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

60 December/January 2018 | Unmanned Systems Technology Primoco 500 four-cylinder UAV boxer engine 54 x 54.5 mm = 499.3 cc Horizontally opposed four cylinder Naturally aspirated Petrol, 95+ RON All-aluminium structure Linerless Four main bearings, ball Steel crankshaft, four pins Steel con rods Light alloy pistons, two rings Two plugs/cylinder Twin carburettors Reed valves Electronic ignition Engine control system 12:1 compression ratio Maximum permitted rpm, 7000 The Primoco 500 is a horizontally opposed four cylinder two-stroke with its cylinders arranged either side of a crankcase that is split transversely into front, centre and rear sections. In effect it is a pair of horizontally opposed twins on a common crankshaft, each pair being supported by two main bearings. Those four main bearings are a push fit into the respective housing formed by the crankcase. Five horizontal bolts run from the front section of the crankcase into the centre section; the same with the attachment of the rear section. Between the front cylinder pair’s inner main bearing and its seal, and the rear cylinder pair’s corresponding items, the centre section of the crankcase simply acts as a spacer, keeping the cylinder barrels on each side the requisite distance apart. A propeller support/thrust bearing on the nose of the crankshaft is housed in an extension of the front of the crankcase. The bulkheads supporting the four main bearings seal the crankcase volumes of the two cylinder pairings; the nose bearing does not come into contact with the lubrication provided by the pre-mix, so it is lubricated with grease. The crankcase and cylinder heads are machined from billet 7075 T651 aluminium alloy. The finned barrels are investment cast in aluminium alloy, a specification chosen to suit the nickel silicon carbide coating applied to the bore of this linerless production. Each head is attached to its barrel by eight M5 bolts and is sealed by a metallic ring. Short M6 K-nuts, four per cylinder, are used to secure a flange at the base of each barrel to the crankcase. The barrel provides five transfer ports. The two-piece crankshaft is machined in-house from case hardened nickel chrome alloy steel and is sent out for case hardening before finish grinding in-house. Each section of the two-piece crankshaft consists of three elements, the centre web being integral with the two individual crankpins, which press into the adjacent web to complete the assembly. A two-ring piston drives an I-section steel con rod that carries a roller bearing at the little end, with a 13 mm diameter piston pin running within it. The pin is 45 mm long (versus 54 mm for the bore). The uncoated steel pin, which runs directly in the piston, is through-drilled and is retained by conventional round wire circlips (lock rings). Thecirclips are both locked against rotation in their grooves. There is also a roller bearing at the big end. The crankshaft bearing journal diameter is 25 mm. The propeller drive hub is attached to the crankshaft nose with a taper fit plus a back-up key. The hub carries the starter ring on its rear side, and is CNC-machined with a lot of lightening. Anatomy Primoco’s 500 cc four-cylinder boxer is notably compact