曝光台 注意防骗
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systems detect the light from flames
or glowing metal (also used by
railways to detect hot wheels). Some
systems in small helicopters use a
very thin wire, with another even
thinner one inside, separated by a
conductor that will vapourise with
heat. When the two wires connect, a
circuit is completed and a warning
will go off. Sometimes, however,
theses can short out, particularly on
misty or otherwise humid days, and
create false alarms.
Airframes, Engines & Systems 63
If there isn't actually an extinguisher
in the engine, at the very least there
should be a firewall shutoff, operated
from the cockpit, that will stop the
flow in fuel and hydraulic lines (PA
31, etc). If there isn't one of those,
try for a fuel valve that stops the flow
to the engine (Bell 206).
An engine-based system will have
CO2 or Halon in cylinders.
The recommended extinguisher for
wheel assemblies is dry powder.
Autopilots, etc
Those in large aeroplanes control
attitudes in pitch, roll and yaw and
are known as 3-axis. You will be able
to maintain an altitude or heading,
intercept and follow a radial or
localiser and keep to a descent
pattern. However, when following a
radial, the results can often be
unsettling to passengers, as the
system chases the needle too much.
A better solution is to use the
heading bug and chase the needles
yourself so you don't spill the coffee.
A flight management system takes
the input from several sources and
interfaces it with the autopilot, aside
from storing commonly used
coordinates and routes.
In helicopters, a Stability Control
Augmentation System (SCAS) is used
for short term control assistance,
just to reduce the workload. It is
found in many machines, particularly
the Hughes 500, which uses a beep
trim on the cyclic control. Rate gyros
generate electrical impulses that
control actuators that resist
movement from where the controls
should be. To stop it going out of
control, there will be a limiter.
The Automatic Flight Control System
(AFCS) is a step above, using a
computer to memories control
positions and keep them there.
Vibrations
In helicopters, vibrations come from
many sources, not least the
operation of the machine itself –
there are many spinning parts that
must be finely balanced. One good
source is being downwind. Various
components can induce vibrations in
others, for example, a tail rotor can
vary them in the main rotors. They
may be felt as lateral, longitudinal or
vertical, or a combination. One way
of testing which types are affecting
you is to sit in the cockpit and rest
your wrist on your knee to see which
way your hand moves.
Vibrations fall into three ranges:
· Low, with large amplitude,
between 100-400 cycles per
minute, generally associated
with the main rotor. They can
usually be felt through the
cyclic. A wumper is one kick per
revolution (one per). A
vibration in the stick and
fuselage is possibly from the
rotors or the rotor support
system, particularly friction
dampers. If felt in the controls
only, look in the linkages.
· Medium, 1,000-2000 cycles per
minute, usually stemming from
the tail rotor after improper
rigging or imbalances.
· High, over 2000 cycles/minute,
from the engine, usually.
64 Canadian Private Pilot Studies
Aircraft Husbandry
You can tell how well pilots treat
their machines by the maintenance
costs. For example, not using brakes
excessively while taxying can save a
lot of money in a large fleet.
Similarly, whilst the manufacturers
tell you that you can use the rotor
brake on a helicopter all the time, do
you really need to? They tell you
that, certainly, but they want to sell
you spare parts! Even something as
small as caging the gyros before you
turn them on can make a lot of
difference long-term.
Aircraft should not be parked on
soft or sloping ground, and suitable
chocks should be placed under the
main gear wheels of aeroplanes.
They should be parked into wind
whenever possible, with the
nosewheel in line with the fore and
aft axis.
Control locks are devices attached to
flying controls (external) or control
columns (internal), to stop them
moving on the ground and protect
them from gust and high wind
damage. Together with covers, they
should be used whenever convenient
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