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run in hard, not less than 65%
power, but preferably 70-75%,
according to Textron Lycoming, so
the rings are forced to seat in
properly. This means not flying
above 8000 feet density altitude for
non-turbocharged engines. Richer
mixtures are important as well. Also,
open the engine compartment after
shutting down on a hot day, as many
external components will have
suddenly lost their cooling. With
some turbine engines (like on the
AStar), you have to keep a track of
the number of times you fluctuate
between a range of power settings
because of the heat stress.
In the cruise, better fuel
consumption may be obtained at
128 The Helicopter Pilot’s Handbook
slower speeds and lower power
settings, at the cost of extended
running time, so you might not really
save that much. For example, leaning
to 10° lean of peak Exhaust Gas
Temperature (EGT), without
exceeding the maximum, loses about
5 knots. Typically, EGT probes are
fitted to one cylinder of the engine,
which is not necessarily the one that
reaches peak temperature first, even
though it may end up as the hottest,
so a margin of 25° rich of peak may
still not be enough to stop another
cylinder from getting too close to
peak for comfort, or even lean.
One consideration with using low
power when it's very cold is that the
engine may not warm up properly
and water that forms from
combustion may not evaporate, so
oil won't lubricate properly.
The reason the temperature cools
either side of the peak reading is that
on the one hand (rich), there is too
much fuel and, on the other (lean),
there is too much air (having said
that, the hottest CHT is between 25-
50° rich of peak EGT, because that's
where the peak cylinder pressure
occurs, with a high rate of heat
transfer to the cylinder head, so you
need to lean past it). However,
although being lean of peak works,
there is much more potential for
causing damage to the engine if it is
mismanaged – it needs more
monitoring to be used effectively, as
the temperature at the exhaust will
still be high, which is not good for
the valves, particularly acute with
high performance turbocharged
engines – Australian authorities
found that leaning causes lead
oxybromide deposits to cling to
various parts inside the combustion
chamber, which could become
hotspots and cause detonation (the
lead appears as a result of chemical
changes in avgas as it burns). At
richer settings, the lead either doesn't
form or is swept out of the cylinder
(this may be true for lower
performance engines, too).
Don't forget to enrich the mixtures
before increasing power when at
peak EGT or when increasing to
more than 75% power. Move the
engine controls slowly and smoothly,
particularly with a turbocharged
engine. Harsh movements that (on
older engines) will result in a cough
and splutter and having no power
can be embarrassing.
Although many flight manuals state
that as soon an engine is running
without stuttering it's safe to use it to
its fullest extent, try warming up for
a few minutes before applying any
load, at least until you get a positive
indication on the oil temperature
(and pressure) gauges. This ensures a
film of oil over all parts.
Even better, warm it before you start
it, because the insides contract at
different rates – in really cold
weather the engine may have the
grip of death on the pistons and
cause some strain when you turn the
starter. Equally important is not
letting an engine idle when it's cold,
as it must be fast enough to create a
splash (about 1,000 RPM is fine).
After flight, many engines have a
rundown period which must be strictly
observed if you want to keep it for
any length of time. As engines get
smaller relative to power output,
they have to work harder. Also, in
turbines, there are no heavy areas to
act as heat sinks, like the fins on a
Techie Stuff 129
piston engine, which results in
localised hotspots which may
deform, but are safe if cooled
properly, with the help of circulating
oil inside the engine (75% of the air
taken into a turbine is for cooling
purposes). If you shut down too
quickly, the oil no longer circulates,
which means that it may carbonise
on the still-hot surfaces, and build
up enough to prevent the relevant
parts from turning. This coking up
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(86)