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simulated), 50 of which must be in
flight, and over 25 in helicopters as
PIC or P1/US (or any combination).
You can only claim up to 25 hours in
a simulator, or 50 on an approved
course.
10 The Helicopter Pilot’s Handbook
Notes
How Helicopters Work
This chapter may be skipped if you
are flying already – it's here for
people who buy the book before
they get their licence, or who feel
they need a refresher.
The helicopter is just a flying
machine that has its wings going
round instead of remaining still,
cynically referred to by some as
50,000 rivets in loose formation –
this means that the lift-producing
surfaces (i.e. rotors) are separate
from the body, unlike an aeroplane.
Another difference is that an
aeroplane engine is directly related to
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 are separate functions on an
aeroplane. In some ways, this is
good, since loss of an engine on a
twin does not require the strength of
a gorilla to keep it straight, but it
certainly makes the flying more
demanding. In a helicopter, lift and
thrust together (or the sum of the lift
of all blades) are often referred to as
Total Rotor Thrust:
Lift is a vertical component, and
thrust is horizontal:
12 The Helicopter Pilot’s Handbook
The Tip Path Plane is the path
described by the tips of the rotor
blades, viewed horizontally. The
Coning Angle is the angle between the
blade and the Tip Path Plane. The
Rotor Disc is the area formed by the
rotation of the blades, inside the
blade tips. As the blades start coning,
it will change its size slightly (the
word rotor includes the blades, the
hub and the shaft). The smaller the
disk gets, the less area there is to
generate lift – a situation that would
arise if the RPM were too low,
causing the coning angle to increase
and centrifugal force to decrease.
Coning is the resultant of centrifugal
and lift forces – as the blades
produce lift, the coning angle increases,
but it decreases as RPM increases -
the blades need centrifugal force for
stiffness, and their ability to support
the machine in flight. It is measured
in tons, against pounds for lift,
thrust weight or drag, which will give
you an idea of its importance.
Main rotor blades obey the same
rules as any aerofoil, with special
shaping on modern machines to suit
peculiar requirements (such as the
Bell 407). Otherwise, they are
generally symmetrical in cross
section, to restrict movement of the
Centre of Pressure.
The pitch angle is that between the
blade's chord line and the spin axis
of the main blades, and the plane of
rotation, which is parallel to it (usually
above). The plane of rotation
contains the centres of mass of the
rotating blades (the axis of rotation is
a vertical element, which the blades
rotate around – it is at right angles to
the plane of rotation, and not
necessarily in line with the rotor
mast. The difference between them
is the flapping angle). The pitch angle
is varied with the collective and
cyclic controls (see Airframes &
Engines, next), and is not the same as
the angle of attack, between the chord
and the relative airflow.
Rotor Profile Drag comes from rotor
blades at zero pitch, occurring purely
because the blades are rotating. Air
flowing through the disc at positive
angles of attack suffers from induced
drag, which is highest in the hover.
The downwards motion of air
through the blades is called the
induced flow.
As the helicopter moves, the blades
will move above and below the plane
of rotation, in a process called
flapping. A flapping hinge allows this
movement to happen, to cope with
different angles of attack around the
disc, and equalise the lift around it.
They are needed when you have
more than two blades, which would
use a teetering head, and work like a
seesaw for the same effect, where
the two blades will flap as a unit.
When the helicopter moves forward,
the blade going forward will develop
more lift because of the added speed
(from the helicopter's forward
movement and that of the blade), so
　
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