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minimum, making necessary
movements at a rate of travel not
faster than one full displacement,
stop to stop, per second. The failure
won't be sudden, so switch off early
to keep fluid in the system.
If you ever have to leave the
controls of a helicopter with the
engine running, do not switch the
hydraulics off, but use the control
locks only, in case the controls
motor by themselves.
Overpitching
In a helicopter, overpitching is
where the rotor RPM are too low to
maintain flight, giving the impression
of "labouring". It's the nearest
equivalent to stalling and is
commonly caused by being
overweight for the particular
conditions. Reduce power to
maintain RPM.
Engine Handling
One of the biggest things to unlearn
when transitioning from piston to
turbine is to keep your finger on the
starter button once things start
happening (with a piston, you tend
to take your finger off straight away
when the engine starts). You take
your finger off when the engine
becomes self-sustaining. Before then, it
relies heavily on the battery or start
trolley to keep it turning. It follows
that, if the battery is weak to start
with, the engine won't spin as fast,
the airflow is reduced, the whole
process becomes hotter and you
could melt the back end with a hot
start. You should always check the
voltage available from the battery
before starting a turbine engine. A
hung start exists when the engine fails
to accelerate to normal idle RPM. It
just sits there, weakening the battery
and leading to a hot start.
Pulling full power just because it's
there is not always a good idea.
Limitations may be there for other
reasons—for example, the
transmission might not be able to
take that much, which is why you
Techie Stuff 127
can’t go faster than 80 kts in a
Jetranger when pulling more than
85% torque (actually, in this case, the
transmission ends up in a strange
attitude). Excessive use of power will
therefore ruin your gearbox well
before the engine (and will show up
as metal particles in the oil). Many
turbine failures are the result of
pulling too many cycles from
minimum to maximum Ng, so if you
don't need 100% torque, it's best not
to use it. It's also best not to reduce
the collective lever of your
helicopter to the bottom when
descending, either, and to make
power changes gently, avoiding overand
undershoots.
Maximum Continuous Power is the
setting that may be used indefinitely,
but any between that and maximum
power (usually shown as a yellow arc
on the instrument) will only be
available for a set time limit.
While I'm not suggesting for a
moment that you should, piston
engines will accept their limits being
slightly exceeded from time to time
with no great harm being done.
Having said that, the speed at which
the average Lycoming engine
disintegrates is about 3450 RPM,
which doesn't leave you an awful lot
of room when it runs normally (in a
Bell 47, anyway) at 3300! Turbines,
however, are less forgiving than
pistons and give fewer warnings of
trouble because of their closer
tolerances. This is why regular power
checks are carried out on them to
keep an eye on their health. The
other difference is that damage to a
piston engine caused by mishandling
tends to affect you, straight away,
whereas that in a turbine tends to
affect others down the line.
In a turbine-engined helicopter,
power is indicated by the torquemeter.
Apart from sympathetic handling,
the greatest factor in preserving
engine life is temperature and its rate
of change. Over and under leaning
are detrimental to engine life, and
sudden cooling is as bad as
overheating—chopping the throttle
at height causes the cylinder head to
shrink and crack with the obvious
results—the thermal shock and extra
lead is worth about $100 in terms of
lost engine life. In other words, don’t
let the machine drive the engine, but
rather cut power to the point where
it’s doing a little work. This is
because the reduced power lowers
the pressure that keeps piston rings
against the wall of the cylinder, so oil
leaks past and glazes on the hot
surfaces, degrading any sealing
obtained by compression. The only
way to get rid of the glaze is by
honing, which means a top-end
overhaul. For the same reasons, a
new (or rebuilt) engine should be
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(85)