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the forward movement of the
aircraft, whereas the engine on a
helicopter isn't – its function is to
drive the rotor system which is really
what makes the machine move
through the air.
In fact, the rotors provide lift, thrust
and directional control in one go –
all three being separate functions on
an aeroplane. In some ways, this is
beneficial, since loss of an engine on
a twin does not need the strength of
a gorilla to keep it straight, but it
does make flying more demanding.
A helicopter's fuselage will have the
tailboom and main rotors attached
to it. The most common landing
gear is skids:
But you may find wheels, too:
28 Canadian Private Pilot Studies
As you probably already know, a
helicopter can move or turn in any
direction, including up and down.
The flight controls are the cyclic and
collective controls, the throttle and the
tail rotor pedals, which all have much
the same effect as they do in
aeroplanes (once out of the hover),
except for the collective, which isn't
used in them at all. As mentioned
before, the main rotors provide lift,
thrust and directional control and,
because it is impractical to change
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.
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.
Principles of Flight 29
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
　
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