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have adjustments for varying the blade pitch. Aconing
angle, determined by projections of blade weight,
rotor speed, and load to be carried, is built into the hub
bar. This minimizes hub bar bending moments and
eliminates the need for a coning hinge, which is used
in more complex rotor systems. A tower block provides
the undersling and attachment to the rotor head
by the teeter bolt. The rotor head is comprised of a
bearing block in which the bearing is mounted and
onto which the tower plates are attached. The spindle
(commonly, a vertically oriented bolt) attaches the
Figure 18-1. The semirigid, teeter-head system is found on
most amateur-built gyroplanes. The rotor hub bar and blades
are permitted to tilt by the teeter bolt.
Tower Plates
Hub Bar
Tower Block
Bearing Block
Teeter Bolt
Spindle Bolt
Torque Tube
Fore / Aft Pivot Bolt
Lateral Pivot Bolt
18-2
rotor blade pitch while in flight. This system is significantly
more complicated than the teeter-head, as it
requires hinges that allow each rotor blade to flap,
feather, and lead or lag independently. [Figure 18-2]
When used, the fully articulated rotor system of a gyroplane
is very similar to those used on helicopters, which
is explained in depth in Chapter 5—Helicopter Systems,
Main Rotor Systems. One major advantage of using a
fully articulated rotor in gyroplane design is that it usually
allows jump takeoff capability. Rotor characteristics
required for a successful jump takeoff must include a
method of collective pitch change, a blade with sufficient
inertia, and a prerotation mechanism capable of approximately
150 percent of rotor flight r.p.m.
Incorporating rotor blades with high inertia potential is
desirable in helicopter design and is essential for jump
takeoff gyroplanes. A rotor hub design allowing the
rotor speed to exceed normal flight r.p.m. by over
50 percent is not found in helicopters, and predicates a
rotor head design particular to the jump takeoff
gyroplane, yet very similar to that of the helicopter.
PREROTATOR
Prior to takeoff, the gyroplane rotor must first achieve
a rotor speed sufficient to create the necessary lift.
This is accomplished on very basic gyroplanes by initially
spinning the blades by hand. The aircraft is then
taxied with the rotor disc tilted aft, allowing airflow
through the system to accelerate it to flight r.p.m.
More advanced gyroplanes use a prerotator, which
provides a mechanical means to spin the rotor. Many
prerotators are capable of only achieving a portion of
the speed necessary for flight; the remainder is
gained by taxiing or during the takeoff roll. Because
of the wide variety of prerotation systems available,
you need to become thoroughly familiar with the
characteristics and techniques associated with your
particular system.
MECHANICAL PREROTATOR
Mechanical prerotators typically have clutches or belts
for engagement, a drive train, and may use a transmission
to transfer engine power to the rotor. Friction
drives and flex cables are used in conjunction with an
automotive type bendix and ring gear on many gyroplanes.
[Figure 18-3]
The mechanical prerotator used on jump takeoff gyroplanes
may be regarded as being similar to the helicopter
main rotor drive train, but only operates while the aircraft
is firmly on the ground. Gyroplanes do not have an
antitorque device like a helicopter, and ground contact is
necessary to counteract the torque forces generated by
the prerotation system. If jump takeoff capability is
designed into a gyroplane, rotor r.p.m. prior to liftoff
must be such that rotor energy will support the aircraft
through the acceleration phase of takeoff. This
combination of rotor system and prerotator utilizes
the transmission only while the aircraft is on the
ground, allowing the transmission to be disconnected
from both the rotor and the engine while in normal
flight.
HYDRAULIC PREROTATOR
The hydraulic prerotator found on gyroplanes uses
engine power to drive a hydraulic pump, which in turn
drives a hydraulic motor attached to an automotive type
bendix and ring gear. [Figure 18-4] This system also
requires that some type of clutch and pressure regulation
be incorporated into the design.
Figure 18-2. The fully articulated rotor system enables the
pilot to effect changes in pitch to the rotor blades, which is
necessary for jump takeoff capability.
Figure 18-3. The mechanical prerotator used by many gyroplanes
uses a friction drive at the propeller hub, and a flexible
　
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