The Art of the Helicopter John Watkinson - Karatunov.net

6.9 The oil system
Engines and transmissions 205
The oil system is responsible for lubrication and cooling of the moving parts of the
engine. Heavy forces are involved in the connecting rods and crankshaft bearings, and
if metal-to-metal contact took place the life of the parts would be very short. Oil
possesses viscosity, which is a resistance to flow. If viscous oil is pumped between two
surfaces it can carry a load without them touching because the viscosity resists the oil
being squeezed out. The relative movement of the surfaces causes shearing of the oil.
Shearing a viscous liquid develops heat, and this heat load adds to the heat picked up
where the oil is used to cool parts of the engine. An oil cooler is necessary to control
the oil temperature.
Figure 6.8 shows a typical engine oil system. Oil is held in the sump below the engine.
The sump contains a filler neck whose cap holds a dipstick so the oil level can be checked.
This should be done before flight. Oil is drawn from the sump through a strainer to
prevent particles of dirt from damaging the pump. The pump may consist of a pair
of meshing gears driven from the camshaft/magneto drive cluster at the rear of the
engine. The capacity of the pump exceeds the requirements of the engine, and surplus
oil is returned to the sump by a spring-loaded relief valve that opens when the correct
Fig. 6.8 Lubrication system for a typical piston engine. See text for details.