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61 adjusting the throttle, the battery voltage and other key variables across the fuel- powered and electrical elements. “We call this device the ‘governor’, owing to its similar functions to the governor unit of a conventional helicopter,” Fuentes notes. The power from the batteries is distributed across four electric lift motors from MAD Components, which have been selected with torque-to-weight ratio in mind, as the motors and their ESCs needed to be as light as possible while still capable of driving 30-32 in T-Motor carbon fibre propellers. The 32 in propellers in particular are a new addition on the HYBRiX 2.1, to take advantage of the torque from the MAD electric motors to lift more payload weight. To use the heavier props effectively, however, Cortes recognises the need for a high-speed, energy- efficient motor controller to maintain stable flight against the increased inertia and drag of the blades. Quaternium has developed its own firmware to control the ESCs, using field- oriented control (FOC) algorithms to provide precise and fast control relative to typical BLDC controller strategies. The basis of Quaternium’s FOC system is the InstaSPIN product from Texas Instruments, developed for controlling three-phase variable-speed motors. Fuentes says, “This is key for meeting our targets and anticipated standards for flight stability. Our plans for the future include further developing our customisations of the motor and ESC software, to fully optimise for the HYBRiX 2.1’s weight and aerodynamics. “When we started developing our ESCs, very few motor designs were compatible with very few models of FOC controller, so we really needed more flexibility than that. Even now, there still aren’t that many options for FOC motor control; developing this capability in- house makes it so much easier for us to tailor our control strategy to whichever motor we’d like to install.” Airframe body and structure The HYBRiX 2.1 is built from carbon composite as well as aluminium CNC- machined parts. The arms are carbon tubes and aluminium fasteners, and are removable from the structure to ease carrying the craft when it is disassembled. In addition to selecting composites and metals to minimise weight while maintaining mechanical strength, the nature of the aircraft as a hybrid-powered system has influenced many other choices on the hull to reduce the impact of the engine’s weight and vibration on the airframe. These include the location of the engine, the isolation of different structural elements, and the distribution of mass in components. “Even from a blank sheet, the airframe body was drawn up with the knowledge that there would be a gasoline engine and a generator cramped in among all the other multi-rotor subsystems,” Fuentes says. “That means component placements, material lightness, balance and loads [vibratory, structural and power consumption] have been considered at every juncture. “As our UAV isn’t overly large, we’ve worked using mainly flight tests for trial and error in deciding where to place all our subsystems on the HYBRiX hub’s racks. We also have a tripod-based test bench for cycling the UAV and engine simultaneously, and tend to work in orderly procedures from calculations, to designs, prototyping, testing and iterating.” The range extender sits on a separate tray at the back to keep its noise and vibration as isolated from the sensitive avionics as possible, with a system of metallic dampers installed to assist here. The fuel tank is in the centre of the vehicle, to keep the centre of gravity balanced as gasoline is consumed. “There is also a large area under the frame for placing the payload, and the electronic devices are in the front, as far as possible from the engine,” Cortes adds. “We’ve been testing a new EO camera payload with a 30x optical zoom recently, and found no significant impact on image quality when the engine was running versus when it was off.” The batteries are also installed in the front, to balance the weight that the engine places on the rear. Also, the mass and inertia of the batteries is used to provide further protection for the electronics against the vibrations produced by the two-stroke engine. Fuentes adds, “The white canopy shell covering the hub is manufactured using a thin layer of polycarbonate, Unmanned Systems Technology | April/May 2020 The HYBRiX 2.1’s batteries and electronics are installed on the front of the hub’s racks, to keep them as far away as possible from the engine’s heat and vibration