rather than engine power. It is the means by which a
helicopter can be landed safely in the event of an
engine failure. In this case, you are using altitude as
potential energy and converting it to kinetic energy dur-
ing the descent and touchdown. All helicopters must
have this capability in order to be certified.
Autorotation is permitted mechanically because of a
freewheeling unit, which allows the main rotor to con-
tinue turning even if the engine is not running. In nor-
mal powered flight, air is drawn into the main rotor sys-
tem from above and exhausted downward. During
autorotation, airflow enters the rotor disc from below
as the helicopter descends. [Figure 3-20]
AUTOROTATION (VERTICAL FLIGHT)
Most autorotations are performed with forward speed.
For simplicity, the following aerodynamic explanation
is based on a vertical autorotative descent (no forward
speed) in still air. Under these conditions, the forces
that cause the blades to turn are similar for all blades
regardless of their position in the plane of rotation.
Therefore, dissymmetry of lift resulting from helicop-
ter airspeed is not a factor.
During vertical autorotation, the rotor disc is divided
into three regions as illustrated in figure 3-21—the
Figure 3-20. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal
speed. In effect, the blades are “gliding” in their rotational plane.
Figure 3-21. Blade regions in vertical autorotation descent.
Figure 3-19. The horizontal component of lift accelerates the
helicopter toward the center of the turn.
Centripetal Force□
(Horizontal Component of Lift)
Vertical□
Component□
of Lift
Bank□
Angle
Resultant□
Lift
Weight
Centrifugal□
Force (Inertia)
Normal Powered Flight Autorotation
Direction
of Flight
Direction
ofFlight
Stall□
Region 25%
Driven□
Region 30% Driving□
Region 45%
driven region, the driving region, and the stall region.
Figure 3-22 shows four blade sections that illustrate
force vectors. Part A is the driven region, B and D are
points of equilibrium, part C is the driving region, and
part E is the stall region. Force vectors are different in
each region because rotational relative wind is slower
near the blade root and increases continually toward
the blade tip. Also, blade twist gives a more positive
angle of attack in the driving region than in the driven
region. The combination of the inflow up through the
rotor with rotational relative wind produces different
combinations of aerodynamic force at every point
along the blade.
The driven region, also called the propeller region, is
nearest the blade tips. Normally, it consists of about 30
Figure 3-22. Force vectors in vertical autorotation descent.
B & D
Rotational□
Relative Wind
Lift
TAF
TAF
Total□
Aerodynamic□
Force Aft□
of Axis of□
Rotation
Total□
Aerodynamic□
Force Forward□
of Axis of□
Rotation
Angle of□
Attack 2ϒ Drag
Chord Line
Inflow Up□
Through Rotor
Resultant□
Relative Wind
Equilibrium
Drag
Inflow TAF
Angle of□
Attack 6ϒ
Drag Driving□
Region
Inflow
□Axis of□
Rotation
Angle of□
Attack 24ϒ
(Blade is Stalled)
TAF
Drag
Stall□
Region
Inflow
Driven□
Range
Driven□
Region
Drag
Point of□
Equilibrium
Point of□
Equilibrium
Driving□
Region
Stall□
Region
Drag
Autorotative Force
B & D
Lift
Lift
Lift
percent of the radius. In the driven region, part A of fig-
ure 3-22, the total aerodynamic force acts behind the
axis of rotation, resulting in a overall drag force. The
driven region produces some lift, but that lift is offset
by drag. The overall result is a deceleration in the rota-
tion of the blade. The size of this region varies with the
blade pitch, rate of descent, and rotor r.p.m. When
changing autorotative r.p.m., blade pitch, or rate of
descent, the size of the driven region in relation to the
other regions also changes.
There are two points of equilibrium on the blade—one
between the driven region and the driving region, and
one between the driving region and the stall region. At
points of equilibrium, total aerodynamic force is
aligned with the axis of rotation. Lift and drag are pro-
duced, but the total effect produces neither acceleration
nor deceleration.
