Introduction
There are three major controls in a helicopter that the pilot
must use during flight. They are the collective pitch control,
the cyclic pitch control, and the antitorque pedals or tail rotor
control. In addition to these major controls, the pilot must also
use the throttle control, which is usually mounted directly
to the collective pitch control in order to fly the helicopter.
In this chapter, the control systems described are not limited
to the single main rotor type helicopter but are employed
in one form or another in most helicopter configurations.
All examples in this chapter refer to a counterclockwise
main rotor blade rotation as viewed from above. If flying a
helicopter with a clockwise rotation, left and right references
must be reversed, particularly in the areas of rotor blade pitch
change, antitorque pedal movement, and tail rotor thrust.
Helicopter Flight Controls
Chapter 3
Figure 3-1. Raising the collective pitch control increases the pitch angle, or angle of incidence, by the same amount on all blades.
Collective Pitch Control
The collective pitch control (or simply “collective” or “thrust
lever”) is located on the left side of the pilot’s seat and is
operated with the left hand. The collective is used to make
changes to the pitch angle of the main rotor blades and does
this 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 3-1]
An adjustable friction control helps prevent inadvertent
collective pitch movement.
Changing the pitch angle on the blades changes the angle
of incidence on each blade. With a change in angle of
incidence comes a change in drag, which affects the speed
or revolutions per minute (rpm) of the main rotor. As the
pitch angle increases, angle of incidence increases, drag
increases, and rotor rpm decreases. Decreasing pitch angle
decreases both angle of incidence and drag, while rotor rpm
increases. In order to maintain a constant rotor rpm, 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 governor,
which automatically adjusts engine power.
Throttle Control
The function of the throttle is to regulate engine rpm. If
the correlator or governor system does not maintain the
desired rpm when the collective is raised or lowered, or if
those systems are not installed, the throttle must be moved
manually with the twist grip in order to maintain rpm. In
most helicopters, rotating the twist-grip throttle away from
the pilot (counter-clockwise), increases engine rpm; rotating
the twist-grip throttle towards the pilot (clockwise) decreases
engine rpm. [Figure 3-2]
Governor/Correlator
A governor is a sensing device that senses rotor and engine
rpm and makes the necessary adjustments in order to keep
rotor rpm constant. In normal operations, once the rotor
rpm is set, the governor keeps the rpm constant, and there
is no need to make any throttle adjustments. Governors are
common on all turbine helicopters (as it is a function of the
fuel control system of the turbine engine) and used on some
piston powered helicopters.
A correlator is a mechanical connection between the
collective lever and the engine throttle. When the collective
lever is raised, power is automatically increased; when
lowered, power is decreased. This system maintains rpm
close to the desired value, but still requires adjustment of
the throttle for fine tuning.
Twist grip throttle
Increasing the throttle increases
manifold pressure and rpm
Lowering the collective pitch
decreases manifold pressure
and increases rpm
Raising the collective pitch
increases manifold pressure and
decreases rpm
Reducing the throttle decreases
manifold pressure and rpm
Solutionand manifold
pressure isif rpm is
HIGH
LOW
LOW
LOW
LOW
HIGH
HIGH
HIGH
Cyclic pitch control
Cyclic pitch control
Figure 3-2. A twist grip throttle is usually mounted on the end of
the collective lever. The throttles on some turbine helicopters are
mounted on the overhead panel or on the floor in the cockpit.
Figure 3-4. The cyclic pitch control may be mounted vertically
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.
Figure 3-3. Relationship between rpm, manifold pressure, collective,
and throttle.
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.
In piston helicopters, the collective pitch is the primary control
for manifold pressure, and the throttle is the primary control
for rpm. However, the collective pitch control also influences
rpm, and the throttle also influences manifold pressure;
therefore, each is considered to be a secondary control of the
other’s function. Both the tachometer (rpm indicator) and
the manifold pressure gauge must be analyzed to determine
which control to use. Figure 3-3 illustrates this relationship.
Cyclic Pitch Control
The cyclic pitch control (or simply “cyclic”) is usually
projected upward from the cockpit floor, between the
pilot’s legs or between the two pilot seats in some models.
[Figure 3-4] This primary flight control allows the pilot to fly
the helicopter in any direction of travel: forward, rearward,
left, and right. As discussed in Chapter 2, Aerodynamics
of Flight, the total lift force is always perpendicular to the
tip-path plane of the main rotor. The purpose of the cyclic
pitch control is to tilt the tip-path plane in the direction of
the desired horizontal direction. The cyclic controls the
rotor disk tilt versus the horizon, which directs the rotor disk
thrust to enable the pilot to control the direction of travel of
the helicopter.
