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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.
With respect to a helicopter, lift and
thrust together (or the sum of the lift
of all blades) are often referred to as
Total Rotor Thrust:
30 JAR Private Pilot Studies
Lift is a vertical component, and
thrust is horizontal:
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 for any aerofoil, with the
exception of special shaping on
modern machines to suit peculiar
requirements (such as the Bell 407).
Otherwise, they are 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
& Systems, 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 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 to
happen, to cope with different
angles of attack around the disc, and
Principles of Flight 31
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 (see below).
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
it will fly higher. As it does so, the
angle of attack reduces because of
the change in relative airflow.
On the other side, the blade going
backwards will generate a lot less lift
because of its reduced speed, in
some areas producing a reverse
effect, which will cause the blade tip
to stall if it gets large enough - on a
Bell 206 at 100 kts, the non-lift
producing area of the retreating
blade is about 25%. This will make it
fly down to increase its angle of
attack, to create more lift (needing
more forward cyclic to compensate).
Disymmetry of lift, therefore, is the
difference in lift between the
advancing and retreating blades,
compensated for by flapping, which,
unfortunately, causes the centre of
mass of the blades to move, making
them speed up or slow down relative
to each other. Limited movement
horizontally is provided with dragging
hinges - dragging is the movement of a
rotor blade forward or backward in
its mounting. However, such hinges
are only found in articulated heads
　
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