Unmanned Systems Technology 019 | Navya Autonom Cab | Batteries | UGVs Insight | UAV Factory UAV28-EFI | Swiss Aerobotics Hummel | UMEX 2018 report | Antennas | Oceanology International 2018 report

64 appropriate action, avoiding engine failure. The operator, who wouldn’t have had time to react appropriately, would then just get a notification that this condition has occurred and could opt to abort the mission.” Data sent to the ground station includes fuel level as read by an optical sensor in the tank. Fuel consumption is measured using a combination of that sensor reading and calculation of injector opening time. The software thus provides the operator with very precise fuel flow as well as the amount of fuel available on board in real time, allowing them to optimise the endurance of the system during the flight. “Also, it is flight-critical to know the weight of fuel remaining, to avoid the danger of stalling the aircraft,” observes Popiks. High-temperature operation Another major step came with development of the power unit to run at temperatures in excess of 50 C, as required by customers in the Middle East. The power unit already had a bespoke carbon fibre top-end cooling duct that is open to the oncoming airflow and exits very close to the propeller sweep, which means that rotation of the propeller helps draw air through it. This is particularly important before launch, when the engine is running but the aircraft is stationary. There is an adjustable carbon fibre flap in the duct, on the exit side; a servo actuator moves the flap from fully open to fully closed as a means of influencing cylinder head temperature. In turn, the servo is controlled by the ECU in response to readings from a cylinder head temperature sensor. To obtain satisfactory cylinder head temperature when running in 50-plus C ambient temperatures, UAV Factory had to revise the design of both the duct and the cylinder head finning. “In a pusher configuration the flow of air around the engine is poor compared to a puller configuration,” remarks Popiks. “We discovered that unless we designed and produced our own head we could not cool the engine sufficiently in very high ambient temperature environments – 50 to 55 C. Now with our own head, with a significantly increased cooling surface area, this problem is completely resolved and we can actually do the pre-flight checks on the ground at that ambient temperature without the assistance of external fans.” Prior to this ‘high temperature’ development programme, fuel was drawn out of the tank (through a filter) by an electric fuel pump of the diaphragm type (in other words, an electrically driven rotor drives an eccentric arm that pushes against a flexible membrane). The pump supplied fuel to a bellows-type accumulator that dampened fluctuations and was effectively the non-return fuel rail, having travelled via a regulator that sets the delivery pressure at 2.5 bar. With high-temperature operation, the diaphragm pump proved susceptible to vapour lock, so the fuel delivery system had to be revised. “Integrating a new type of pump was a complicated project,” reports Popiks. The solution was found in an electrically driven impeller pump that runs at constant rpm. That in turn feeds a mechanical fuel pressure regulator that automatically ensures that the supply to the fuel rail is at 2.5 bar, sending excess fuel back to the tank. Generator development The UAV28-EFI includes a fully integrated generator system. The most recent development has been the introduction of a brand new generator, replacing the original 100 W item. April/May 2018 | Unmanned Systems Technology Development of this new cylinder head was fundamental to operation at temperatures in excess of 50 C Since our main concern is MTBF, anything that increases complexity is unwelcome. Any complication will fail at some time