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the speed of the blades, or their
shape, the pitch is altered instead
when varying the thrust. Blade speed
is constant, and the minimum and
maximum speeds are close together,
because of engine limitations on
piston machines, and transmission
limits for turbines (the minimum
limits are for the coning angle).
The collective, to the left of the
pilot's seat, is called that because it
changes the pitch of all the rotor
blades at the same time, that is,
collectively, thus changing their
angles of attack all at once.
Unfortunately, this also increases
drag, which will tend to decrease the
engine and rotor RPM, so some
throttle needs to be applied when
the collective is moved to keep them
up (in fact, the throttle in a piston
machine should be applied just before,
so the engine doesn't lag behind,
called leading with throttle). There is
some sort of automatic linkage
between the collective and throttle
on most machines, but with pistons,
this is rudimentary at best, and may
not exist at all, as on the Hiller 12E.
Traditionally, the throttle is mounted
on the end of the collective lever. Its
function is to regulate engine RPM, and
it is moved by the left hand
outwards (away from the thumb) to
increase power, and the other way to
reduce it. Where (turbine) engine
RPM is maintained by a Fuel Control
Unit (FCU) or FADEC (Full
Authority Digital Electronic Control –
see below), it isn't moved at all,
except in some emergencies where it
can be used to control the direction
of the fuselage. Because they are
usually left in one position, turbine
helicopters may also have the
throttles mounted in the roof or on
the floor which, of course, restricts
their use when problems occur.
The cyclic control is the equivalent
of the central column in a plane, and
only changes the pitch of one blade
at a time, to raise the rotor disk (or,
rather, the tip path plane) at that point
and tilt it in the direction you want
to go. In other words, it changes the
direction of the lifting force, and not
its magnitude, except in the one place
required to lift the blade.
Principles of Flight 29
Like the rudder in an aeroplane, the
tail rotor pedals are not used to turn
the ship (except in the hover), but to
stop it turning the wrong way when
you are turning, or to provide fine
tuning for trimming purposes. In
straight and level flight, or above
about 60 knots, they can, to all
intents and purposes, be ignored, as
the tail boom does all the work.
The normal tail rotor, as found on
JetRangers, requires a large number
of components and sits in the dirty
airflow from the main rotors – it
therefore lives a stressful life. The
fenestron, as used by Eurocopter, is a
different solution, consisting of a
series of very small blades enclosed
in a shroud:
The blades are not equally spaced, to
help with noise, and the shroud
prevents tip losses, for more
efficiency. Because the blades are
rigid, they are less susceptible to
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
　
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