Main rotor system
Landing gear
Tail rotor system
Airframe
Fuselage
Transmission
Powerplant
Figure 4-1. The major components of a helicopter are the airframe, fuselage, landing gear, powerplant, transmission, main rotor
system,and tail rotor system.
Fuselage
The fuselage, the outer core of the airframe, is an aircraft’s
main body section that houses the cabin that holds the crew,
passengers, and cargo. Helicopter cabins have a variety of
seating arrangements. Most have the pilot seated on the right
side, although there are some with the pilot seated on the
left side or center. The fuselage also houses the engine, the
transmission, avionics, flight controls, and the powerplant.
[Figure 4-1]
Main Rotor System
The rotor system is the rotating part of a helicopter which
generates lift. The rotor consists of a mast, hub, and rotor
blades. The mast is a hollow cylindrical metal shaft which
extends upwards from and is driven and sometimes supported
by the transmission. At the top of the mast is the attachment
point for the rotor blades called the hub. The rotor blades are
then attached to the hub by any number of different methods.
Main rotor systems are classified according to how the main
rotor blades are attached and move relative to the main rotor
hub. There are three basic classifications: semirigid, rigid,
or fully articulated. Some modern rotor systems, such as the
bearingless rotor system, use an engineered combination of
these types.
Semirigid Rotor System
A semirigid rotor system is usually composed of two blades
that are rigidly mounted to the main rotor hub. The main rotor
hub is free to tilt with respect to the main rotor shaft on what
is known as a teetering or flapping hinge. This allows the
blades to flap together as a unit. As one blade flaps up, the
other flaps down. Since there is no vertical drag hinge, lead/
lag forces are absorbed and mitigated by blade bending. The
semirigid rotor is also capable of feathering, which means that
the pitch angle of the blade changes. This is made possible
by the feathering hinge. [Figure 4-2]
If the semirigid rotor system is an underslung rotor, the center
of gravity (CG) is below where it is attached to the mast. This
underslung mounting is designed to align the blade’s center
of mass with a common flapping hinge so that both blades’
centers of mass vary equally in distance from the center of
rotation during flapping. The rotational speed of the system
tends to change, but this is restrained by the inertia of the
engine and flexibility of the drive system. Only a moderate
amount of stiffening at the blade root is necessary to handle
this restriction. Simply put, underslinging effectively
eliminates geometric imbalance. [Figure 4-3]
Static stops
Pitch horn
Main rotor mast
Feathering hinge
Teetering hinge
Figure 4-2. The teetering hinge allows the main rotor hub to tilt, and
the feathering hinge enables the pitch angle of the blades to change.
Center of gravity
Center of gravity
Mast axis
Figure 4-3. With an underslung rotor, the center of gravity (CG) remains in the same approximate location relative to the mast before
and after rotor tilt.
The underslung rotor system mitigates the lead/lag forces by
mounting the blades slightly lower than the usual plane of
rotation, so the lead/lag forces are minimized. As the blades
cone upward, the center of pressures of the blades are almost
in the same plane as the hub. Whatever stresses are remaining
bend the blades for compliance.
Helicopters with semirigid rotors are vulnerable to a
condition known as mast bumping which can cause the rotor
flap stops to shear the mast. The mechanical design of the
semirigid rotor system dictates downward flapping of the
blades must have some physical limit. Mast bumping is the
result of excessive rotor flapping. Each rotor system design
has a maximum flapping angle. If flapping exceeds the design
value, the static stop will contact the mast. The static stop is
a component of the main rotor providing limited movement
of strap fittings and a contoured surface between the mast
and hub. It is the violent contact between the static stop and
the mast during flight that causes mast damage or separation.
This contact must be avoided at all costs.
Mast bumping is directly related to how much the blade
system flaps. In straight and level flight, blade flapping is
minimal, perhaps 2° under usual flight conditions. Flapping
angles increase slightly with high forward speeds, at low
rotor rpm, at high-density altitudes, at high gross weights, and
when encountering turbulence. Maneuvering the aircraft in a
sideslip or during low-speed flight at extreme CG positions
can induce larger flapping angles.
Rigid Rotor System
The rigid rotor system shown in Figure 4-4 is mechanically
simple, but structurally complex because operating loads
must be absorbed in bending rather than through hinges. In
this system, the blade roots are rigidly attached to the rotor
hub. Rigid rotor systems tend to behave like fully articulated
systems through aerodynamics, but lack flapping or lead/
lag hinges. Instead, the blades accommodate these motions
by bending. They cannot flap or lead/lag, but they can be
feathered. As advancements in helicopter aerodynamics
Good control
response
Simple, easy to
hangar due to two
blades
Simple design,
crisp response
High aerodynamic
drag. More complex,
greater cost.
