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

53 Northwest UAV NW-44 | Dossier programme into minimising propeller noise. Located just behind the propeller is the generator. In theory this could be used as a starter motor but in practice most users are happy to flip the propeller to fire the engine. The generator is a permanent magnet device, the output from which varies with operating speed. It is therefore fed through the GCU, which provides a continuous 28, 12 and 6 V supply (tuneable up or down as required) with a 30 W supply to the engine and a 250 W supply to the craft. Harris notes that to flatline the voltage irrespective of generator speed was challenging in terms of the power electronics in the GCU (which is finned for cooling but does not require a forced air supply). “It was a big challenge to make the regulator light and efficient, but every gramme counts,” he says. He adds, “The NWUAV generator is capable of producing its maximum designed output at what would be considered ‘flight idle’. That way the operator doesn’t need to turn off their payload to descend or change altitude.” The vibration inherently imparted by a single-cylinder two-stroke is a key issue to address for the UAV market, Harris admits. It is tackled at source by crankshaft counterbalancing and also by the way in which the core engine is installed in the airframe. Harris notes that a (radio-controlled aircraft) “hobby grade-derived” UAV two-stroke typically has a crankshaft balance factor of about 30% in an effort to save weight, whereas the NW-44 has one “of the mathematical ideal” of 61%, specifically to reduce the vibration potentially imparted into the craft and thence to its payload. He explains that, since the vibration signature is a function of engine speed, the location of the centre of the moment of inertia through which the engine vibrates changes with rpm. In general, while the engine is running at normal cruising speed, that centre of the moment of inertia is located towards the generator. As such, the three engine mounting legs, which are integrated with the aluminium crankcase, are aligned towards that position as they spread out running rearwards to their airframe pick- up points. That helps to minimise the vibration imparted to each leg’s foot, which in turn is designed to decouple vibration where it attaches to a mounting plate behind the engine. As far as possible, vibration- sensitive electronics are mounted aft of that T6061 aluminium mounting plate. The electronics are housed in a dedicated 3D-printed Nylon 12 ‘puck’ attached to the back of that plate or optionally elsewhere on the craft. For example, each spark plug has its own coil, and these are mounted off the engine core in this remote puck to avoid the effects of vibration. There is no issue of torsional resonance associated with engine operation that calls for crankshaft dampening, reports Harris. He says vibration imparted into the airframe is so low that it does not affect the operation of any payload camera. He adds that the NW-44’s top end was designed for easy maintenance access, given that carbon build-up on the piston crown is the Achilles heel of this type of two-stroke. “Maintainability was key,” he remarks. “We wanted to minimise the logistics footprint – to keep it flying.” NW-44 development Overseen by Harris and NWUAV chief technical officer Jeff Ratcliffe, design of the NW-44 began in the summer of 2012, with the first prototype running on the test bench a few months later. The initial focus was on optimising the cylinder design, especially the combustion chamber and the piston, then that in turn influenced the detail design of the bottom end. Transfer port development was Unmanned Systems Technology | February/March 2016 The secret of our atomisation, which results in tiny droplets, is not in the fuel pressure but in the injector itself. That is a secret we want to keep! Dimensions of the very compact NW-44