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efficiency. Because the blades are
rigid, they are less susceptible to
198 Canadian Professional Pilot Studies
vibration. They can also work closer
to the stall, and their service life is
longer because they are not so
stressed – this also applies to the rest
of the transmission.
The point about flying controls is
that they should always be moved
smoothly. Good helicopter flying is
essentially downwash management,
which has some lag built in. If you
jerk the controls, you will get all the
drag without the lift when the blades
get into position before the airflow
has a chance to catch up.
Control Orbit
The movements of the collective
and cyclic controls are transmitted to
the main rotors through a swashplate
or spider system on the main rotor
drive shaft, and their plane of
rotation is called the control orbit.
With the swashplate, two circular
plates are on top of each other,
directly connected. Movement of the
bottom plate is reflected directly in
the upper one. The bottom plate is
also stationary, except for sliding up
and down the drive shaft, and
connected to the cockpit controls.
The top one rotates with the main
blades and transmits the movements
of the bottom plate to them.
The spider system uses an operating
arm in a sleeve inside the main drive
shaft with a ball at the end of it, that
moves around a socket rotating with
the blades, that itself is inside a
stationary socket connected to the
flying controls. Movement of the
lower socket shows in the upper
one, causing the operating arm to tilt
and alter the pitch of the blades.
Both sockets can move up and down
inside the shaft so that collective
movements are transmitted.
The inverted spider is as above, but
upside down, on older machines.
Engines
Engines, turbine or piston, all work
on more or less the same principle –
a quantity of air is sucked in and
mixed with fuel, compressed, set on
fire and slung out of the back (suck,
push, bang, blow in other words or,
more technically, induction, compression,
power and exhaust). The difference is
that the power comes from the
ignition stage in the piston, and the
exhaust stage in the turbine, which is
always ignited, whereas the piston
only does so when the spark plugs
operate. The jet is also a whole lot
lighter, and spins a lot faster. In
short, engines convert heat energy
into mechanical energy, and not very
efficiently at that (if they were,
exhausts would be cold – the
thermal efficiency of a piston engine
is only 30%, though it does increase
with altitude). The mechanical
energy may be to drive electrical,
hydraulic and pneumatic systems as
well, which is why engines are also
called powerplants.
For short range aeroplanes, the
propeller is the most economical
method of propulsion, which can be
driven by a piston or turbine engine
(piston-driven ones are generally
found on aircraft below 5700 kg).
Because of inefficiencies in design,
there is a loss of energy in the
process, and the thrust horse power is
about 80% of the shaft or brake horse
power actually coming out of the
engine. Sometimes, engine output is
too great for the transmission, so the
manufacturer will derate it to make
Airframes, Engines & Systems 199
sure it doesn't damage anything.
Often, derating provides a power
reserve in emergency (as with the
Bell 407 helicopter).
Reciprocating Engines
A typical piston engine consists of a
series of identical cylinders which can
be arranged in many ways, according
to what the engine is going to be
used for. The Beaver, for example
had a Pratt & Whitney R985, which
was a radial engine that had the
cylinders arranged in a circle,
attached to the crankshaft in the
centre (with the propeller bolted to
it). In this case, the cylinders stayed
still, and the crankshaft moved, but
earlier engines made the crankshaft
stay still while the cylinders moved.
As you can imagine, this was a pain
for maintenance.
Most modern light aircraft, including
helicopters, have their cylinders
opposite each other (horizontally or
vertically opposed), to cancel some
forces out, but many have them in
line and even in a V formation.
Some engines are even upside down,
as found with the Gypsy Major in
the Chipmunk (since the pistons in
this case are at the bottom, there is a
　
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