Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

66 Dossier | Insitu N20 UAV 50 cc two-stroke single the tank once a given pressure has been obtained. This system has the effect of warming the fuel, which helps overcome the problem of waxing when using kerosene-based fuel at very low temperatures in flight. Fuel delivery pressure is a function of the regulated pressure created by the delivery pump and the fuel regulator, which can be anything up to 6.5 bar, which is referenced (or related to) the pressure of the compressed air supply, which is normally in the region of 4 bar. The compressed air is supplied by a crankshaft-driven air compressor, which uses a piston pump. Matching the reciprocation of the combustion piston, the air compressor piston supplies a given amount of air at any given engine speed. The increase in air supply with increasing rpm is not a linear relationship, given the design of the porting that transfers air from the crankcase into the piston pump working chamber. Moreover, the air supply system has an accumulator effect, such that the compressed air supply is at constant pressure rather than pulsed. On the ignition side, while the N20 has provision for a second spark plug, that is not the initial offering – it is in recognition of the fact that in some instances it will be a requirement of certification. The long-life iridium spark plug is fired by a high-energy, long-duration inductive coil. The coil is directly fired by the ECU, which determines spark timing and dwell, thereby controlling ignition energy. In theory the coil – or pair of coils in the case of a twin-plug arrangement – could be directly mounted on the plug, but in practice it is remotely located for reason of overall craft streamlining. N20 mechanicals The N20 features a single-piece cantilever- mounted crankshaft in a one-piece crankcase (in other words, it is supported only on the drive side of the crankpin). The crankcase has a rear panel for access, which houses the reed valve. The propeller is directly driven by the front of the longitudinally oriented crankshaft (with main bearings between it and the crankpin). The terms ‘front’ and ‘rear’ refer to use in a puller configuration; as noted, in the ScanEagle the N20 is swapped around to work as a pusher. A generator fits onto the front of the crankshaft, just behind the propeller, which is bolted to its rotor. Integral with the crankcase is the housing for the air compressor, which is located between the generator and the power cylinder. An eccentric lobe on the crankshaft drives the piston used for air compression. This is sandwiched by the two main bearings (a roller and a ball bearing). The big-end bearing is of the roller type, and Cathcart recognises that the fact the crankpin is cantilevered “avoids the need for a split crankcase that can be the source of a leak”. The cylinder head is integral with the barrel, which makes plating and honing the bore more challenging. However, the base of the cylinder can then be formed as a flange with four short bolts attaching it to the top of the crankcase. The combined head and barrel thus saves the weight of head bolts as well as of a head sealing system, and where it attaches via its flange to the crankcase is a low-pressure seal, so an O-ring suffices. The only gasket in the engine is that of the exhaust, which is pressed metal stainless steel with a two-bolt fixing. The single crankshaft web has a heavy metal plug for reason of counterbalance; the balance factor is undisclosed. Cathcart notes that the air compressor lobe incorporated into the crankshaft and the generator’s rotor contribute to engine balance. “The air compressor and the rotor help minimise the vibration couple,” he says. The con rod is of tapered section since that type is considered to minimise weight while maximising stiffness and durability. The rod is hardened steel, forged and machined, and the big-end bearing runs directly in it, as does a needle roller bearing in the small end. Enhanced lubrication for the big or small end is not considered necessary, as both are served by general oil mist in the crankcase. The piston pin is hardened steel, machined from solid and through- drilled with tapered ends to the drilling. Uncoated, the pin runs directly in the small-end bearing and in the piston. The pin is retained by steel wire C-clips, which are a sufficiently tight fit in the respective pin bore groove to stop rotation of them, although the pin itself rotates. The piston is described as “die cast from a high silicon-aluminium alloy”, the actual percentage of silicon being June/July 2016 | Unmanned Systems Technology ScanEagle launch – field testing the N20