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

46 Dossier | Northwest UAV NW-44 crankshaft with twin, often full-circle webs that sandwich the crankpin. Ideally the charge is then fed into the middle of the crankcase between the webs (the intake port consequently running perpendicular to the crankshaft axis). That is the case with many reed valve and some rotary valve two-strokes. A reed valve responds to a pressure differential either side of it, opening and closing the intake port passageway accordingly, whereas a rotary valve is a spinning disc with a segment cut out to open and close the port at specific crank angles. This disc is driven directly by the crankshaft, consequently the rotary valve controls the intake timing more positively than a reed valve, albeit without any scope to alter timing with increasing rpm. In applications with a lot of transient operation, such as motorcycle road racing, reed valves are often preferred. NWUAV did not have that performance consideration though, and found that the more predictable operation of a rotary valve provided enhanced fuel efficiency for its UAV-specific design. A rotary valve is sometimes located such that the charge runs through it into the middle of the crankcase, but this implies that it is spinning on an axis perpendicular to the crankshaft axis. That in turn means that a drive has to be taken from one end or the other of the crankshaft, ‘around the corner’ to the valve’s axle. Some two-strokes use a straightforward belt drive for this, but even that solution represents extra complexity and weight that NWUAV wished to avoid. In the case of the NW-44, driving a rotary valve spinning on an axis perpendicular to the longitudinal crank axis would also spoil the packaging of the engine, as the induction system would then need to protrude out of the side, right where the curved silencer actually runs and possibly out into the airstream depending on the configuration of the craft. The induction system is instead set at the rear of the engine, where the air filter is submerged in the overall power plant package, which in turn is buried in the fuselage. The filter then feeds forward through a throttle body and on via a rotary valve spinning on the crankshaft axis. The concept of a ram intake snorkel is eschewed in favour of this, keeping the charge air protected from any airborne debris, dust, water or ice and as clean as possible. Air finds its way into the filter housing with no dedicated ducting but this does not, Harris assures us, cause excessive charge air temperature or a lack of charge density. It isn’t unusual to have a rotary valve-controlled inlet port feed into the crankcase against the flank of the web on its side, but this is where the NW-44 is very different: it has only one crank web. The main shaft driving the propeller runs in two main bearings ahead of the single web which, with the crankpin projecting behind, is thus cantilevered. The rotary valve disc then spins where the second web would otherwise be situated, driven directly by a peg on the end of the crankpin. Thus, with no web to interrupt its path, the charge feeds directly into the crankcase volume within which the big end circulates. This simple but very effective cantilevered crankshaft arrangement makes for a very compact bottom end. It is also in the interest of maintenance – the crankshaft is February/March 2016 | Unmanned Systems Technology The core engine, shown here shrouded with the electronics package exposed behind it With no web to interrupt its path, the charge air feeds directly into the crankcase volume within which the big end circulates