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minimum RPM to let it circulate.
264 Operational Flying
Synthetic oils have come from
turbine oil development, but they
have one drawback, in that the
sludge in tends to centrifuge out
inside the dome of a constant speed
propeller and make cycling a bit
difficult. They also hold
contaminants longer.
Many Flight Manuals recommend
not changing brands of oil (e.g. the
JetRanger), but if you check with the
engine manufacturer’s manual (in
this case Allison), you may find that
it is permitted on a top-up basis, that
is, if you already have one brand in
there, just start using the new stuff
until eventually the contents change
completely. Do not drain your
present oil, and replace it in one go,
because oils have different cleaning
characteristics—your new brand may
be more efficient and you get bits of
coke and carbon floating around that
could cause a seizure. However, this
is something you should discuss with
your maintenance people.
Carburettor Icing
This is actually one aspect of induction
system icing. The other two are fuel
icing, arising from water suspended in
fuel, and impact ice, which builds up
on the airframe around the various
intakes that serve the engine. Even
on a warm day, if it's humid,
carburettor icing is a danger,
especially with small throttle
openings where there's less area for
the ice to block off in the first place
(as when descending, etc.). Also, the
temperature drop (between the OAT
and that in the venturi) can be
anywhere between 20-30°C, so icing
(in an R22, anyway) can happen even
when the OAT is as high as 21°C
(70°F), or more. Tests have
produced icing at descent power at
temperatures above 30°C, with a
relative humidity below 30%, in clear
air. Because it is more volatile, and
likely to contain more water, you can
expect more fuel and carb icing with
MOGAS than AVGAS.
It usually arises from the action of
the venturi in the throat, just before
the butterfly valve, which regulates
the amount of fuel into the engine.
You will remember the venturi's
purpose is to accelerate airflow by
restricting the size of the
passageway, which has the effect of
reducing the pressure and pulling the
fuel in. Unfortunately, this process
also reduces the temperature, as does
the fuel vapourisation, hence the
problem (the lower temperature
means greater relative humidity, and
closeness to the dewpoint, and the
vapourisation takes its latent heat
from the surroundings, making the
situation worse). In fact, the
vapourisation (and cooling) can carry
on most of the way to the cylinders,
causing the problem to persist,
especially with the butterfly semiclosed,
which produces another
restriction and more of the same.
Any water vapour under those
conditions will sublimate, or turn
directly to ice. Note also that warm
air will produce more ice because it
can hold more moisture.
With smaller engines, use full
settings—that is, either on or off, with
no in-betweens - the greatest risk is
at reduced power. Out of Ground
Effect hover performance charts for
helicopters usually assume the carb
air is cold (the R22 requires heat
below 18" MP). In fact, when heat is
applied, an engine will typically lose
around 9% of its rated power.
Techie Stuff 265
Rough running may increase as
melted ice goes through the engine.
Also, be careful you don't get an
overboost or too much RPM when
you reselect cold. Of course,
aeroplanes have some advantage if
the engine stops from carb icing, as
the propeller keeps the engine
turning, giving you a chance to do
something about it.
In a helicopter, due to the freewheel
that allows autorotation, the practice
of only selecting hot air when you
actually get carb ice may not be such
a good idea – usually, a gauge is used
with a yellow arc on it, showing the
danger range.
Use carb heat as necessary to keep
out if it. The other peculiarity with
regard to helicopters is that they
tend to use power as required on
takeoff, whereas aeroplanes use full
throttle. This makes them more
vulnerable, as the butterfly opening
is smaller, and is particularly
apparent on the first takeoff of the
day, when the engine and induction
system are still cold. If it is filtered,
your carb heat may be used to
preheat the induction system during
　
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