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direction of rotation of the aft rotor and the method
of keeping directional control. The forward rotor
turns in a counterclockwise direction viewed from
below, and the aft rotor rotates in a clockwise
direction. A separate antitorque system is not
needed because the rotor systems rotate in opposite
directions (counteract each other’s torque). Advantages
of the tandem-rotor system are a larger
center-of-gravity range and good longitudinal
stability also, the counter-rotating rotors do away
with the need for an antitorque rotor. Because there
is no antitorque rotor, full engine power can be applied
to load lifting. Disadvantages of the tandemrotor
system are a complex transmission and more
drag due to its shape and excessive weight.
FLIGHT CONTROLS
As a helicopter maneuvers through the air, its attitude
in relation to the ground changes. These
changes are described with reference to three axes
of flight: lateral, vertical, and longitudinal. Movement
about the lateral axis produces a nose-up or
nose-down attitude; this is accomplished by moving
the cyclic pitch control fore and aft. Movement
about the vertical axis produces a nose swing (or
change in direction) to the right or left; this movement
is called yaw. This is controlled by the directional
control pedals. These pedals are used to
increase or decrease thrust in the tail rotor of a
single-rotor helicopter and to tilt the rotor discs in
opposite directions on a tandem-rotor helicopter.
Movement about the longitudinal axis is called roll.
This produces a tilt to the right or left. The movement
is accomplished by moving the cyclic pitch
control to the right or left. Some other helicopter
flight controls are discussed below.
Cyclic Pitch Control
The cyclic pitch control looks like the control stick of
a common aircraft. It acts through a mechanical
linkage to cause the pitch of each main rotor blade to
change during a cycle of rotation. To move a helicopter
forward from a hovering height, the rotor disc
must be tilted forward so that the main rotor provides
forward thrust. This change from hovering to flying
is called transition and is done by moving the cyclic
control stick. Moving the cyclic control stick changes
the angle of attack of the blades this change tilts the
rotor disc. The rapidly rotating rotor blades create a
disc area that can be tilted in any direction relative to
3-1
FM 1-514
the supporting rotor mast. Horizontal movement is
controlled by changing the direction of tilt of the
main rotor to produce a force in the desired direction.
Collective Pitch Control
Collective pitch control varies the lift of the main
rotor by increasing or decreasing the pitch of all
blades at the same time. Raising the collective pitch
control increases the pitch of the main rotor blades.
This increases the lift and causes the helicopter to
rise. Lowering the control decreases the pitch of the
blades, causing a loss of lift. This produces a corresponding
rate of descent. Collective pitch control
is also used in coordination with cyclic pitch control
to regulate the airspeed. For example, to increase
airspeed in level flight, the cyclic is moved forward
and the collective is raised at the same time.
Control Plate
Forces from the cyclic and collective pitch sticks are
carried to the rotor by a control plate usually located
near the bottom of the rotor drive. Control plates
used by various builders are different in appearance
and name, but they perform the same function. The
control plate is attached to the rotor blades by pushpull
rods and bell cranks. The collective pitch stick
changes the pitch of the blades at the same time by a
vertical deflection of the entire control plate. The
cyclic pitch stick allows angular shifting of the control
plate to be sent to a single blade. This causes flapping
and small angles of pitch change to make up for
unequal lift across the rotor disc. The direction of tilt
of the control plate decides the direction of flight:
forward, backward, left, or right.
Throttle Control
By working the throttle control, pilots can keep the
same engine and rotor speed, even if a change in
blade pitch causes them to increase or decrease engine
power. When the main rotor pitch angle is
increased, it makes more lift but it also makes more
drag. To overcome the drag and keep the same rotor
RPM, more power is needed from the engine. This
added power is obtained by advancing the throttle.
The opposite is true for a decrease in main rotor pitch
　
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