The Art of the Helicopter John Watkinson - Karatunov.net

206 The Art of the Helicopter
oil pressure is reached. The oil then passes through a filter that removes microscopic
particles of metal due to engine wear and combustion products that have found their
way past the piston rings into the oil. The filter needs to be replaced periodically. If the
filter becomes clogged, a spring-loaded valve opens allowing oil to bypass the filter.
The oil next passes to the oil cooler. This is a small heat exchanger that is often fitted
in the engine cooling baffles so that it gets a cold air feed from the engine driven fan.
The cooler is bypassed by a thermostatic valve that is open at low temperatures to allow
the oil to warm up quickly. The valve can also be forced open by oil pressure if the
cooler blocks up. At the outlet of the cooler the oil is ready to be fed to the engine. At
this point the oil pressure and temperature are monitored and displayed on gauges in
the cockpit. Some of the oil is fed to the main crankshaft bearings. The crankshaft is
hollow and oil is pumped in from the main bearings and flows to lubricate the big ends.
The connecting rods may also be hollow and oil carries on up the rods to lubricate
the little end. An oil jet is drilled in the top of the connecting rod to spray oil on the
piston for cooling and to lubricate the cylinder wall. Oil is also supplied to the camshaft
bearings and the valve mechanism as well as to the gear train driving the camshaft and
the magnetos. The oil will eventually fall back into the sump.
The engine oil has a finite life primarily because it builds up an increasing load
of acids, which are by-products of combustion, and water from condensation. Oil is
characterized by its viscosity. Viscosity is measured by the time taken in seconds for
a standard quantity of oil to flow through an orifice standardized by the Society of
Automotive Engineers (SAE) at a standard temperature. Oil may also be classified by
a commercial aviation number that is twice the SAE number.
The viscosity of oil falls with increasing temperature. So-called multi-grade oils have
a smaller viscosity loss with temperature. The correct viscosity of oil is essential and
it must be used at the correct temperature. If the oil is too viscous or used at too low
a temperature, it may not flow readily to all the necessary parts of the engine. If the
oil is too thin or used at too great a temperature, it may be squeezed from bearings
and permit contact. The engine oil must be warmed up before flight so that correct
lubrication is achieved when full power is demanded.
The flight manual will specify the viscosity of oil that must be used in temperate
conditions. A mandatory placard on the machine adjacent to the filler will state the
viscosity needed. If the machine is consistently operated at elevated ambient temperatures,
it may not be possible to keep the oil down to the required temperature. In this
case thicker oil will be used which will thin down to the right viscosity at the elevated
temperature. In a similar way if the machine is to be used in cold conditions the oil
may run too cold and thinner oil will be specified to get the right viscosity at a lower
temperature. It is important that the oil level is checked regularly and kept topped up
with the same type of oil. Oil grades should not be mixed and it is better not to mix
brands. Oils contain a host of additives to improve performance and sometimes the
additives in one kind of oil react with those in another. Turbine engines need oil with
different additives and viscosity. Turbine oil cannot be used in piston engines.
The oil pressure and temperature gauges tell a great deal about the health and working
conditions of the engine and should be monitored regularly as part of the pre-flight
check and whilst airborne. On starting the engine, it may take some time for oil pressure
to be established as the oil is cold and thick and the pump has difficulty dragging it from
the sump. Expect the delay to be longer in cold conditions. Generally if oil pressure has
not established in 30 seconds the engine should be shut down and the cause investigated.
When cold the oil resists flowing through the engine and once pressure is established
the pump has no difficulty supplying enough pressure. Paradoxically, once the pressure
has established, it may become excessive in cold conditions or if using straight