曝光台 注意防骗
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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.
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
Techie Stuff 131
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
the engine warm-up.
Oil Cans
Whatever you use to open them,
don't bang it down on the lid, but
gently prise it open, so you don't get
slivers of metal in the oil which may
disagree with your engine.
Schermuly Flares
Flares should be stored for a
maximum of 4 yrs between 0-25º C,
and only be installed for 1 year or
1000 flying hours. Check that the
centre contact in the mounting
moves freely and is clean, and the
locking and securing ring threads for
damage. When not carrying flares,
the plastic cover plate should be
fitted and the securing ring locked
with the locking ring.
Test the firing circuit with a suitable
50W 24V bulb and holder in a cutdown
fired flare case. Before
installation of any flares a "no volts"
check of the firing circuit (using a
millivoltmeter) should be made.
The securing ring must make a good
electrical contact with the flare case,
achieved by rotating the flare with
the securing ring partially tightened.
When tightening the ring, rotating
the flare will help give the required
tightness. The locking ring must be
tightened (if in doubt, the
flare/circuit resistance should be less
than 5 ohms). If a circuit resistance
check is performed, a safety
ohmmeter must be used and the
measured resistance should be in the
range .8-1.2 ohms.
Height/Velocity Curve
Otherwise known as the Dead Man's
Curve, this is a chart for helicopters
that compares speeds against heights
to give you an idea of where not to
be if you don't want to kill yourself if
an engine fails, that is, you don't want
to be at high altitudes with low
speeds, or low ones with high
speeds, so the best place to be is in
the gap between the shaded areas:
In other words, the graph shows
combinations of speed and height
132 The Helicopter Pilot’s Handbook
that the average pilot would find it
impossible to make a safe emergency
landing from. For more of a chance,
a one-second delay is factored in for
minimum skill levels.
A couple of points to note first of
all: one is that you should check to
see if the chart is in the Limitations or
Performance section of the Flight
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The Helicopter Pilot’s Handbook(88)
