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

66 harsh operating conditions, with UAVs naturally demanding more in terms of altitude performance. One supplier offers tanks that meet requirements including resistance to various fuels and additives, collapsibility for ease of installation, puncture resistance and weight savings. The tanks are made from rubberised materials reinforced with proprietary woven ballistic fabrics using generic polymer fibres such as nylon or proprietary fibres such as Kevlar, Zylon and Vectran. Fuel compatibility is the first priority when choosing components for a fuel system, as fuels have different chemical compositions, which affect the materials that can safely be used for the tank, lines, pumps, seals and connectors. Saving weight is a key motivation for using collapsible tanks, so they dominate UAV applications. They tolerate being squeezed and manipulated during installation and removal, minimising the need for dismantling, but generally have shorter service lives, and each flexible material tolerates a narrower range of fuels than their rigid counterparts. Fuel tanks have internal components including baffles to reduce sloshing, collectors that cause fuel to gather around a pick-up point for a hose, and drainable sumps to collect water, all of which can be integrated and made from the same material as the tank. In flexible tanks, for example, baffles are often attached using bi-axial or tri-axial fabrics, which are made from two or three sets of yarns respectively that cross and interlace at various angles. Other components include internal hangars, loops and other mounts to secure fuel lines, filters and pumps. Rigid polymer tanks require complex tools and patterns for their manufacture, but the result is seamless containers that fix complex spaces, tolerate all fuels and cost less in larger numbers. One supplier puts the cut-off point at 100 units. Altitude limits on fuel tanks are mostly determined by the materials’ temperature tolerance and their ability to cope with air pressure changes through venting. Some bladder tanks can operate safely up to 60,000 ft and withstand temperatures between +95 and -50 C. Electrical power Engines rely on electrical power for fuel pumps, ignition systems and engine control units (ECUs), and must recharge the batteries that power control systems and payloads, so they need dc generators or ac alternators, some of which double as starters. For example, one dc generator is described by its supplier as “an inside- out alternator”, which was developed in the 1990s as the first practical generator for model aircraft. These days it is also offered for small UAVs that need between 5 and 20 W of power. The most basic standard version of it is smaller still, putting out 800 mA at 4.8 V, which works out at 3.84 W. That is enough to run a radio and servos while charging the battery and enabling it to run some 4.8 V accessories as well. Power output can be doubled or tripled by adding one or two more coils, while its central control unit can be uprated to 13.5 V dc to power other payloads that require a nominal 12 V. Further customisation with more coils and larger magnet rings that provide the moving magnetic field that generates current in the coils can take outputs up to 20 W. Direct drive eliminates the need for a drivetrain and involves mounting the generator on a propeller shaft or rear output shaft, improving weight distribution and minimising the size of the physical envelope. If space constraints preclude direct drive, the generator can be mounted on a secondary shaft and driven by belts or gears. Rare earth magnets ensure high power density, while the only moving part is the rotor and there are no sliding contacts such as brushes or slip rings to wear. The alternators’ ac output varies in amplitude and frequency with rpm and their power is proportional to rpm, so they must be used with regulator/rectifier units to convert the output to dc of the correct voltage. Many of these alternators can reverse their function to work as starter motors, so long as they can generate enough torque to turn the engine over and the June/July 2017 | Unmanned Systems Technology Flexible UAV fuel cells can be made in complex shapes, and collapse for ease of installation and removal (Courtesy of Aero Tec Laboratories)