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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.
The basic system will have a jack,
with a control to direct the fluid into
whichever end of the jack is desired
60 Canadian Private Pilot Studies
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
should it decide not to work. This
can be operated manually, or by an
air bottle.
Otherwise, landing gear is made up
of struts, that are attached to the
fuselage, and which have the wheels
attached to them (in tailwheel
aircraft, struts are fitted slightly
ahead of the C of G – otherwise,
they will be slightly behind). For
shock absorption, an oleo strut
contains a piston and a cylinder,
moving together inside hydraulic
fluid. There are holes in the piston to
allow the fluid through and damp
down the shocks. There may be
nitrogen or dry air instead of fluid.
Another function of a strut is to
force the tyre on to the ground.
With wheels, there will be brakes,
used for stopping, and sometimes
steering (not recommended in a
multi-engined aeroplane – use
differential engine power instead).
There will be a master cylinder for
each brake pedal, a reservoir, the
brakes themselves and connecting
pipes. The toe pedal activates the
master cylinder directly. A piston
inside forces fluid along the lines to
activate the brakes on the wheels.
Transmissions
This is how you get the power from
the engines to the propellers or rotor
blades. Because some engines work
at higher speeds than other
components, there will also be an
element of reduction involved.
Transmission systems will also have
their own, self-contained oil supply.
In a helicopter, a clutch may be fitted
between the engine and main rotor
gearbox. A centrifugal clutch (as found
on the Bell 47) is automatic and will
have more effect as speed is
increased so, if the engine stops, the
blades are free to rotate. A friction
or belt drive clutch (like in the
Enstrom) is manually operated by
the pilot, which allows the engine to
Airframes, Engines & Systems 61
be warmed up without the blades
rotating (safer for passengers, too).
Some turbine helicopters have
clutches, too, like the Gazelle.
Others have a freewheel unit to let the
blades spin without engine power.
The engine RPM and rotor RPM
gauges are normally combined, with
the needles superimposed on each
other:
They are split in the above diagram.
Pneumatics
Compressed air is used for many
purposes, such as operating the
landing gear, doors, flaps, etc.
Nitrogen will sometimes be used for
backup pneumatics for landing gear.
Flight instruments are also operated
by air, and for deicing boots.
Heating Systems
The simplest system uses a muff
round the exhaust that makes air
flowing through it warmer, on its
way to the cabin. As a result, there is
a danger of carbon monoxide
poisoning if the exhaust is faulty.
Slightly more complicated is what
　
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