Unmanned Systems Technology 017 | AAC HAMR UAV | Autopilots | Airborne surveillance | Primoco 500 two-stroke | Faro ScanBot UGV | Transponders | Intergeo, CUAV Expo and CUAV Show reports

62 structure and with their crankshafts bolted together in the middle to form a single power output (to drive the propeller at the front and generator via belt from the rear). Since this is a two-stroke engine, each pair of opposed twins has its own sealed crankcase compartment, in which the initial compression of the mixture feeding those two cylinders takes place. Each cylinder has its own crankpin, the opposing cylinders ‘boxing’ towards each other and away again on each rotation of the crankshaft. UST has investigated a number of two-stroke UAV engines in the past but they have all been single- or twin-cylinder units, the twins invariably being boxers. The Primoco 500 is the first flat four (two- or four-stroke) that we have put under the spotlight. Flat fours are not inevitably boxers but that configuration is dominant, as championed in the automotive world by the original, long- lived Volkswagen Beetle and more recently by Subaru with its World Rally Championship-winning engine. In the world of aviation, Lycoming, Continental and Rotax flat-four engines are a staple of lightweight manned aircraft. The flat four’s open and exposed layout lends itself to efficient air cooling with excellent thermal balance. The configuration results in inherently good balance of the reciprocating parts, and it runs smoother than a flat twin. It is superior to an inline four in terms of secondary vibration, and is also shorter in length and with an inherently lower centre of gravity. Compared with an I4, a flat four also has inherently less secondary imbalance at the expense of larger rocking vibrations, which cause it to rotate back and forth around a vertical axis. That is because the cylinders cannot be directly opposed, but must be offset so that the con rods can be on separate crankpins, which results in the forces being slightly off-centre. In the case of the notably compact 500 two-stroke, this rocking vibration is not pronounced – indeed, it has no vibration suppressant beyond its crankshaft webs, which offer a balance factor of about 50%, Smat reports. No other vibration damping is anticipated. Smat also notes that there has been no issue with vibration from the smaller displacement four-cylinder December/January 2018 | Unmanned Systems Technology Dossier | Primoco 500 four-cylinder, 500 cc two-stroke The Primoco 500’s crankcase is split transversally into three sections Key to durability of the engine is the integrity of its crankshaft and the stiffness of its crankcase, which to that end is very well ribbed