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

64 June/July 2016 | Unmanned Systems Technology Multi-fuel, spark-ignited two-stroke Single cylinder 42.6 x 35.0 mm = 49.89 cc Naturally aspirated Gasoline, Avgas or kerosene-based fuel (Jet A, Jet A1, JP5, JP8 and so on) Aluminium structure Linerless, nickel silicon carbide- coated bore Two main bearings, roller and deep groove ball Steel crankshaft, one pin Steel con rod Reed valve Light alloy piston; single ring Electronic ignition, single (or twin) plug Electronic, compressed-air assisted direct injection Engine management system 10.7:1 compression ratio Maximum rpm, 6500 The N20 is an air-cooled single cylinder two-stroke. Its cylinder with integral head is die cast aluminium and is attached to the crankcase via four bolts. The crankcase is machined from billet aluminium, as is its endplate. Three fasteners attach the endplate to the main unit with an O-ring seal. The endplate carries the billet aluminium throttle body with a two- bolt fixing. Those same fasteners attach the aluminium-and-foam air filter assembly to the throttle body, again with an O-ring seal. The throttle body carries the butterfly-type throttle, the servo that operates it and also a throttle position sensor. The reed valve body is a pressure- moulded plastic component sandwiched between the endplate and the throttle body, clamped by the latter’s attachment bolts. It is wedge-shaped in cross-section such that it presents a pair of reeds each at 45 º to the inlet face. Fibreglass composite reeds are used. Housed in a remote tank (actually integrated with the fuel tank, in the case of the ScanEagle), engine lubricant is supplied via an electric pump submerged in the tank at a pressure in the 0.05-0.10 bar range. The cantilevered crankshaft runs in a roller bearing positioned in front of its single web, and ahead of that a deep-groove ball bearing; these two main bearings sandwich the air compressor drive lobe. Between the lobe and the front main bearing is a lip seal. The main and big-end bearings are all-metal while the carburised forged steel crankshaft is machined on all surfaces to very tight tolerances. The crankshaft is formed in one piece from the generator attachment at the front to the crankpin at the rear. Its cantilevered arrangement allows it to have its roller and ball main bearings slipped over from the front, the single web running just behind the former. A cap screws into the end of the crankpin to retain the big end. A single axial bolt screws into the nose of the crankshaft to retain the generator rotor, to which in turn the propeller is attached. The stator is attached to the crankcase by three bolts. The rotor is aluminium and carries rare earth permanent magnets, while the stator is steel with copper coils. The air compressor piston is driven by an aluminium rod, the lower end of which fits over the lobe formed by the crankshaft with a needle roller bearing between the two, retained by the former. This bearing is lubricated by the oil mist in the crankcase. The compressor rod is subject to only a very short stroke and low pressure compared with the main engine con rod, and is isolated from the heat of combustion. It is connected to its piston by a hardened steel pin that runs directly in its small-end bore. The pin bore in the piston is anodised for longevity. The aluminium piston carries retainer clips for the pin. The air compressor assembly is enclosed by a cap with O-ring sealing and three bolts attaching it to the crankcase. The air compressor piston, which has a single ring of undisclosed material, reciprocates in a separate aluminium sleeve, atop which is a steel disc that can lift to exit compressed air – a coil spring keeps it closed until the required pressure has been generated. In effect, this is a one-way check valve with a high flow area. Transfer ports in the cap that closes the air compressor assembly feed air from the crankcase to the compression chamber. A further port takes the compressed air through the crankcase to a tube that in turn leads up to the fuel rail assembly. The compressed air is fed to the back of the direct injector. The fuel rail assembly consists of a housing, the injector, a fuel pressure regulator and the fuel and compressed air supply lines. The injector has a high-pressure Teflon seal where it provides a combustion seal to the engine cylinder. An O-ring seal on the opposite end seals the injector to the main fuel rail assembly; the assembly housing is formed of front and rear sections that are machined from billet aluminium. Two fasteners join the two halves, Anatomy of the N20