King Air C90GTi, Pilot Training Manual

Page: 179 / 557

The position of the low pitch stop is con-
trolled from the cockpit by the power lever.
W h e n ev e r t h e p owe r l ev e r i s a t I D L E o r
above, this stop is set at approximately 12°.
But bringing the power lever aft of IDLE
prog ressively repositions the stop to lesser
blade angles.
Before reversing can take place, the pro-
peller must be on the low pitch stop. As the
propellers reach approximately 12°, the Beta
valve is repositioned, creating the low pitch
stop. The primary governor is sensing an un-
derspeed and is directing oil pressure into
the propeller dome. The Beta valve is con-
trolling oil flow into the primary governor,
and is def ining the low pitch stop through
oil pressure.
When blade angles less than approximately
12°, the linkage pulls the Beta valve actua-
tor, readjusting the propeller blade angle as
the Beta valve allows more oil into the pro-
peller dome. The slip ring moves with the
prop dome and will def ine the low pitch stop
at a lower, or negative, blade angle. If blade
angles less than approximately 12° are re-
quested before the propeller blades are on
t h e l ow p i t c h s t o p , t h e s l i p r i n g w i l l n o t
move, and the reversing cable and linkage
may be damaged.
The region from 12° to –10° blade angle is
refer red to as the Beta range.
The Ground Fine range extends from +12° to
+3°, and the engine’s compressor speed (N
1
)
remains at the value it had when the power
l eve r wa s a t I D L E ( l ow i d l e t o h i g h i d l e )
based on condition lever position. From +3°
to –10° blade angle, the N
1
speed progres-
sively increases to a maximum value at –10°
blade angle of approxi85% ±3%.
Low Pitch Stop Operation
During non-reversing operations, the low
pitch stop prevents the propeller blades from
reducing the airflow over the empennage of
the aircraft.
The low pitch stop uses a mechanical link-
age to hydraulically control propeller blade
angle. As the propeller blades reduce angle
t h r o u g h a p p r ox i m a t e ly 2 0 ° o f p i t c h , t h e
flange mounted on the propeller dome con-
tacts the nuts located on the rods mounted
on the slip ring. The propeller dome moves
the slip ring forward, which in turn activates
the Beta valve, which controls oil pressure
into the propeller dome.
Riding in the slip ring is linkage which con-
nects the Beta valve with the slip ring, and
the power levers via a cable. As the slip ring
moves, the linkage pivots about the end with
the cable attached to it, with the Beta valve
in the middle. For reversing, the pilot repo-
sitions the linkage with the power levers,
which resets the low pitch stop.
When the Beta valve is controlling blade
angle, oil pressure supplied from the gov-
e r n o r o i l p u m p i s s u p p l y i n g p r e s s u r e
t h r o u g h t h e B e t a v a l v e t o t h e p r o p e l l e r
dome. The Beta valve modulates the amount
o f p r e s s u r e e n t e r i n g t h e p r o p e l l e r d o m e ,
c o n t r o l l i n g t h e bl a d e a n g l e . T h e p r i m a r y
gover nor must be in the underspeed condi-
tion, allowing all of the pressure flowing
from the Beta valve into the propeller dome.
If the underspeed condition did not exist
when lower blade angles are requested, the
Beta valve could not fully control the pro-
peller blade angle, and the slip ring would
not move without help from the propeller
blades. Since the propeller blades only con-
tact the slip ring when the blades are at the
low pitch stop, the request for lower blade
angles when the propellers are not on the
low pitch stop will result in damage to the
c o n t r o l c a b l e , a s i t c a n n o t e f f e c t t h e s e
changes alone.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-34
FOR TRAINING PURPOSES ONLY
7
POWERPLANT