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rotor. Because of the greater weight of a rotor in relation
to the power of the engine, as compared to the
weight of a propeller and the power in an airplane, the
rotor must be disconnected from the engine when you
engage the starter. A clutch allows the engine to be
started and then gradually pick up the load of the rotor.
On free turbine engines, no clutch is required, as the
gas producer turbine is essentially disconnected from
the power turbine. When the engine is started, there is
little resistance from the power turbine. This enables
the gas producer turbine to accelerate to normal idle
speed without the load of the transmission and rotor
system dragging it down. As the gas pressure increases
through the power turbine, the rotor blades begin to
turn, slowly at first and then gradually accelerate to
normal operating r.p.m.
On reciprocating helicopters, the two main types of
clutches are the centrifugal clutch and the belt drive clutch.
CENTRIFUGAL CLUTCH
The centrifugal clutch is made up of an inner assembly
and a outer drum. The inner assembly, which is connected
to the engine driveshaft, consists of shoes lined
with material similar to automotive brake linings. At
low engine speeds, springs hold the shoes in, so there is
no contact with the outer drum, which is attached to the
transmission input shaft. As engine speed increases,
centrifugal force causes the clutch shoes to move outward
and begin sliding against the outer drum. The
transmission input shaft begins to rotate, causing the
rotor to turn, slowly at first, but increasing as the friction
increases between the clutch shoes and transmission
drum. As rotor speed increases, the rotor tachometer
needle shows an increase by moving toward the engine
tachometer needle. When the two needles are superimposed,
the engine and the rotor are synchronized,
indicating the clutch is fully engaged and there is no
further slippage of the clutch shoes.
BELT DRIVE CLUTCH
Some helicopters utilize a belt drive to transmit power
from the engine to the transmission. A belt drive consists
of a lower pulley attached to the engine, an upper
pulley attached to the transmission input shaft, a belt
or a series of V-belts, and some means of applying
tension to the belts. The belts fit loosely over the
upper and lower pulley when there is no tension on
the belts. This allows the engine to be started without
any load from the transmission. Once the engine is
running, tension on the belts is gradually increased.
When the rotor and engine tachometer needles are
superimposed, the rotor and the engine are synchronized,
and the clutch is then fully engaged.
Advantages of this system include vibration isolation,
simple maintenance, and the ability to start and warm
up the engine without engaging the rotor.
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 r.p.m. is less than main
rotor r.p.m. This allows the main 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 engine gearbox in a turbine helicopter.
When the engine is driving the rotor, inclined surfaces in
the spray clutch force rollers against an outer drum. This
prevents the engine from exceeding transmission r.p.m. 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.
MAIN ROTOR SYSTEM
Main rotor systems are classified according to how the
main rotor blades move relative to the main rotor hub.
As was described in Chapter 1—Introduction to the
Helicopter, there are three basic classifications: fully
articulated, semirigid, or rigid. Some modern rotor systems
use a combination of these types.
FULLY ARTICULATED ROTOR SYSTEM
In a fully articulated rotor system, each rotor blade is
attached to the rotor hub through a series of hinges,
which allow the blade to move independently of the
others. These rotor systems usually have three or more
blades. [Figure 5-5]
Pitch Change

Axis

(Feathering)
Flapping

Hinge
　
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