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

61 of the primary airflow for scavenging. Arranging two in series lifts the overall efficiency to between 95% and 99%, but needs 15-20% of the primary airflow for scavenging. Zero-scavenge tubes are between 87% and 91% efficient. While inertial separators are simple and reliable enough, they can become clogged with debris such as straw, leaves or insects, while the more efficient types that use bleed air are only effective when there is enough bleed air available. Also, fine dust particles are very light and therefore less susceptible to centrifugal force, so a secondary filter is sometimes necessary. When the drawbacks of inertial separators render them unsuitable for an application, barrier filters are the alternative. Filtration media are often fibrous, with some having diameters in the nanometre range. There is a wide variety of filter materials, including cellulose (paper), cotton, glass and a range of synthetic fibres that might be dry, oiled or coated with a dust-retaining adhesive. Nano-fibres form very fine meshes that stop extremely small particles while still allowing air through, and they are very effective at trapping the particles on the filter’s surface to prevent deeper contamination. This kind of technology is usually intended for UAVs operating in the dustiest environments, although electric motive power may ultimately be a better all-round option. Polyester foams with open cell structures are widely used, as they are inexpensive and flame-retardant, and resist water, fuel and detergents, mineral oils and greases, although organic solvents will cause swelling. They are available in grades defined by the number of pores per inch. Simple and low-cost filters tend to be suitable for small UAV engines, but sometimes installations have to be more sophisticated, particularly with UAVs derived from manned aircraft, which inevitably means greater size and weight. The Bell 407 helicopter for example, on which the Northrop Grumman MQ-8C Fire Scout is based, uses an inlet barrier filter in a housing below and behind the rotor. Here, the primary air path passes through the filter, which is mounted vertically in a duct that allows dirt or snow to drop past the filter and out through vents in the cowling. The duct includes a bypass door that opens if the filter becomes clogged, providing an alternative path into the engine. Fuel supplies Fuel pumps are obviously critical components, and while redundancy through duplication is a sound way to improve overall reliability, the extra weight and cost can make that impractical. For small UAVs, pumps manufactured for model aircraft are viable options, but with caveats. While small and light, they generally lack the configuration UAV engine ancillary systems | Focus Unmanned Systems Technology | June/July 2017 A two-stage centrifugal particle separator showing the helical swirl vanes, scavenge airflow supply and central tubes that feed the cleaned air to the engine or a secondary barrier filter (Courtesy of Pall Aerospace) Currawong’s Corvid 29 features a positive-displacement fuel pump, capacitor discharge ignition, an integrated generator and generator drive, and a high-volume custom muffler (Courtesy of Currawong Engineering)