Reaction to control
input not as quick
as articulated head.
Vibration can be
higher than multi-
bladed articulated
systems.
Higher vibration than
articulated rotor.
DisadvantagesSystem Type Advantages
Articulated
Semirigid
(Teetering,
Underslung, or
See-Saw)
Rigid
Figure 4-5. Differences in handling between the types of rotor
systems.
Main rotor blades
Main rotor hub
Blade pitch horns
Main rotor blades
Main rotor mast
Pitch change links
Figure 4-4. Four-blade hingeless (rigid) main rotor. Rotor blades are comprised of glass fiber reinforced material. The hub is a single
piece of forged rigid titanium.
and materials continue to improve, rigid rotor systems may
become more common because the system is fundamentally
easier to design and offers the best properties of both
semirigid and fully articulated systems.
The rigid rotor system is very responsive and is usually
not susceptible to mast bumping like the semirigid systems
because the rotor hubs are mounted solid to the main rotor
mast. This allows the rotor and fuselage to move together
as one entity and eliminates much of the oscillation usually
present in the other rotor systems. Other advantages of the rigid
rotor include a reduction in the weight and drag of the rotor
hub and a larger flapping arm, which significantly reduces
control inputs. Without the complex hinges, the rotor system
becomes much more reliable and easier to maintain than the
other rotor configurations. A disadvantage of this system is
the quality of ride in turbulent or gusty air. Because there are
no hinges to help absorb the larger loads, vibrations are felt
in the cabin much more than with other rotor head designs.
There are several variations of the basic three rotor head
designs. The bearingless rotor system is closely related to
the articulated rotor system but has no bearings or hinges.
This design relies on the structure of blades and hub to absorb
stresses. The main difference between the rigid rotor system
and the bearingless system is that the bearingless system has
no feathering bearing—the material inside the cuff is twisted
by the action of the pitch change arm. Nearly all bearingless
rotor hubs are made of fiber-composite materials. The
differences in handling between the types of rotor system
are summarized in Figure 4-5.
Fully Articulated Rotor System
Fully articulated rotor systems allow each blade to lead/lag
(move back and forth in plane), flap (move up and down
about an inboard mounted hinge) independent of the other
blades, and feather (rotate about the pitch axis to change lift).
[Figures 4-6 and 4-7] Each of these blade motions is related
Rotor blade
Lagging positionLeading position
Rotor hub Center of rotation
Lead/lag hinge
(Vertical hinge)
Pure Radial Position
Lead/lag or drag hinge
Lagging position
Leading position
Figure 4-6. Lead/lag hinge allows the rotor blade to move back
and forth in plane.
Figure 4-7. Fully articulated flapping hub. Figure 4-8. Fully articulated rotor blade with flapping hinge.
to the others. Fully articulated rotor systems are found on
helicopters with more than two main rotor blades.
As the rotor spins, each blade responds to inputs from the
control system to enable aircraft control. The center of lift
on the whole rotor system moves in response to these inputs
to effect pitch, roll, and upward motion. The magnitude of
this lift force is based on the collective input, which changes
pitch on all blades in the same direction at the same time. The
location of this lift force is based on the pitch and roll inputs
from the pilot. Therefore, the feathering angle of each blade
(proportional to its own lifting force) changes as it rotates
with the rotor, hence the name “cyclic control.”
As the lift on a given blade increases, it tends to flap upwards.
The flapping hinge for the blade permits this motion and is
balanced by the centrifugal force of the weight of the blade,
which tries to keep it in the horizontal plane. [Figure 4-8]
Either way, some motion must be accommodated. The
centrifugal force is nominally constant; however, the flapping
force is affected by the severity of the maneuver (rate of
climb, forward speed, aircraft gross weight). As the blade
flaps, its CG changes. This changes the local moment of
inertia of the blade with respect to the rotor system and it
speeds up or slows down with respect to the rest of the blades
and the whole rotor system. This is accommodated by the
lead/lag or drag hinge, shown in Figure 4-9, and is easier to
visualize with the classical ‘ice skater doing a spin’ image.
As the skater moves her arms in, she spins faster because her
inertia changes but her total energy remains constant (neglect
friction for purposes of this explanation). Conversely, as
her arms extend, her spin slows. This is also known as the
conservation of angular momentum. An in-plane damper
typically moderates lead/lag motion.
Following a single blade through a single rotation beginning
at some neutral position, as load increases from increased
feathering, it flaps up and leads forward. As it continues
Figure 4-10. Tandem rotor heads.
