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above the engine to work properly
(as with the Bell 47). Modern design
requirements, however, mean that
the fuel cells are in all manner of
strange places, and come in many
different shapes and sizes (together
with a C of G system all of their
own). Because of this, various
methods are used to get the fuel
from them to the engine, all
involving fuel pumps and filters. Each
engine will have its own pump, but
there will also be an inline backup,
just before the carburettor or
actually at the fuel tank. Note that
boost pumps are lubricated by the
fuel they work on, so don’t run them
dry or you will burn them out.
A fuel primer is a small hand pump
designed to put neat fuel directly
into either the induction manifold
(near the combustion chamber) or
the inlet valve port before you start
in the cold to promote the presence
of fuel vapour that will ignite to start
the engine (very rarely do you need
to prime a warm engine). They are
not there with fuel injection systems.
Fuel tanks will be vented to
atmosphere, to prevent a vacuum
forming inside the tanks as the fuel
level is reduced. The vents might be
in the fuel cap, or be an overflow pipe
in the tank.
Although many fuel gauges are
accurate, they should never be relied
upon as the final guide to what you
have. Reading the book Free Fall,
about the Gimli Glider is very
instructive about this - a 757 had to
make a dead stick landing at Gimli
after running out of fuel in the
cruise, due to a combination of
circumstances, including misleading
fuel gauges. Actually, the episode is
also instructive with regard to CRM
procedures.
Hydraulics
Liquids have minimal compressibility,
meaning that, when pressure is
applied, it will be pretty much taken
up throughout the whole system.
This makes it a useful way of
transferring movement round
corners and into strange places, as
the forces produced by a hydraulic
system can be very powerful indeed,
which is why they are used in
helicopters to reduce the forces that
would otherwise be required to
move the flying controls.
Keeping to the helicopter theme,
some (such as the AStar) may have
an accumulator instead of a second
hydraulic system, to save weight.
Aside from smoothing out
fluctuations in hydraulic pressure,
the accumulator's job is to store
pressure that can be used for a short
time if the main system fails. That is,
it can be used in emergency. You can
think of an accumulator as a shock
absorber, since a valve opening in a
highly pressurised system causes
quite a shock to the lines. It is a
cylinder in which a piston separates
hydraulic pressure from air, which is
pressurised on the ground.
Speaking of shocks to the system,
hydraulic fluids are specially made to
withstand high pressures and
temperatures without vapourising, so
make sure you use the proper stuff.
60 JAR Private Pilot Studies
The basic system will have a jack,
with a control to direct the fluid into
whichever end of the jack is desired
to move, a pump and a reservoir. The
engine drives the pump, which
moves the fluid out of the reservoir
and applies pressure to it. When the
jack gets as far as it can go, the
increased pressure forces a relief valve
open so the fluid can be dumped
back to the reservoir. To keep
everything clean there will also be a
filter somewhere.
Some helicopters have a rotor brake
that is hydraulically operated, but is
nothing to do with the main system.
It consists of a disk around the main
drive shaft that is gripped with brake
pads operated by the lever in the
cockpit. There will be a range of
rotor RPM that the brake must be
operated within.
Undercarriages
These can be skids, skis, wheels or
floats. Helicopters can also have
inflatable floats that are used when
over water – they are packed tightly
inside a covering and are inflated
when the pilot operates an air bottle
inside the cabin. Wheeled helicopters
typically taxi like aeroplanes, and
rarely hover for long periods.
Landing gear is there to take the
shock of landing, so it isn't
transferred to the airframe. Of
course, it also helps you get around
on the ground. The retractable variety
produces less drag in flight, at the
expense of complication. As with
transmissions, the system will have
its own oil system, in this case
hydraulics, and a backup system
　
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