Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

62 Focus | UAV engine ancillary systems control, quality control planning and traceability that vehicle fleet operators want, and may not cope well if turbulence shakes the pick-up out of the fuel, causing the pump to ingest air. Hard manoeuvring and challenging flight profiles can also cause fuel flow and vapour lock problems, particularly if tanks lack sufficient internal baffles to stop excessive fuel movement. Fuel pumps also potentially need to cope with decentralised fuel tank systems, moving fuel between tanks to keep the centre of gravity where it should be. Most small UAV pumps are not designed to combine a self-priming capability with a high compression ratio. Self-priming – pumping air until fuel flows again without vapour locking – is difficult, according to one supplier, which offers self-priming, single-cylinder, positive-displacement piston pumps for electronic fuel injection and carburettor applications requiring flow rates from 35 to 20 g/minute. Working at a pressure of 4 bar, they require about 2 W to drive them and accept electrical power at 8-20 V, drawing currents of 80-90 mA at 4 bar. They weigh 65 g. UAVs comparable to manned aircraft commonly use pumps built for general aviation aircraft, which are light and reliable and whose extra cost is easier to absorb in larger UAVs. Vehicles requiring engine power between about 25 and 150 hp are harder to cater for with pumps from other sectors of the aerospace industry, however, but there are automotive options. While relatively heavy, these pumps tend to be inexpensive, well supported and reliable within their design parameters, although clearly those don’t include UAV-specific conditions such as g -loadings, altitude and turbulence. Like UAVs, racecars and motorcycles can put severe demands on their fuel pumps, and they even share some important general requirements such as low weight and high performance – otherwise known as specific weight – and reliability, so some manufacturers who have built a reputation in automotive competition are entering the UAV business. If anything, fuel pumps in racing vehicles can be pushed harder because relatively short operational lives are accepted, says one supplier, while running at lower flow rates and/or pressures in UAVs allows them to last longer. While few competition vehicles still use carburettors, many UAV power plants do. The basic options here are for gravity feed from the fuel tank to the carburettor or a pump whose flow rate closely matches the engine’s consumption and runs at low pressure, controlled by a regulator, to ensure it does not force fuel past a closed float valve. Another option for medium-sized UAVs is to use purpose-built pumps designed to work efficiently throughout the vehicle’s operating envelope, and to cope with the altitude, acceleration and vibration environments they will encounter. They are expensive, however, while applications outside the UAV industry that might bring economies of scale are limited. One way to reduce their cost though is to blend ‘near-COTS’ and custom components. A typical example runs on 28 V dc and can be configured for outputs from 275 to 1150 cc/min with a continuous operating pressure of up to 100 psi. This ‘semi-standard’ electronic fuel metering pump is used in a variety of UAVs with a range of fuel system architectures. It can be used in a simple open-loop arrangement with a pressure regulator, or as a servo pump to control flow and pressure in fuel injection systems without return lines. It even controls the speed of one small turbine engine by directly controlling the flow of fuel to the combustor. Fuel injection is becoming more common, primarily because it enables better optimisation across the whole flight envelope instead of forcing designers to optimise for just a part of it. Furthermore, g -forces during launch and operational manoeuvring make life harder for carburettor-based fuel systems with gravity feeds into float bowl reservoirs. The supplier of the pump above is also developing a larger unit that is designed to be configurable for specific applications and offer better suppression of electromagnetic interference. Both pumps use aluminium where possible, although iron-based alloys such as stainless or tool steels are used for more highly stressed and wear-prone components. Both are available as generated June/July 2017 | Unmanned Systems Technology Prototype high-reliability electronic servo gerotor pump for fuels, oil and refrigerants, with its first application as a fuel metering pump in a TDI turbojet engine for a developmental L-3 UAV (Courtesy of Cascon)