Pitch horn
Pitch change axis (feathering)
Drag hingeFlapping hinge
Damper
Figure 4-9. Drag hinge.
around, it flaps down and lags backward. At the lowest
point of load, it is at its lowest flap angle and also at its most
‘rearward’ lag position. Because the rotor is a large, rotating
mass, it behaves somewhat like a gyroscope. The effect of
this is that a control input is usually realized on the attached
body at a position 90° prior to the control input displacement
in the axis of rotation. This is accounted for by the designers
through placement of the control input to the rotor system
so that a forward input of the cyclic control stick results in a
nominally forward motion of the aircraft. The effect is made
transparent to the pilot.
Older hinge designs relied on conventional metal bearings. By
basic geometry, this precludes a coincident flapping and lead/
lag hinge and is cause for recurring maintenance. Newer rotor
systems use elastomeric bearings, arrangements of rubber
and steel that can permit motion in two axes. Besides solving
some of the above-mentioned kinematic issues, these bearings
are usually in compression, can be readily inspected, and
eliminate the maintenance associated with metallic bearings.
Elastomeric bearings are naturally fail-safe, and their wear
is gradual and visible. The metal-to-metal contact of older
bearings and the need for lubrication is eliminated in this design.
Tandem Rotor
Tandem rotor (sometimes referred to as dual rotor)
helicopters have two large horizontal rotor assemblies; a twin
rotor system, instead of one main assembly, and a smaller
tail rotor. [Figure 4-10] Single rotor helicopters need an
anti-torque system to neutralize the twisting momentum
produced by the single large rotor. Tandem rotor helicopters,
however, use counter-rotating rotors, with each canceling
out the other’s torque. Counter-rotating rotor blades will not
collide with and destroy each other if they flex into the other
rotor’s pathway. This configuration also has the advantage
of being able to hold more weight with shorter blades, since
there are two sets. Also, all of the power from the engines can
be used for lift, whereas a single rotor helicopter uses power
to counter the torque.
Coaxial Rotors
A coaxial rotor system is a pair of rotors mounted on the same
shaft but turning in opposite directions. This design eliminates
the need for a tail rotor or other antitorque mechanisms, and
since the blades turn in opposite directions, the effects of
dissymmetry of lift are avoided. The main disadvantage
of coaxial rotors is the increased mechanical complexity
of the rotor system. Numerous Russian helicopters, such
as the Kaman Ka-31 and Ka-50, along with the Sikorsky
experimental X2 use a coaxial rotor design.
Intermeshing Rotors
An intermeshing rotor system is a set of two rotors turning
in the opposite directions with each rotor mast mounted on
the helicopter with a slight angle, so the blades intermesh
without colliding. This design also eliminates the need for an
antitorque system, which provides more engine power for lift.
However, neither rotor lifts directly vertical which reduces
each rotor’s efficiency. The Kaman HH-43, which was used
by the USAF in a firefighting role and the Kaman K-MAX
are examples of an intermeshing rotor systems.
Swash Plate Assembly
The purpose of the swash plate is to convert stationary
control inputs from the pilot into rotating inputs which can be
connected to the rotor blades or control surfaces. It consists
of two main parts: stationary swash plate and rotating swash
plate. [Figure 4-11]
Blade rotation
Blade rotation
Tail rotor thrust
to compensate
for torque
Torque
Torque
Rotating swash plate
Control rod
Stationary swash plate
Pitch link
Drive link
Inner and outer parts turning at same rpm
Outer turning much faster than inner
Figure 4-11. Stationary and rotating swash plate.
Figure 4-13. Antitorque rotor produces thrust to oppose torque.
Figure 4-12. Freewheeling unit in normal drive position and
freewheeling position. Note that in the top example, the engine
output shaft (inner part) drives the rotor shaft (outer part) at the
same speed (normal flight). In the bottom example, the rotor shaft
(outer part) breaks free under autorotation, as it turns faster than
the driver shaft (inner part).
changes, or to make heading changes while hovering. Most
helicopters drive the tail rotor shaft from the transmission
to ensure tail rotor rotation (and hence control) in the event
that the engine quits. Usually, negative antitorque thrust is
needed in autorotations to overcome transmission friction.
The stationary swash plate is mounted around the main rotor
mast and connected to the cyclic and collective controls by a
series of pushrods. It is restrained from rotating by an anti-
drive link but can tilt in all directions and move vertically.
The rotating swash plate is mounted to the stationary swash
plate by means of a uniball sleeve. It is connected to the mast
by drive links and must rotate in constant relationship with
the main rotor mast. Both swash plates tilt and slide up and
down as one unit. The rotating swash plate is connected to
the pitch horns by the pitch links.
