Unmanned Systems Technology 022 | XOcean XO-450 l Radar systems l Space vehicles insight l Small Robot l BMPower FCPS l Prismatic HALE UAV l InterDrone 2018 show report l UpVision l Navigation systems

70 Digest | Prismatic HALE UAV for pitot tubes on both of the systems. This theory will be tested as development of the PHASA-35 proceeds, and if necessary an angle-of-attack vane may be added to help derive airspeed. As Dixon notes, “We expect to undertake a variety of missions, with very different payloads, power configurations and technologies. So we have opted for a design that is insensitive to the variations in payload mass, and power, and which allows us to provide a simple and consistent interface control document to our customers.” Brooks adds, “All high-efficiency aircraft tend to follow this conventional configuration, and we are always surprised when other HALE groups seem determined to force a novel configuration, apparently ‘just to be different’.” The wing is a carbon-skinned foam monocoque structure, separated into five parts to enable low-cost transportation. “As we require a high degree of performance, the construction is tightly controlled with CNC-cut cores and moulded wing sections,” says Dixon. Payload and avionics A tubular carbon fibre composite fuselage connects the wing to the tailplane and fin tail assembly, while payloads are allocated to the nose pod, allowing rapid changes to payloads without affecting the platform. Given the range of anticipated payloads, the pod provides just a power interface, with the company expecting payloads to manage themselves in terms of control and data downlinks. “That very much follows on from our space heritage, where the concept of independent platform and payload operations is standard for communication satellites,” explains Brooks. “We support our customers with the aerodynamics, low mass and thermal control of the payload, but we keep it separate from the flight systems.” The principal payloads the company anticipates hosting are critical technologies such as power systems, which vary significantly according to solar power level, endurance, altitude and other qualities unique to a HALE UAV. For application payloads, Prismatic expects to support area-specific surveillance and comms, where the PHASE-8’s low speeds make it better suited to persistent operations rather than covering large areas and long distances. The slow speeds and simple mechanics of the PHASE-8 has allowed development of automatic flight control to proceed smoothly, although early testing also made use of manual control. The onboard comms can use a direct line-of-sight link or Iridium for BVLOS, but the ultimate aim of the HALE-proving platform is to produce a largely autonomous aircraft with comms geared primarily towards updating navigation waypoints. Solar power For flights capable of lasting multiple days, triple-junction GaAs solar cells are installed on the wings to recharge the battery pack. The cells give efficiency levels of over 30% while providing more than 1500 W per kg. The actual persistence available is defined by the cloud cover and the period of daylight, as the PHASE-8 operates at altitudes below the clouds. Also, the design is intended to enable the aircraft to fly through the night on battery power alone, giving the potential for persistent operations through the summer. The solar cell array is almost always worth flying cost-wise, but in winter Prismatic might use the higher specific energy of primary non-rechargeable batteries augmented by the solar cells to provide a 24-hour capability. As Brooks explains, “As the variation in cloud has a significant impact on the available energy from the solar panels, it is power management, not flight control, that forms the most significant challenge with developing this aircraft. The power required to fly increases with the cube of the flight speed, so we manage speed as part of the power budget to make sure endurance is prioritised over speed.” The exact proportion of solar array used depends largely on the UAV’s location and time of year, but it also has a high cost compared to the rest of the vehicle, so the amount of array is defined by the cost benefit of duration rather than any fixed metric such as wing surface area or UAV weight limits. “For that reason we have made the PHASE-8 modular in terms of its power system,” says Dixon. “We can add extra batteries or solar cells without October/November 2018 | Unmanned Systems Technology The tail assembly holds a rudder and elevator, and is connected to the wings by the tubular carbon fibre fuselage