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

54 assisted by the additive manufacturing of prototype cylinders using T6 aluminium alloy. Since the ports are otherwise cast in, this enabled NWUAV to produce different port geometries much more rapidly for evaluation on the dyno. Development then progressed through evaluation of induction and exhaust configurations, cooling and vibration isolation. Harris explains that NWUAV’s policy is to run new engines under conditions of simulated flight for prolonged periods, seeking out any slight flaw in the design. He notes that in developing the NW-44, “we broke a lot of stuff” as it was pushed to extremes. “For example, we tested it under conditions of a 32 g launch, far beyond what it might see in normal operating conditions. This was necessary to obtain a highly reliable product,” he says. After what is described as “thousands of hours of ground testing” and having been selected by an undisclosed initial customer, the NW-44 made its maiden flight in 2014. By May 2015 the customer in question had completed initial flight tests and had been sufficiently impressed to order a further six examples. NW-44 performance The NW-44 operates through the 3000- 7500 rpm range, with 3000-4000 rpm idle, 4000-6500 rpm the normal cruising band and with higher crankshaft speeds used for climbs. Measured on a propeller dyno, torque and power both steadily build through the operating speed range, peaking at 7500 rpm (we are told that the torque curve is flatter when measured on a water brake dyno). Maximum power is 4.1 bhp at 8150 rpm but when married to an appropriate flight propeller, 3.6 bhp at 7500 rpm is the outcome. Maximum rpm and throttle response are influenced by the choice of propeller. Harris says that in all instances the engine goes from idle to 7500 rpm in “well under a second”. Ignition timing is described by Harris as “in the high 20s [BTDC]” regardless of fuel type. He remarks that, even when running on heavy fuel, the NW-44 has a significant margin before detonation becomes an issue. “You would have to run it hotter than 350 C to start to get detonation,” he says. “At normal operating temperature, even increasing the compression ratio won’t cause detonation.” This insensitivity to detonation appears to be a function of the compact chamber and the work done by NWUAV to develop the engine to run on all fuel types. Interestingly, increasing the compression ratio beyond the specified 9.5:1 doesn’t increase power (for reasons that are not yet fully understood) but does make the engine run hotter and make ignition timing more critical. Fuel consumption is strongly influenced by the choice of propeller and the overall form of the craft into which the NW-44 is fitted. Measured on a propeller dyno using a variety of propellers, brake- specific fuel consumption (bsfc) is in the 0.6-1.0 lb/hp.h range, with cruise bsfc in the 0.65-0.75 lb/hp.h range. In August 2015, NWUAV announced February/March 2016 | Unmanned Systems Technology Dossier | Northwest UAV NW-44 The NW-44 seen from the front – the nose intake is for cooling rather than charging the engine The NW-44 powertrain package, ready to slot into a UAV platform