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63 1 Rocker cover 2 Cylinder head 3 Drive sprocket 4 Drive hub 5 Propeller washer 6 Propeller nut 7 Lock nut 8 Crankshaft 9 Driven sprocket 19 Crankshaft rotation sensor 20 Generator support (R) 21 OGR-100 generator 22 Generator support (F) 23 Injector 24 Throttle valve 25 Injector joint 26 Manifold pressure sensor 27 Throttle servo 10 Intake manifold 11 Cylinder 12 Connector (injector) 13 Intake air temperature sensor 14 Air inlet 15 Engine bracket 16 Mounting plate (R) 17 Mounting plate (F) 18 Crankcase 28 Throttle linkage rod 29 Spark plug (CM-6) 30 Cylinder head temperature sensor 31 F-6040 silencer 32 Generator mount 33 Reinforcement beam 34 Engine mount weigh no more than 2 kg complete with exhaust, electronics and generator. On the other hand, that weight advantage comes at the cost of inherently greater thirst. Based on the state-of-the- art engines we have investigated, we can expect such a two-stroke to have a best specific consumption in the region of 475 g/kW/h at 7000 rpm, whereas the GF40U-FI registers 436 g/kW/h, falling to 418 g/kW/h at 7700 rpm. That was the main consideration when OS Engines opted for a four-stroke. On top of that though, the GF40U-FI offers dependability, since it is derived from the company’s well-proven, carburettor- equipped GF40 aero engine. The GF40 was already manufactured to a quality level consistent with UAV power plants, so most of its internals went unchanged when the newly created OS Professional division designed this first UAV power plant. Air cooling and pre-mix lubrication were retained to maintain simplicity. The basic layout of the GF40 is defined by a longitudinal crankshaft directly driving a propeller from its nose, plus an upright cylinder. The crankcase, barrel and head are separate items, and the piston/rod assembly drives a cantilevered crankpin attached to a counterbalance web at the rear of the crankshaft. This arrangement means the crankshaft can be inserted from the rear through the two roller main bearings carried by a crankcase that is a single piece aside from a rear end plate. The valvetrain has pushrod and rocker operation of the two overhead, upright poppet valves, the (equal length) rocker for each sharing a shaft that is perpendicular to the crankshaft axis. The camshaft is similarly perpendicular and is gear-driven directly from the crankshaft. Flat tappets transfer cam action to the pushrods located in front of the barrel, while a single spring closes each valve. A single spark plug is located behind the two valves. The head (a portion of which is closed by a rocker cover) provides 90 º porting for each valve. The intake entry faces towards the rear, while the exhaust exit faces towards its side. In the standard GF40 the intake is fed by a rear-located carburettor; the exhaust system, complete with silencer, is mounted high and parallel to the crankshaft axis (or optionally angled) on the right-hand side – the side where the exhaust valve is located. The GF40 (from which the UAV engine is derived) has a bore of 40 mm and a stroke of 31.8 mm for a displacement of 39.96 cc. It runs from 1800 rpm through to 9000 rpm, with maximum power of 3.75 bhp (2.8 kW) at 8600 rpm, and is fed unleaded gasoline at a 50:1 ratio with two-stroke lubricant. A positive crankcase ventilation (PCV) system recirculates blow-by gas and spent lubricant to minimise emissions. In this port-injected standard trim the GF40 weighs 1.17 kg, with a silenced exhaust adding 113 g and a battery-powered ignition module adding 95 g. OS Engines GF40U-FI | Dossier Unmanned Systems Technology | December/January 2019