ROTORCRAFT FLYING HANDBOOK1(23)

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causes the point of equilibrium to move inboard along
the blade’s span, thus increasing the size of the driven
region. The stall region also becomes larger while the
driving region becomes smaller. Reducing the size of
the driving region causes the acceleration force of the
driving region and r.p.m. to decrease.
The inner 25 percent of the rotor blade is referred to as
the stall region and operates above its maximum angle
of attack (stall angle) causing drag which tends to slow
rotation of the blade. Part E of figure 3-22 depicts the
stall region.
Aconstant rotor r.p.m. is achieved by adjusting the collective
pitch so blade acceleration forces from the driving
region are balanced with the deceleration forces
from the driven and stall regions.
AUTOROTATION (FORWARD FLIGHT)
Autorotative force in forward flight is produced in
exactly the same manner as when the helicopter is
descending vertically in still air. However, because forward
speed changes the inflow of air up through the
rotor disc, all three regions move outboard along the
blade span on the retreating side of the disc where angle
of attack is larger, as shown in figure 3-23. With lower
angles of attack on the advancing side blade, more of
that blade falls in the driven region. On the retreating
side, more of the blade is in the stall region. A small
section near the root experiences a reversed flow, therefore
the size of the driven region on the retreating side
is reduced.
Figure 3-23. Blade regions in forward autorotation descent.
Forward
Driven

Region
Driving

Region
Retreating

Side
Stall

Region
Advancing

Side
3-12
4-1
Note: In this chapter, it is assumed that the helicopter has
a counterclockwise main rotor blade rotation as viewed
from above. If flying a helicopter with a clockwise rotation,
you will need to reverse left and right references,
particularly in the areas of rotor blade pitch change, antitorque
pedal movement, and tail rotor thrust.
There are four basic controls used during flight. They
are the collective pitch control, the throttle, the cyclic
pitch control, and the antitorque pedals.
COLLECTIVE PITCH CONTROL
The collective pitch control, located on the left side of
the pilot’s seat, changes the pitch angle of all main rotor
blades simultaneously, or collectively, as the name
implies. As the collective pitch control is raised, there
is a simultaneous and equal increase in pitch angle of
all main rotor blades; as it is lowered, there is a simultaneous
and equal decrease in pitch angle. This is done
through a series of mechanical linkages and the amount
of movement in the collective lever determines the
amount of blade pitch change. [Figure 4-1] An
adjustable friction control helps prevent inadvertent
collective pitch movement.
Changing the pitch angle on the blades changes the
angle of attack on each blade. With a change in angle
of attack comes a change in drag, which affects the
speed or r.p.m. of the main rotor. As the pitch angle
increases, angle of attack increases, drag increases,
and rotor r.p.m. decreases. Decreasing pitch angle
decreases both angle of attack and drag, while rotor
r.p.m. increases. In order to maintain a constant rotor
r.p.m., which is essential in helicopter operations, a
proportionate change in power is required to compensate
for the change in drag. This is accomplished
with the throttle control or a correlator and/or governor,
which automatically adjusts engine power.
THROTTLE CONTROL
The function of the throttle is to regulate engine r.p.m.
If the correlator or governor system does not maintain
the desired r.p.m. when the collective is raised or lowered,
or if those systems are not installed, the throttle
Figure 4-1. Raising the collective pitch control increases the pitch angle the same amount on all blades.
4-2
has to be moved manually with the twist grip in order
to maintain r.p.m. Twisting the throttle outboard
increases r.p.m.; twisting it inboard decreases r.p.m.
[Figure 4-2]
COLLECTIVE PITCH / THROTTLE
COORDINATION
When the collective pitch is raised, the load on the
engine is increased in order to maintain desired r.p.m.
The load is measured by a manifold pressure gauge
in piston helicopters or by a torque gauge in turbine
helicopters.
In piston helicopters, the collective pitch is the primary
control for manifold pressure, and the throttle is the primary
control for r.p.m. However, the collective pitch
control also influences r.p.m., and the throttle also
　
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