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

52 straightforward, with two side and one rear – “Simple but effective,” observes Harris. The transfer ports are cast into the linerless aluminium cylinder, and although difficult to access, they do not need to be further worked by hand or machine, such is the integrity and accuracy of the casting process. The NW-44 avoids the complexity of a variable geometry exhaust port. This is a feature of many small-displacement two-strokes optimised for applications requiring good performance under transient conditions, such as motorcycle road racing. Harris points out that in the aviation world, consistent smooth throttle running is the primary requirement, and as such NWUAV has found no net benefit from this technology. The exhaust design is inevitably a compromise between performance and noise suppression. At the same time, Harris says the usual two-stroke approach of using exhaust pressure wave reflection to assist cylinder charging in pursuit of outright power is inappropriate, as it leads to overly peaky performance. For the same reason, the intake system isn’t tuned in that manner. “Our mindset is 180º from that of someone developing a two-stroke for motorcycle road racing,” he says. Harris acknowledges that a larger exhaust would release more power but, he says, the implicit increase in engine package size would impact on the overall aerodynamic drag of the craft, to the extent that there would be a net loss of fuel efficiency. He notes that in a UAV application, something like 90% of the running time is spent in cruise mode rather than at wide-open throttle. Moreover, the compact design of the silencer helps to integrate the propulsion unit with the rest of the craft. On the electronics front it is significant that the ignition, injection and engine management systems are all in-house productions, as is the generator and the generator control unit (GCU), the wiring loom and many of the sensors. The loom uses Mil Spec connectors, is guarded against electrical interference and is described by Harris as “very reliable yet very light – it is highly sophisticated”. Direct fuel injection was ruled out on a number of grounds. It requires a much higher supply pressure, implying extra weight and complexity, and with greater danger of fuel leakage. Also, there would still need to be lubricant introduced with the charge air to avoid the use of a more complex oiling system. In addition, having an injector in the head can potentially spoil the geometry of the overall unit in packaging terms. For the same reason, NWUAV developed low- profile spark plug caps. Fuel is supplied from the tank to the single solenoid-type injector by a miniature electric pump, whose speed of operation is controlled by the ECU to maintain the required 3 bar delivery pressure. This is backed up by a pressure regulator that sends any excess fuel back to the tank. Fuel pressure is constantly monitored, and the ‘active pump control’ operates on a closed-loop basis, with the regulator only returning fuel in the event of a fault. “This save precious electrical power for payloads, comms and avionics,” Harris notes. He adds, “The secret of our atomisation – which results in tiny droplets, even of heavy fuel – is not in fuel pressure, which is only 3 bar, but is within the injector itself. That is a secret we want to keep!” The ECU has two boards (a mother and a daughter board) and boasts a self-diagnosis capability. Via the daughter board, it interfaces directly with state- of-the-art autopilots such as Cloud Cap Technology’s Piccolo. Harris notes that it isn’t straightforward to make any such control unit interface directly with an autopilot but that “integration such as that is a speciality of ours”. The NW-44 package includes NWUAV’s own propeller, and the company has an ongoing research February/March 2016 | Unmanned Systems Technology The NW-44 seen from the rear, where the air intake feeds into the rotary valve housing