The driving region, or autorotative region, normally
lies between 25 to 70 percent of the blade radius. Part
C of figure 3-22 shows the driving region of the blade,
which produces the forces needed to turn the blades
during autorotation. Total aerodynamic force in the
driving region is inclined slightly forward of the axis of
rotation, producing a continual acceleration force. This
inclination supplies thrust, which tends to accelerate
the rotation of the blade. Driving region size varies
with blade pitch setting, rate of descent, and rotor r.p.m.
By controlling the size of this region you can adjust
autorotative r.p.m. For example, if the collective pitch
is raised, the pitch angle increases in all regions. This
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.
A constant rotor r.p.m. is achieved by adjusting the col-
lective pitch so blade acceleration forces from the driv-
ing 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 for-
ward 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, there-
fore 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
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 rota-
tion, you will need to reverse left and right references,
particularly in the areas of rotor blade pitch change, anti-
torque 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 simul-
taneous 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 com-
pensate for the change in drag. This is accomplished
with the throttle control or a correlator and/or gover-
nor, 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 low-
ered, 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.
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 pri-
mary control for r.p.m. However, the collective pitch
control also influences r.p.m., and the throttle also
influences manifold pressure; therefore, each is consid-
ered to be a secondary control of the other’s function.
Both the tachometer (r.p.m. indicator) and the manifold
pressure gauge must be analyzed to determine which
control to use. Figure 4-3 illustrates this relationship.
CORRELATOR / GOVERNOR
A correlator is a mechanical connection between the
collective lever and the engine throttle. When the col-
lective lever is raised, power is automatically increased
and when lowered, power is decreased. This system
maintains r.p.m. close to the desired value, but still
requires adjustment of the throttle for fine tuning.
A governor is a sensing device that senses rotor and
engine r.p.m. and makes the necessary adjustments in
order to keep rotor r.p.m. constant. In normal operations,
once the rotor r.p.m. is set, the governor keeps the r.p.m.
constant, and there is no need to make any throttle adjust-
ments. Governors are common on all turbine helicopters
and used on some piston powered helicopters.
Some helicopters do not have correlators or governors
and require coordination of all collective and throttle
movements. When the collective is raised, the throttle
must be increased; when the collective is lowered, the
throttle must be decreased. As with any aircraft control,
large adjustments of either collective pitch or throttle
should be avoided. All corrections should be made
through the use of smooth pressure.
CYCLIC PITCH CONTROL
The cyclic pitch control tilts the main rotor disc by
changing the pitch angle of the rotor blades in their
cycle of rotation. When the main rotor disc is tilted, the
horizontal component of lift moves the helicopter in
the direction of tilt. [Figure 4-4]
Figure 4-2. A twist grip throttle is usually mounted on the end
of the collective lever. Some turbine helicopters have the
throttles mounted on the overhead panel or on the floor in
the cockpit.
If□
Manifold□
Pressure□
is
and□
R.P .M.□
is
Solution
Low
Low
Low
LowHigh
High
High
High
Increasing the throttle increases manifold□
pressure and r.p.m.
Lowering the collective pitch decreases□
manifold pressure and increases r.p.m.
Raising the collective pitch increases□
manifold pressure and decreases r.p.m.
Reducing the throttle decreases manifold□
pressure and r.p.m.
Figure 4-3. Relationship between manifold pressure, r.p.m.,
collective, and throttle.
Figure 4-4. The cyclic pitch control may be mounted verti-
cally between the pilot’s knees or on a teetering bar from a
single cyclic located in the center of the helicopter. The cyclic
can pivot in all directions.
The rotor disc tilts in the direction that pressure is applied
to the cyclic pitch control. If the cyclic is moved forward,
the rotor disc tilts forward; if the cyclic is moved aft, the
disc tilts aft, and so on. Because the rotor disc acts like a
gyro, the mechanical linkages for the cyclic control rods
are rigged in such a way that they decrease the pitch angle
of the rotor blade approximately 90° before it reaches the
direction of cyclic displacement, and increase the pitch
angle of the rotor blade approximately 90° after it passes
the direction of displacement. An increase in pitch angle
increases angle of attack; a decrease in pitch angle
decreases angle of attack. For example, if the cyclic is
moved forward, the angle of attack decreases as the rotor
blade passes the right side of the helicopter and increases
on the left side. This results in maximum downward
deflection of the rotor blade in front of the helicopter and
maximum upward deflection behind it, causing the rotor
disc to tilt forward.
