LOW PITCH STOP
It is easy for the pilot to determine when the
propeller blade angle is at the low pitch stop.
Assuming the propeller is not feathered or in
the process of being feathered,
whenever the
propeller rpm is below the selected governor
rpm, the propeller blade angle is at the low
pitch stop.
T h i s a s s u m e s t h a t m o m e n t a r y p e r i o d s o f
u n d e r - s p e e d a r e n o t b e i n g c o n s i d e r e d .
Rather, the propeller r pm is below and stay-
ing below the selected gover nor r pm.
For example, if the propeller control is set
at 1,900 r pm but the propeller is tur ning at
less than 1,900 r pm, the blade angle is at the
low pitch stop.
On many types of air planes, the low pitch
stop is simply at the low pitch limit of travel,
deter mined by the propeller’s constr uction.
But with a reversing propeller, the extreme
travel in the low pitch direction is past 0°,
into reverse or negative blade angles (Figure
7-32). Consequently, the low pitch stop on
this propeller must be designed in such a
way t h a t i t c a n b e r e p o s i t i o n e d wh e n r e -
versing is desired.
The low pitch stop is created by mechanical
linkage sensing the blade angle. The link-
age causes a valve to close, which stops the
flow of oil pressure coming into the propeller
dome. Since this pressure causes low pitch
and reversing, once it is blocked, a low pitch
stop has been created. The low pitch stop is
commonly refer red to as the “Beta” valve.
Furthermore, the valve is spring-loaded to
cause the propeller to feather in the event of
mechanical loss of Beta valve control.
KING AIR C90GTi/C90GTx
PILOT TRAINING MANUAL
7-33
FOR TRAINING PURPOSES ONLY
7
POWERPLANT
COUNTERWEIGHT
CARBON
BLOCK
LOW-PITCH
STOP COLLAR
REVERSE
RETURN
SPRING
LOW-PITCH
STOP NUT
RING, ROD END
FEATHER RETURN
SPRINGS
Figure 7-32. Low Pitch Stop Diagram
Summary of Contents for C90GTi
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