Freewheeling Unit
Since lift in a helicopter is provided by rotating airfoils,
these airfoils must be free to rotate if the engine fails. The
freewheeling unit automatically disengages the engine from
the main rotor when engine revolutions per minute (rpm)
is less than main rotor rpm. [Figure 4-12] This allows the
main rotor and tail rotor to continue turning at normal in-
flight speeds. The most common freewheeling unit assembly
consists of a one-way sprag clutch located between the engine
and main rotor transmission. This is usually in the upper
pulley in a piston helicopter or mounted on the accessory
gearbox in a turbine helicopter. When the engine is driving
the rotor, inclined surfaces in the sprag clutch force rollers
against an outer drum. This prevents the engine from
exceeding transmission rpm. If the engine fails, the rollers
move inward, allowing the outer drum to exceed the speed
of the inner portion. The transmission can then exceed the
speed of the engine. In this condition, engine speed is less
than that of the drive system, and the helicopter is in an
autorotative state.
Antitorque System
Helicopters with a single, main rotor system require a
separate antitorque system. This is most often accomplished
through a variable pitch, antitorque rotor or tail rotor.
[Figure 4-13] Pilots vary the thrust of the antitorque system to
maintain directional control whenever the main rotor torque
Air jet
Downwash
Lift
Rotating nozzle
Main rotor wake
Air intake
Figure 4-15. While in a hover, Coanda effect supplies approximately
two-thirds of the lift necessary to maintain directional control.
The rest is created by directing the thrust from the controllable
rotating nozzle.
Figure 4-14. Fenestron or “fan-in-tail” antitorque system. This design
provides an improved margin of safety during ground operations.
Fenestron
Another form of antitorque system is the Fenestron or “fan-
in-tail” design. This system uses a series of rotating blades
shrouded within a vertical tail. Because the blades are located
within a circular duct, they are less likely to come into contact
with people or objects. [Figure 4-14]
NOTAR®
Using the natural characteristics of helicopter aerodynamics,
the NOTAR ® antitorque system provides safe, quiet,
responsive, foreign object damage (FOD) resistant directional
control. The enclosed variable-pitch composite blade fan
produces a low pressure, high volume of ambient air to
pressurize the composite tailboom. The air is expelled
through two slots which run the length of the tailboom on the
right side, causing a boundary-layer control called the Coanda
effect. The result is that the tailboom becomes a “wing,”
flying in the downwash of the rotor system, producing up to
60 percent of the antitorque required in a hover. The balance
of the directional control is accomplished by a rotating direct
jet thruster. In forward flight, the vertical stabilizers provide
the majority of the antitorque; however, directional control
remains a function of the direct jet thruster. The NOTAR ®
antitorque system eliminates some of the mechanical
disadvantages of a tail rotor, including long drive shafts,
hanger bearings, intermediate gearboxes and 90° gearboxes.
[Figure 4-15]
Antitorque Drive Systems
The antitorque drive system consists of an antitorque drive
shaft and a antitorque gearbox mounted at the end of the tail
boom. The drive shaft may consist of one long shaft or a
series of shorter shafts connected at both ends with flexible
couplings. This allows the drive shaft to flex with the tail
boom. The tail rotor gearbox provides a right-angle drive
for the tail rotor and may also include gearing to adjust the
output to optimum tail rotor rpm. [Figure 4-16] Tail rotors
may also have an intermediate gearbox to turn the power up
a pylon or vertical fin.
Engines
Reciprocating Engines
Reciprocating engines, also called piston engines, are
generally used in smaller helicopters. Most training
helicopters use reciprocating engines because they are
relatively simple and inexpensive to operate. Refer to the
Pilot’s Handbook of Aeronautical Knowledge for a detailed
explanation and illustrations of the piston engine.
Turbine Engines
Turbine engines are more powerful and are used in a wide
variety of helicopters. They produce a tremendous amount
of power for their size but are generally more expensive
to operate. The turbine engine used in helicopters operates
differently from those used in airplane applications. In most
applications, the exhaust outlets simply release expended
gases and do not contribute to the forward motion of the
helicopter. Approximately 75 percent of the incoming airflow
is used to cool the engine.
The gas turbine engine mounted on most helicopters is
made up of a compressor, combustion chamber, turbine,
and accessory gearbox assembly. The compressor draws
filtered air into the plenum chamber and compresses it.
Common type filters are centrifugal swirl tubes where debris
is ejected outward and blown overboard prior to entering
the compressor, or engine barrier filters (EBF), similar to
the K&N filter element used in automotive applications.