Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

68 the lower halves – with straight-line slots running between the centre and outer parts of the case for the cross-members to run in. Mounted on each cross- member are four ball bearings, with the outermost being around half the diameter of the inner bearings. The drive cams and pistons are oriented so that when the piston head fires downwards (towards the centre of the engine), the larger ball bearings on the cross-members push down against the two separate cams in their respective engine chambers. Each bearing rolls against its cam’s downward-sloping surface, to push them apart. From above, this effect looks rather like scissor blades being opened by a round metal object pushing into them and forcing them apart. By this method, rotational momentum is given to the drive cams, the linear motion of the pistons being converted into rotational motion for the output shafts and propellers. After pushing apart the downward- sloping cam surfaces and reaching the cams’ bottom dead centres (BDCs), the piston is then driven back up into its cylinder for the exhaust stroke, by the immediately following upward slopes of the cams, which carry forward their rotational momentum and push up against the large bearings. To ensure the piston is carried up steadily, an additional, downward- facing cam surface is designed along the opposite inner rim of each driving cam. This effectively creates a raceway between the cam lobes and the rim, and helps the overall efficiency of the system by ensuring that no kinetic energy is ‘lost’ as the pistons and cams push each other up and down. The smaller bearings fixed to the outer part of the piston rods’ cross-members run along and against this upper rim. The reason for that is to prevent the need for the larger bearings to run within this raceway – doing so would mean the upper and lower tracks of the raceway acting on the bearing in opposite directions. That could cause the large bearing to spin in both directions and generate unnecessary friction, meaning more oil would be needed and negating the reason for having rolling bearings in the first place. Once the cross-member bearings reach the top dead centre (TDC) of the next lobe (for the cylinder’s next intake stroke), the next cylinder along fires and pushes the cams, with the upper track of the cam raceway then pulling the first cylinder’s piston down to draw in the fuel-air mixture. Typically, in the NorEaster (with the four-lobe drive cam), four of the eight pistons stroke downwards at once. One opposing pair of cylinders will be firing, while the other pair will be taking in fuel and air. As this happens, the other four pistons are being pushed upwards for compression and exhaust. Once the first four pistons’ cross- members reach the BDC of their respective cam lobes, and the second four have reached TDC, they alternate their movement, with the first four now stroking upwards as the second four stroke down. By having two horizontally opposing cylinders firing on each stroke, the directional force of each cylinder’s combustion is cancelled out, reducing engine vibration. The firing order of the cylinders follows clockwise around the engine when viewed from above, with an even firing time between them. This means that, initially, the first and fifth cylinders will fire, followed by the second and sixth cylinders, and so on. It follows from this that each horizontally opposed pair of lobes corresponds to similar halves of the power cycle. Going around the cam, the first and third lobes’ surfaces contact the piston bearings during their exhaust and intake strokes, while the second and fourth lobes correspond to the compression and combustion phases. All these contacting surfaces and bearings mean oiling is crucial. “In its current design iteration the oil pump is external, but we hope for the production model to integrate the pump inside the cam case,” O’Neill comments. “Right now, the cam case itself is also the oil pan, so oil is pumped up at the top of the driving cams, with gravity and centrifugal motion providing for the August/September 2019 | Unmanned Systems Technology Dossier | O’Neill Power Systems NorEaster Exploded view of the NorEaster engine