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

45 aspirated or forced induction. There are other alternatives to those options but embracing unproven alternative technology would not have ensured a key criteria, which was dependable power. Along with reliability, other key criteria were engine weight, size and fuel efficiency. Harris notes that, from the perspective of power plant requirement, the 40-80 lb platform range is a wide one, and that at 40 lb “every last gramme counts”. Also, he notes that the smaller and lighter the craft, the more significant is the power unit in terms of its impact on the overall form factor, particularly aerodynamic drag. That in turn impacts on the UAV’s fuel consumption and hence its range. Overall, NWUAV considered what it terms the “net efficiency” of the NW-44 when installed in a craft. NWUAV reasoned that a naturally aspirated, spark-ignited two-stroke could offer adequate performance from an air-cooled single-cylinder providing a displacement of 44.2 cc. This option offered simplicity and small overall package size, the former promising reliability and both characteristics keeping weight low. Further, conceptual studies suggested that this option provided the best possible power- to-weight ratio together with the best possible fuel efficiency. While unnecessary given the displacement requirement, in theory the alternative of a horizontally opposed twin would provide a ‘get you home’ possibility in the event that one cylinder failed. In practice, the scenario of one cylinder failing but not the other is considered by Harris to be most unlikely, certainly not enough of a concern to justify the implicit additional complexity, size and weight in a craft of this size range. “We tested a twin and found that it quit running when we were simulating a failed/fouled plug at altitude,” he adds. “The normal perception with a twin is that you have increased reliability and redundancy, when really you have only added weight and fuel consumption.” In addition, Harris considers that it is extremely difficult to obtain equal combustion from both cylinders of a twin when cruising at part-throttle. He likewise considers the complication of turbo- supercharging unnecessary given the power requirement. NWUAV’s naturally aspirated single was a simple yet very effective solution given the overarching requirements of dependability, weight, size and fuel efficiency. NW-44 design The NW-44 was designed to be mounted at the front or rear of the airframe with its crankshaft axis aligned with the main fuselage axis, directly driving a propeller or gearbox. Alternatively, a pair could be used, one on each wing. Its air-cooled cylinder is mounted upright with the exhaust port exiting to one side, a wide but low-profile silencer then looping more than 180 º around the base of the unit to the other side. When the engine is installed, this ‘muffler’ is normally left exposed by an engine shroud formed in the same curve – the outside of the ‘conformal’ silencer becomes an integral part of the craft’s streamlining. The exhaust flow runs to the far end of the curved unit and then turns back, to exit next to the exhaust port. Thus the exhaust path is actually twice as long as it appears to be. Interestingly, NWUAV has looked at the concept of an IC-electric hybrid UAV from the point of view of stealth. That study revealed that there is no significant reduction in noise when running on electric power alone; its silencer is able to keep IC engine noise well below that caused by propeller operation. That in turn is a function of the silencer’s internal design and the packing material (which is undisclosed). At the same time, Harris says, the silencer doesn’t significantly compromise engine output. Since the NW-44 is a two-stroke, induction is into the crankcase, with piston-controlled internal transfer ports then sending the charge up into the combustion chamber. The conventional single-cylinder two-stroke has a Northwest UAV NW-44 | Dossier Unmanned Systems Technology | February/March 2016 Normally a twin is perceived as having increased reliability and redundancy, when really you have only added weight and fuel consumption Components of the NW-44 powertrain system excluding the propeller. Note the three-pronged engine mount towards the rear