King Air C90GTi, Pilot Training Manual

Page: 154 / 557

The power section’s job is to convert the gas
flow from the gas generator section into me-
chanical action to drive the propeller. This
is done through an integ ral planetar y gear-
box, which conver ts the high speed and low
torque of the power turbine to the low speed
and high torque required at the propeller.
The reduction ratio from power turbine shaft
r pm to propeller r pm is approximately 15:1.
COMPRESSOR BLEED VALVE
At low N
1
r pm, the axial compressors pro-
d u c e m o r e c o m p r e s s e d a i r t h a n t h e c e n -
trifugal compressor can effectively handle
(accept). A compressor bleed valve com-
pensates for this excess airflow at low r pm
by opening, to relieve this pressure. As com-
pressor speed increases, the valve closes
propor tionally until, at 80% N
1
, the valve
is fully closed (Figure 7-10). This pressure
relief helps prevent compressor stall of the
centrifugal stage.
The compressor bleed valve is a pneumatic
piston which references the pressure dif-
ferential between the axial and centrifugal
stages. Looking forward, the valve is located
at the 6 o’clock position. The function of
this valve is to prevent compressor stalls
and surges in the low N
1
r pm range (75 to
80% N
1
).
At low N
1
r pm, the valve is in the open po-
sition. At takeoff and cr uise N
1
r pm, above
approximately 80%, the bleed valve will be
closed. If the compressor bleed valve sticks
closed, a compressor stall will result. If the
valve sticks open, the ITT would be notica-
bly higher as the power lever is advanced
above 80% N
1
.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-9
FOR TRAINING PURPOSES ONLY
7
POWERPLANT
LEGEND
CONTROL PRESSURE
PISTON
DISCHARGE AIR
AMBIENT PRESSURE
INLET AIR P
3
COMPRESSOR BLEED AIR PRESSURE P
2.5
PISTON
ROLLING
DIAPHRAGM
ROLLING
DIAPHRAGM
Figure 7-10. Compressor Bleed Valve