude in cruise mode. Optimised for hovering, the Alpha 800 has a vertical mast, a choice driven by the judgement that many customers’ machines will spend most of their time in this flight regime. For example, a typical mission might involve a half-hour transit to a target 25 km away, 90 minutes on station in a hover or at low speed, and a half-hour return leg. In some applications, such as precision agriculture with infrared payloads, Escarpenter points out that the camera has an aperture time that limits flight speeds to 10 kph. “The same happens with Lidar – the higher the speed the lower the density of the point cloud – thus speeds are kept low in most operational scenarios.” A level fuselage attitude is also important for landing on moving platforms such as boats, a capability that the company is putting significant effort into developing, he adds. The Alpha 800 hovers at 0° in both pitch and roll. That would come naturally in an aircraft with a tail rotor thrust line positioned at the same height as its centre of mass. If however the tail rotor thrust line is higher, it will induce a slight roll angle that would either be tolerated or corrected by the autopilot. Rotor servos The Futaba servos that change the pitch of the main rotor blades, both collectively and cyclically, are connected to the non- rotating half of the swash plate by ball- June/July 2018 | Unmanned Systems Technology Commercial rotor components include the spindle, hub, blade grips, pitch change bearings and dampers (Courtesy of Alpha) The tail rotor is mounted on an aluminium boom and protected from ground strikes by a carbon fibre-reinforced plastic skid (Courtesy of Alpha)