King Air C90GTi, Pilot Training Manual

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PRIMARY GOVERNOR
T h e p r i m a r y g ov e r n o r ( F i g u r e 7 - 2 8 ) i s
needed to convert a variable-pitch propeller
into a constant-speed propeller. It does this
by changing blade angle to maintain the pro-
peller speed the operator has selected. The
primary governor can maintain any selected
propeller speed from approximately 1,600
r pm to 1,900 r pm.
Suppose an air plane is in nor mal cr uising
flight with the propeller turning 1,900 r pm.
If the pilot trims the air plane down into a
descent without changing power, the air-
s p e e d w i l l i n c r e a s e . T h i s d e c r e a s e s t h e
angle of attack of the propeller blades, caus-
ing less drag on the propeller, thus begin-
ning to increase its rpm. Since this propeller
h a s a v a r i a b l e - p i t c h c a p a b i l i t i e s a n d i s
equipped with a gover nor set at 1,900 r pm,
t h e g ove r n o r w i l l s e n s e t h i s “ ove r s p e e d ”
c o n d i t i o n a n d i n c r e a s e s bl a d e a n g l e t o a
h i g h e r p i t c h . T h e h i g h e r p i t c h i n c r e a s e s
the blade’s angle of attack, slowing it back
to 1,900 r pm, or “onspeed.”
Likewise, if the air plane moves from cr uise
to climb airspeeds without a power change,
the propeller r pm tends to decrease, but the
governor responds to this “underspeed” con-
dition by decreasing blade angle to a lower
p i t c h , a n d t h e r p m r e t u r n s t o i t s o r i g i n a l
value. Thus the gover nor gives “constant-
speed” characteristics to the variable-pitch
propeller.
Power changes, as well as airspeed changes,
cause the propeller to momentarily experi-
ence overspeed or underspeed conditions,
but again the gover nor reacts to maintain
the onspeed condition.
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-29
FOR TRAINING PURPOSES ONLY
7
POWERPLANT
LEGEND
OIL UNDER PRESSURE
RETURN OIL
FROM
OIL PUMP
COUNTERWEIGHTS
TO
PROPELLER
UNDERSPEED
BETA VALVE
ONSPEED
TO TANK
OVERSPEED
PILOT
VALVE
Figure 7-28. Primary Governor Diagram