The rotor disk tilts in the same direction the cyclic pitch control
is moved. If the cyclic is moved forward, the rotor disk tilts
forward; if the cyclic is moved aft, the disk tilts aft, and so on.
Because the rotor disk acts like a gyro, the mechanical linkages
for the cyclic control rods are rigged in such a way that they
Figure 3-5. Antitorque pedals compensate for changes in torque
and control heading in a hover.
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 AOA; a decrease in pitch angle
decreases AOA. For example, if the cyclic is moved forward,
the AOA 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 disk 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 or other antitorque system. See Chapter 5, Helicopter
Components, Sections, and Systems, for a discussion on
these other systems. [Figure 3-5] Newton’s Third Law was
discussed in Chapter 2, General Aerodynamics, stating that
for every action there is an equal and opposite reaction.
This law applies to the helicopter fuselage and its rotation
in the opposite direction of the main rotor blades unless
counteracted and controlled. To make flight possible and
to compensate for this torque, most helicopter designs
incorporate an antitorque rotor or tail rotor. The antitorque
pedals allow the pilot to control the pitch angle of the tail
rotor blades, which in forward flight puts the helicopter in
longitudinal trim and, while at a hover, enables the pilot to
turn the helicopter 360°. The antitorque 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
The tail rotor is used to control the heading of the helicopter
while hovering or when making hovering turns, as well as
counteracting the torque of the main rotor. Hovering turns
are commonly referred to as “pedal turns.”
At speeds above translational lift, the pedals are used to
compensate for torque to put the helicopter in longitudinal
trim, so that coordinated flight can be maintained. 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, negative,
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. The maximum
positive pitch angle of the tail rotor is generally greater than
the maximum negative 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 transmission tends to yaw
the nose to the left, or in the same direction the main rotor
is turning.
From the neutral position, applying right pedal causes the
nose of the helicopter to yaw right and the tail to swing to
the left. Pressing on the left pedal has the opposite effect:
the nose of the helicopter yaws to the left and the tail swings
right. [Figure 3-6]
With the antitorque pedals in the neutral position, the tail rotor
has a medium positive pitch angle. In medium positive pitch,
the tail rotor thrust approximately equals the torque of the
main rotor during cruise flight, so the helicopter maintains
a constant heading in level flight.
A vertical fin or stabilizer is used in many single-rotor
helicopters to help aid in heading control. The fin is designed
to optimize directional stability in flight with a zero tail rotor
thrust setting. The size of the fin is crucial to this design. If
the surface is too large, the tail rotor thrust may be blocked.
Heading control would be more difficult at slower airspeeds
and at a hover and the vertical fin would then weathervane.
Helicopters that are designed with tandem rotors do not have
an antitorque rotor. The helicopter is designed with both
rotor systems rotating in opposite directions to counteract the
torque rather than a tail rotor. Directional antitorque pedals
Tail moves
Tail moves
Negative or Low Positive Pitch Medium Positive Pitch High Positive Pitch
Figure 3-6. Tail rotor pitch angle and thrust in relation to pedal positions during cruising flight.
are used for directional control of the aircraft while in flight,
as well as while taxiing with the forward gear off the ground.
In intermeshing rotor systems, which are a set of two rotors
turning in opposite directions with each rotor mast mounted
on the helicopter with a slight angle to the other so that
the blades intermesh without colliding, and a coaxial rotor
systems, which are a pair of rotors mounted one above the
other on the same shaft and turning in opposite directions, the
heading pedals control the heading of the helicopter while at
a hover by imbalancing torque between the rotors, allowing
for the torque to turn the helicopter.
Chapter Summary
This chapter introduced the pilot to the major flight controls
and how they work in relation to each other. The chapter also
correlates the use of flight controls and aerodynamics and
how the two work together to make flight possible.
Introduction
This chapter discusses the components, sections, and systems
found on most modern helicopters. Helicopters come in a
variety of sizes and shapes, but most share the same major
components. The chapter introduces the major components/
sections of the helicopter and the systems that correlate
with each. Knowing how the components and systems
work on the helicopter enables the pilot to more easily
recognize malfunctions and possible emergency situations.
Understanding the relationship of these systems allows the
pilot to make an informed decision and take the appropriate
corrective action should a problem arise.
Airframe
The airframe, or fundamental structure, of a helicopter can be
made of either metal, wood, or composite materials, or some
combination of the two. Typically, a composite component
consists of many layers of fiber-impregnated resins, bonded
to form a smooth panel. Tubular and sheet metal substructures
are usually made of aluminum, though stainless steel or
titanium are sometimes used in areas subject to higher
stress or heat. Airframe design encompasses engineering,
aerodynamics, materials technology, and manufacturing
methods to achieve favorable balances of performance,
reliability, and cost. [Figure 4-1]
Helicopter Components,
Sections, and Systems
Chapter 4