96 Focus | Fuel systems energy stored in a capacitor formed by the probe’s geometry. The fluid forms the insulative dielectric, and the capacitance measured is therefore proportional to the fluid level in the probe. They deliver a reliable and fineresolution data output, which can be communicated over CAN, and in addition to being solid-state sensors and therefore suitable for high-vibration environments, in recent times they have been upgraded to enable their hardware to meet Mil-Std 461F for the control of EMI characteristics in subsystems. That level of standards compliance will become a pressing need for many autonomous aircraft in the future, as engineers integrate more and more EMemitting electronics for various purposes. The growing popularity of hybrid powertrains could add to the risk of EMI inside aircraft hulls as well. Fuel filters and lines Transporting fuel to the engine depends on several key components. Filtering for instance is critical to keeping the fuel clear of sand, dust, metals or cleaning fluids for example. Many fuel filtering systems are now standardised systems, but the range of filter types available run from sock-type pre-filters designed to catch anything over 30-60 microns in size, to inventions such as the Holly HydraMat, a 15-micron system that also serves to collect fuel bunching up at any corner of the tank. Although this latter solution is currently only compatible with gasoline, it ensures that during hard cornering or other instances of highg-forces, the fuel does not slosh about or flow into the edges of the tank and out of reach of the collection nozzle, nor can any air bubbles force their way into the fuel and past the filter. It can thus make collectors and check valves unnecessary to the continued supply of clean fuel to the engine. And once filtered, various industry-standard fuel hoses are available for the UAVmarket. Fuel couplings and fittings provide safe, secure connections of fuel lines as well as overall weight saving and reduction of vibration resonances. Key design points to note in this regard lie in a few areas, one obvious one being careful to ensure couplings are well-matched to the chosen fuel hoses, with an optimised fit and which are grip-tested across different temperature, liquid, shock and vibration conditions. On top of that, stretching the fuel tubing up the inside of the coupling’s hose barb – a cylindrical fluid connector component, typically featuring barb-like rings on the male part that allow an easy push connection of flexible plastic or rubber tubing. It can mitigate the flow restriction point needed by the internal barb fitting, and thus increase the nominal flow diameter. That enables the use of smaller and lighter tubing. Fuel pumps The UAV industry has not always had access to quality fuel pumps. Before 2000, many UAVs built by defence primes would crash because of poor choices of pumps, with few if any being tailored for the specific needs of UAVs. A conventional automotive EFI system uses a gerotor pump submerged in the fuel tank, sending fuel to a rail fittedwith a pressure regulator; that in turnwould have a bypass filter to send excess fuel back to the engine. In such a system, the gerotor would always be activated and pumping, the pressure regulator would always be open due to the quantity of fuel in constant June/July 2023 | Uncrewed Systems Technology A three-cylinder fuel pump developed for high-pressure supply of heavy fuel (Courtesy of Currawong) The UAV industry has not always had access to quality fuel pumps. Before 2000, few if any of themwere tailored for the specific needs of UAVs
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