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changes in current. An inductor uses
energy stored in a magnetic field.
Any conductor can be one. Its action
in an electrical circuit is rather like a
flywheel, in that, when voltage is
applied to an inductor, it takes time
for the current to reach a steady
value. Similarly, when voltage is
taken away, it takes time for it to die
down, so it smooths out variations.
However, a voltage is produced as
this happens, its amount dependent
on the rate at which the magnetic
flux changes. It therefore affects the
flow of AC by producing a reverse
flow, called back emf (the conductor
is cut by its own magnetic field).
Back emf is why an electric drill
stops at a certain RPM.
Inductors are the basis of transformers,
a principle used in magnetos (or the
coil in a car) to boost voltage from
24v to the 25,000 volts or so need to
jump across the gap of a spark plug.
A change in current in the primary
windings induces one in the
secondaries through mutual inductance.
The unit of inductance is the Henry
(H).
Resonance
A resonant, or tuned, circuit is one
where the inductive and capacitative
reactances (that is, concerning
voltage and current) are equal, and
therefore cancel each other out. This
is made use of in radio to tune in to
a particular frequency (the tuning
knob is actually a variable capacitor).
Ohm's Law
This describes the relationship of the
three elements mentioned above.
Electricity & Radio 273
The symbols for them are I for
current (amps), R for resistance and
V for voltage. they come together in
this formula:
V = I x R
Ohm's Law states that the voltage
drop across a resistance is equal to
the current flowing through it,
multiplied by its value. So, if you
have a 24 volt battery (as found in
most aircraft), and a load applied to
it has 12 ohms of resistance, there
would be 2 amps of current.
Busbars
The lighter an aircraft is, the better,
so it's impractical (if only for weight
saving) to run a wire from the
battery to every component it
supplies. A better solution is to run a
single (big) wire to a collection of
electrical appliances and serve all of
them from the end of that line,
which is what a busbar is all about.
Physically, an electrical busbar (bus
for short) is a metal bar with
provisions to make electrical contact
with a number of devices that use
electricity. Electrically, it's a conduit
between components, as in the
memory bus in a computer. There's
nothing to stop you having main
buses supplying secondary ones.
Devices are connected in parallel to
the busbar to keep voltages equal.
Most multi-engined aircraft have left
and right main buses, and a battery
master bus for a few essential items,
but there the similarity ends. There
are so many variations on a theme
that it's quite difficult to keep track
of them all, and getting acquainted
with a new type of aeroplane can be
quite difficult, especially when the
Flight Manual is less than perfect. It
helps in these cases to understand
the philosophy behind buses (which
are notoriously complicated).
Essential things to know about buses
are what they power, how to reroute
power to them and how to isolate
them, a bit like fuel tanks. All aircraft
must have standby electrical power
systems, in case the normal one goes
down. For small ones, this is usually
the main battery, which is oversized
for this reason. The problem is, it's
time-limited, as mentioned above.
Usually, each power source will
power its own set of buses, but there
will be ways of rerouting that power
to others if the source fails.
Duplicated items will usually be
powered singly from different buses.
Vital services, like emergency lights, or
engine temperature gauges (in the
Twinstar) will be wired directly to
the battery bus, so they will work
when the battery relay is switched
off (there may be a secondary battery
switch for just these items to stop
the battery running down when the
ship is unattended). Essential services,
like those required for flight, will be
powered if a generator or alternator
fails. Non-essential services are things
like galleys.
The term hot battery bus means the
bus is live even when the generators
are not working, so you must switch
devices attached to it off when you
close down (as with the left fuel
boost pumps in Bell 206L or 407
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Canadian Professional Pilot Studies2(44)
