The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 207
(non-multi-grade) oil in a new engine. In this case the engine revs must be kept to a
minimum after starting until the oil has warmed up. Another cause of excessive oil
pressure at idle is if the relief valve has stuck. In any case of excess pressure, engine revs
must not be increased as a further increase in pressure could burst the oil filter or the
cooler.
In a hot engine the oil flows more freely and worn bearings will increase the oil flow.
At idle speed the oil pump may not be able to meet the flow demand and the pressure
will fall. Thus low oil pressure with hot idle may indicate bearing wear.
After a cold start, the engine should be run on the ground until the oil temperature
has reached a minimum level for flight. If it takes an excessive time for this temperature
to be reached the thermostatic valve in the oil cooler could be stuck and making the
cooler work continuously.
Sudden and total loss of oil pressure in flight is rare and will be due to a mechanical
failure such as a sheared pump drive or a burst pipe. The engine will be destroyed
unless it is shut down immediately. If a suitable landing site is available, the engine
should be stopped and an autorotating landing will be needed. A conscious decision
to autorotate is safer than trying to land using power because without oil the engine
could sieze up without warning and this might occur in the avoid curve. If no suitable
forced landing site is near, it will be necessary to sacrifice the engine in order to attempt
to fly to safety. Flying at minimum power speed will buy time.
A more likely fault situation in flight is where the oil temperature slowly rises and
the oil pressure slowly falls. This is an indication of oil loss due to a leak or oil being
burned in the cylinders. A landing should be made promptly so that an investigation
can be made.
When assessing the oil pressure and temperature gauges, the stress the machine has
been under should be considered. Following a lengthy climb at maximum all up weight
(MAUW) one might expect hot engine oil. The stress on the machine would be reflected
in elevated transmission oil temperature and cylinder head temperature. If all of these
readings are high, the machine is being driven hard and should be flown in a way that
will reduce the power requirements.
On landing the engine should be run at idle for a short while in order to cool it. If
this is not done oil inside the engine will be exposed to heat that it cannot carry away
and the oil could be carbonized and lose effectiveness. This is known as heat soak.
6.10 The carburettor
Engines produce power by burning charge. The purpose of the carburettor is to mix air
and fuel to create charge. Charge will burn over the following range of fuel/air mixes.
A ratio of 1 part fuel to 8 parts air is called a rich mixture, and not all the fuel will
be burned. A ratio of 1:20 is called a lean mixture and not all of the oxygen will be
burned. A ratio of about 1:12 is called a stoichiometric ratio as there is just enough
fuel to combine with all the oxygen.
Clearly a chemically accurate ratio will give the most power and a lean mixture will
give the best fuel economy. Unfortunately it is not possible to take full advantage of this
because of a number of practical considerations. At high power the engine temperature
cannot be kept at a safe level unless a rich mixture is used. The surplus fuel absorbs
heat by evaporation.
The four-stroke engine compresses the charge before combustion to increase the
power output. The greater the compression ratio, the more power can be released.
When charge is ignited it should proceed to burn smoothly as a flame front travelling