Unmanned Systems Technology 011 | C-Astral Bramor ppX | IMUs | Autonomous farming | UAV Turbines UTP50R | London Show report | Advanced materials | Las Vegas Expo report

25 C-Astral Bramor ppX | Dossier electronic flight control system, the eventual solution emerging from a combination of aerodynamic refinements and active control. The result is a UAV with a very wide static margin and a gentle stall. So benign is the handling, Trost says, it can be recovered from an incipient spin in just half a turn when flown manually. 
 Composite structure The fully composite structure of the ppX uses only carbon- and Kevlar-reinforced plastic, in this case the matrix being certified general aviation-grade epoxy resin, to form skins separated and stiffened by a non-metallic honeycomb. Most of the airframe is carbon fibre- reinforced plastic (CFRP), with Kevlar used in areas that have to allow RF signals to pass to and from the aircraft’s antennas. The winglets are made from Kevlar for this reason and for durability, and weigh just 20 g each. Composite materials come from undisclosed suppliers in Germany and Italy. While there is no structural metal, aluminium and copper are used in applications such as small moving parts and electrical connectors and cables, for RF shielding and in the electronics. Manufacturing involves a combination of CAD-CAM and vacuum moulding of structural elements and advanced laboratory-grade measuring equipment to build and test the electronics, and provide quality control. Propulsion and power Trost describes electric propulsion technology as disruptive because of its high rate of improvement, pointing to ongoing work by manufacturers of motors for remote controlled aircraft to increase efficiency and durability while also reducing weight. “If you look at the other elements, like batteries, they are improving all the time,” he says. “Their capacity is twice as high as in 2006, and developers of the lithium- sulphur batteries that will be available in several years’ time are promising energy densities of up to 500 Wh/kg.” Energy densities in the best lithium battery packs at the moment are about 250-260 Wh/kg. “From that perspective, that’s why the technology is disruptive – it’s changing and it will keep changing,” Trost says. The electric motor is a brushless type made specifically for the ppX by an undisclosed supplier from the EU, and is rated at about 1 kW. It drives a folding propeller made from CFRP by Aeronaut. Power for the propulsion, control systems and the sensors comes from a main battery, while a second battery is optionally fitted as an independent failsafe to deploy the parachute in case for example the main power supply fails (a requirement set by some civil aviation authorities). Solar power represents a particularly disruptive element of electrical propulsion, and in November C-Astral announced its choice of Alta Devices to provide the gallium arsenide (GaAs) based solar panels for the Long Range Solar (LRS) variant of the ppX. (For more details of these see our report on the Commercial UAV Expo 2016 on page 76.) “We are in the testing phase, and the results are really promising,” Trost says. “This technology will enable us to fly for about two hours more, which is quite something when you look at the aircraft’s size.” Integrated into the wings, the panels are connected to a solar controller, via which they charge the batteries and bring the UAV’s endurance up to about 5.5 hours while adding less than 2 oz in weight. The solid-state power electronics are C-Astral’s own development. GaAs, a semiconductor, makes a good solar cell material because its band gap of 1.43 eV is in the range that allows it to achieve the maximum possible efficiency for a solar cell with a single p-n junction under natural sunlight (not fed through a concentrator). The maximum theoretical efficiency for a single-junction solar cell is known as the Shockley Queisser limit, and is about 34%. Alta Devices claims an efficiency of 28.8% for a single-junction cell and 31.6% for a dual-junction cell with its third- and fourth- generation products. Their efficiency is improved by what Alta Devices calls ‘photon recycling’, in which photons bounce around within the cell, providing a greater chance that they will be absorbed. The company also claims its Unmanned Systems Technology | December/January 2017 Sensor options include RGB, NIR and CIR cameras as well as multi-spectral sensors and a hyperspectral imager