King Air C90GTi, Pilot Training Manual

Page: 180 / 557

GROUND FINE AND REVERSE
CONTROL
T h e g e o m e t r y o f t h e p owe r l eve r l i n k a g e
through the cam box is such that power lever
increments from idle to full forward thr ust
have no effect on the position of the Beta
valve. When the power lever is moved from
idle into the reverse range, it positions the
Beta valve to direct gover nor oil pressure
t o t h e p r o p e l l e r p i s t o n , d e c r e a s i n g bl a d e
angle through zero into a negative range.
The travel of the propeller ser vo piston is
fed back to the Beta valve to null its posi-
tion and, in effect, provide inf inite negative
bl a d e a n g l e s a l l t h e way t o m a x i m u m r e -
ve r s e . T h e o p p o s i t e w i l l o c c u r wh e n t h e
power lever is moved from full reverse to
any forward position up to idle, therefore
providing the pilot with manual blade angle
control for g round handling.
Ground Fine and Reverse
Control Operation
W h e n t h e b l a d e a n g l e r e a c h e s a p p r o x i -
mately 20°, the flange extending from the
d o m e m a k e s c o n t a c t w i t h t h e B e t a n u t s
(Figure 7-33). As the propeller pitch angle
continues to decrease, each flange on the
propeller dome pushes the nut and the at-
tached Beta rod forward. As the rod moves
forward, it pulls the slip ring forward. In
t u r n , a B e t a va l ve i n s i d e t h e g ove r n o r i s
pulled into the oil pressure cutoff position.
The linkage is set to control the oil pressure
supply to the dome when the blade angle
reaches low pitch stop.
If this system were f ixed at the low pitch stop,
the propeller could not be reset throughout
the Beta range. However, the low pitch stop
can be adjusted to allow access to the Ground
Fine and Reverse ranges on the ground. The
hydraulic low pitch stop can be reset to allow
the propeller to operate in the Ground Fine
and Reverse ranges while the aircraft is on the
ground and the engines are operating.
When the power levers are lifted up and over
the idle detent into the Ground Fine range, the
Beta valve is repositioned. As the Beta arm
moves back, the Beta valve is opened, re-es-
tablishing oil flow to the propeller dome.
This allows the propeller blade to move to a
flatter pitch. As the propeller blades move to
a flatter pitch, the propeller dome and slip
ring continue forward, eventually moving the
Beta valve back into position to stop pro-
peller blades. In summary, the position of the
low pitch stop is controlled by the power
levers. When the power levers are set at idle
or above, the stop is set at approximately 12°.
When the power levers are moved aft of idle,
however, the low pitch stop is repositioned to
lesser blade angles.
The propeller can be feathered by moving the
propeller lever full aft past the detent into the
feather range. The feathering action raises
the pilot valve to the full up position. The oil
pressure is released from the propeller pitch
mechanism and the propeller feathers. In this
type of turbine engine, the propeller shaft and
N
1
shaft are not connected. Thus, the pro-
peller can be feathered with the engine run-
ning at idle power. Without an autofeather
system, in flight, the propeller will maintain
rpm unless it is manually feathered when the
engine is shut down.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-35
FOR TRAINING PURPOSES ONLY
7
POWERPLANT