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consists of a main transmission (main gearbox), a
main drive shaft, and a series of tail rotor drive shafts
with two gearboxes. The main transmission includes
input drive with freewheeling provisions if no clutch
assembly is required, output drive, and main rotor
mast. The main drive shaft between the engine and
main transmission drives the main transmission. A
series of tail rotor drive shafts with two gearboxes
(transmissions) – intermediate and tail rotor – between
the main transmission and tail rotor drive the
tail rotor.
transmission input drive on the other end. The clutch
assembly provides freewheeling (Figure 5-3). On
systems not requiring a clutch assembly, the shaft is
attached to an adapter on the engine output shaft on
one end and to the freewheel coupling of the transmission
input drive assembly on the other end.
Clutch Assembly
The clutch assembly allows for a smooth engagement
of the engine to the power train system. The clutch
is used to stop possible blade damage and shaft
shearing due to sudden torque loading. Some
MAIN DRIVE SHAFT clutches are designed to let the engine start and run
The main drive shaft (Figure 5-2) transmits torque
from the engine to the main transmission. The shaft
is a hollow, statically balanced tube. In addition to
required fittings, bolts, nuts, and washers are
provided with flexible splined or rubber couplings
for installation between the engine and transmission.
On systems using a clutch assembly, the main
shaft is attached to the clutch on one end and to the
without the rotor turning. This is very useful for
warm-up and maintenance procedures. Due to the
free power system in all gas turbine engines used by
the Army, a clutch assembly is not needed on aircraft
with gas turbine engines.
The centrifugal clutch assembly is used only with
engines of low horsepower output. When the engine
speed is increased, centrifugal force throws the
5-1
FM 1-514
clutch shoe against the inner surface of a drum,
completing the drive to the rotor. This type of clutch,
because of its slippage at low and medium speeds,
generates heat, which is harmful to the life of clutch
parts.
Freewheeling Unit
All rotary-wing aircraft have a freewheel unit located
between the engine and the main rotor or rotors.
Three basic types of freewheel units are roller, sprag
freewheel unit is to free the power train drive system
from the drag made by the dead or idling engine. By
doing this the freewheel unit makes autorotation
possible. This allows an aircraft to land safely
without engine power. All types of freewheel units
generally work in the same manner. They provide a
positive lock of the power train drive system to the
engine at any time engine speed equals rotor speed.
When rotor speed is faster than engine speed, the
freewheel unit unlocks the power train drive system
clutch, and overrunning clutch. The purpose of the from the engine.
5-2
FM 1-514
5-3
FM 1-514
MAIN TRANSMISSION
A typical main transmission
functions (Figure 5-4). It—
• Drives the main rotor
Input Drive
performs a number of Engine torque is transmitted through the main drive
shaft to the input drive, which drives the main transmission
gear trains. On systems not using a clutch
• mast assembly. assembly, a freewheel coupling is provided in the
•
Changes the angle of drive from the engine to input drive assembly, which automatically engages to
the main motor assembly. allow the engine to drive the rotor or disengages the
Provides RPM reduction through a train of idling engine during autorotational descent. On
spiral bevel gears and planetary gears. dual-engine, single-rotor power train systems, the
Provides a means of driving the tail rotor and transmission has two input drive assemblies.
the transmission accessories.
Supports the main rotor assembly. Tail Rotor Drive
•
•
The main transmission is mounted in a variety of ways
according to a particular manufacturer’s design.
Some transmissions contain a support case. The case
is an integral part of the transmission mounted
directly to the transmission deck. The transmission
may be secured to the transmission deck by a system
of tubular support assemblies. In one power train
system, the transmission is secured to the main rotor
mast support structure. In the power train system of
a reciprocating-engine-powered, observation-type
helicopter where neither shafting from engine to
transmission nor drive angle change is necessary, the
　
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