ANTITORQUE PEDALS
The antitorque pedals, located on the cabin floor by the
pilot’s feet, control the pitch, and therefore the thrust,
of the tail rotor blades. [Figure 4-5] . The main purpose
of the tail rotor is to counteract the torque effect of the
main rotor. Since torque varies with changes in power,
the tail rotor thrust must also be varied. The pedals are
connected to the pitch change mechanism on the tail
rotor gearbox and allow the pitch angle on the tail rotor
blades to be increased or decreased.
HEADING CONTROL
Besides counteracting torque of the main rotor, the tail
rotor is also used to control the heading of the helicopter
while hovering or when making hovering turns. Hovering
turns are commonly referred to as “pedal turns.”
In forward flight, the antitorque pedals are not used to
control the heading of the helicopter, except during por-
tions of crosswind takeoffs and approaches. Instead they
are used to compensate for torque to put the helicopter in
longitudinal trim so that coordinated flight can be main-
tained. The cyclic control is used to change heading by
making a turn to the desired direction.
The thrust of the tail rotor depends on the pitch angle of
the tail rotor blades. This pitch angle can be positive, neg-
ative, or zero. A positive pitch angle tends to move the tail
to the right. A negative pitch angle moves the tail to the
left, while no thrust is produced with a zero pitch angle.
With the right pedal moved forward of the neutral posi-
tion, the tail rotor either has a negative pitch angle or a
small positive pitch angle. The farther it is forward, the
larger the negative pitch angle. The nearer it is to neu-
tral, the more positive the pitch angle, and somewhere
in between, it has a zero pitch angle. As the left pedal is
moved forward of the neutral position, the positive pitch
angle of the tail rotor increases until it becomes maxi-
mum with full forward displacement of the left pedal.
If the tail rotor has a negative pitch angle, tail rotor
thrust is working in the same direction as the torque of
the main rotor. With a small positive pitch angle, the
tail rotor does not produce sufficient thrust to overcome
the torque effect of the main rotor during cruise flight.
Therefore, if the right pedal is displaced forward of
neutral during cruising flight, the tail rotor thrust does
not overcome the torque effect, and the nose yaws to
the right. [Figure 4-6]
With the antitorque pedals in the neutral position, the tail
rotor has a medium positive pitch angle. In medium pos-
itive pitch, the tail rotor thrust approximately equals the
torque of the main rotor during cruise flight, so the heli-
copter maintains a constant heading in level flight.
Figure 4-5. Antitorque pedals compensate for changes in
torque and control heading in a hover.
Tail MovesTail Moves
Negative or Low□
Positive Pitch
Medium□
Positive Pitch
High Positive□
Pitch
Figure 4-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.
If the left pedal is in a forward position, the tail rotor
has a high positive pitch position. In this position, tail
rotor thrust exceeds the thrust needed to overcome
torque effect during cruising flight so the helicopter
yaws to the left.
The above explanation is based on cruise power and air-
speed. Since the amount of torque is dependent on the
amount of engine power being supplied to the main rotor,
the relative positions of the pedals required to counteract
torque depend upon the amount of power being used at
any time. In general, the less power being used, the
greater the requirement for forward displacement of the
right pedal; the greater the power, the greater the forward
displacement of the left pedal.
The maximum positive pitch angle of the tail rotor is
generally somewhat greater than the maximum nega-
tive pitch angle available. This is because the primary
purpose of the tail rotor is to counteract the torque of
the main rotor. The capability for tail rotors to produce
thrust to the left (negative pitch angle) is necessary,
because during autorotation the drag of the transmis-
sion tends to yaw the nose to the left, or in the same
direction the main rotor is turning.