*TM 1-1500-204-23-5
3-4.4
Change 3
b.
Main Transmission. A typical main transmission
performs a number of functions (Figure 3-3.D). It
D Drives the main rotor mast assembly.
D Changes the angle of drive from the engine to the
main motor assembly.
D Provides RPM reduction through a train of spiral
bevel gears and planetary gears.
D Provides a means of driving the tail rotor and the
transmission accessories.
D Supports the main rotor assembly.
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 neces-
sary, the main transmission is mounted directly on the
engine.
(1)
Input Drive. 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 assembly, a freewheel coupling is pro-
vided in the input drive assembly, which automatically
engages to allow the engine to drive the rotor or disen-
gages the idling engine during autorotational descent.
On dual--engine, single--rotor power train systems, the
transmission has two input drive assemblies.
(2)
Tail Rotor Drive. The tail rotor is mounted on
the end of the main transmission and is driven by the
accessory gear train. A flexible splined coupling provides
a means of attaching the tail rotor drive shaft.
(3)
Generator Drive. The generator drive is driv-
en by the main transmission accessory gear train. The
generator is driven off the main transmission so that,
when the helicopter goes into autorotation and the en-
gine is idling or stopped, enough electrical power will be
left to operate instruments, radio, and electrical fuel
pumps.
Figure
3-3.D. Typical Main Transmission.
