Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

49 Northwest UAV NW-44 | Dossier Unmanned Systems Technology | February/March 2016 Incorporated into one side of the finned head are a pair of small tubular housings, one for the glow plug, the other for a temperature sensor. Neither item reaches into the combustion chamber. The combustion chamber is surrounded by a subtle recess for the circular copper gasket that seals it to the cylinder. The exhaust is all contained in the curved (more than 180 º ) wide and low-profile ‘muffler’ that wraps tightly around the base of the engine. This silencer is attached to the cylinder’s exhaust port by a pair of bolts, then is supported by the three engine mounting legs formed integrally with the crankcase. It is cast in A356 T6 aluminium alloy. Initially the internal form is made in polystyrene using an EOS additive manufacturing machine, then this is burned out after the emerging unit has been dipped in a ceramic slurry and left to harden. Acoustic damping material is then inserted. Cooling air is collected via a duct running above the engine, its inlet located at the front of the nose shroud, facing the airflow coming over the top of the propeller snout. This duct directs air so that it flows over the cylinder and cylinder head. The rate of airflow is controlled by a flap just behind the inlet, the position of which is controlled by the engine control unit (ECU) according to the temperature reading it receives from the probe in the cylinder head. The main crankcase section forms the three legs, which extend rearwards (behind the rotary valve cover) and outwards, to attach the engine to an aluminium plate located behind it (situated parallel with the craft’s airframe bulkheads). Behind the plate is the electronics puck that includes the ECU, the generator control unit (GCU) and the twin ignition coils. CD ignition is used. Each of the two spark plugs is 10 mm and is fired by its own 25 kV coil. The plugs have secured cap provision with shielding. A ‘micro atomising’ injector is used by the sequential port injection system, which incorporates active pump control. The key outputs from the ECU are throttle butterfly position (via its attendant servomotor), ignition coil and injector operation, fuel pump speed control (direct in all those cases) and cooling intake flap position (via its attendant servomotor). Key inputs to the ECU are crankshaft speed (via a Hall effect sensor in the front of the generator), engine load (confirmation of the butterfly position that it sets) plus readings of operating temperatures and pressures including intake charge temperature, generator temperature and so on. The ECU calculates fuel consumption as it logs 31 parameters, and the data can be sent via telemetry to the ground station in real time. The data is also recorded on a 5 Gbyte memory card on board the craft, and is routinely downloaded at the end of each flight. The data is then used to monitor the ongoing operation of the engine, with the trend monitoring software developed by NWUAV able to calculate updated component life expectancy and identify any parameters of concern. The NW-44’s one-piece billet steel crankshaft has its bearing surfaces heat treated. The generator slides over the nose of the crankshaft, its rotor secured by a single bolt on the central axis. In turn, the propeller is attached by six bolts to the generator. As standard, the NW-44 propulsion system package incorporates NWUAV’s own ground- adjustable variable pitch propeller developed for a low noise signature with high efficiency. The permanent magnet generator is frameless, brushless and is 90% efficient. Its stator weighs 314 g, its rotor 144 g and it will operate at temperatures up to 250 C. The outside diameter of its stator is just 77 mm. It interfaces with a GCU that weighs 400 g while the ECU weighs 175 g. The NW-44 engine core weighs 1800 g while the entire propulsion system, including propeller, generator and electronic control unit as turnkey package weighs 5100 g. It takes only a couple of hours to strip and rebuild the engine core. Key suppliers Head/ cylinder/crankcase: in-house Bore coating: US Chrome Crankshaft: undisclosed Pistons: Wiseco Rings: Wiseco Con rods: in-house Big-end bearings: SKF Main bearings: SKF Spark plugs: NGK Engine management system: in-house Connectors: Lemo Connectors: Glenair Servos: undisclosed Machine tools: DMG Mori Rapid prototyping: EOS/Northwest Rapid Manufacturing Quality control: Zeiss