Constant-Speed Propeller
Systems
by Derek W Beck
June 2008
Constant Speed Propeller
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Adjusting propeller pitch
allows for better efficiency
(more engine power
converted into thrust) in a
variety of flight attitudes
(climb, cruise, etc)
On the C172RG, the range
is 12.0º to 26.5º
Greater pitch demands
greater torque from the
engine, thus for a given
power setting, the result is
lower RPM’s
Source: FAA-H-8083-3A Airplane Flying Handbook p. 11-4
Propeller Control
Pitch is controlled by the
(blue) pitch control lever
Increasing Pitch
Oil is supplied through the
propeller piston to a piston
head, which is pushed
down due to the oil
pressure, pushing it
against a spring and
holding it into a high-pitch
position
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Maintaining Pitch
The pressure is maintained
in the system to hold the
pitch on the propeller
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Decreasing Pitch
Oil is allowed to drain
from the system, allowing
the spring to “win” against
the oil, forcing the pitch to
high pitch
Spring
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
The Governor
Source: http://flash.aopa.org/asf/engine_prop/swf/flash.cfm?
The Governor – Controlling Pitch
Governor
controls the
oil pressure to
the propeller
(thus,
controlling
pitch)
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Control of the Governor
The propeller
control lever
tightens a
threaded shaft that
in turn tightens
the speeder
spring, which
increases
resistance against
spinning
flyweights, which
are mechanically
connected directly
to the engine…
To Prop Lever
Speeder
Spring
Flyweights
Prop Lever Forward
Prop Lever Back
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Governor Underspeed
When engine RPM is
slower than the
propeller control
setting, the speeder
spring holds the pilot
valve down and oil
flows from the
propeller…
To Prop Lever
Speeder Spring
Spinning
Flyweights
This is called
“underspeed”
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Governor On Speed
As engine RPM
increases, the tops of
the weights are
thrown outward by
centrifugal force; the
lower legs then pivot
up, raising the pilot
valve against the
force of the speeder
spring so no oil can
flow to or from the
propeller…
To Prop Lever
Speeder Spring
Spinning
Flyweights
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Governor Overspeed
The faster the
flyweights spin, the
further out they are
thrown, causing the
pilot valve to be
raised (against the
spring) and allowing
more oil to flow to
the propeller…
To Prop Lever
Speeder Spring
Spinning
Flyweights
This is called
“overspeed”
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Putting It All Together
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
On Speed
On Speed condition results when
the RPM is constant. Movement of
the cockpit controls have set the
speeder springs at the desired
RPM. The flyweights have
positioned the pilot valves to direct
oil to or from the propellers. This,
in turn, has positioned the propeller
blades at a pitch that absorbs the
engine power or RPM selected.
When the moment of RPM balance
occurs, the force of the flyweights
equals the speeder spring load.
This positions the pilot valves in
the constant RPM position with no
oil flowing to or from the
propellers.
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Overspeed
At constant speed, an Overspeed
condition results and airspeed
increases when the airplane begins
a descent or engine power is
increased. Since the pitch of the
propeller blades is too low to
absorb engine power, the engine
RPM begins to increase. At the
instant this happens, however, the
flyweights move out and raise the
pilot valves, causing oil to flow to
the propeller, increasing the pitch
of the blades in both cases. Engine
speed then slows to the original
RPM setting.
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Underspeed
If the airplane begins to climb or
engine power is decreased, an
Underspeed condition results.
Airspeed is reduced and, since the
pitch of the propeller blades is too
high, the engines begin to slow
down. At the instant this happens,
the flyweights will droop, causing
the pilot valves to move down.
Simultaneously, oil flows from the
propeller, reducing the pitch of the
blades. This automatically
increases the speed of the engines
to maintain the original RPM
setting.
Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
Other Notes
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The governor boosts the oil pressure sent to the
propeller by 3 to 6 times (e.g. to 300 PSI)
The governor range sets the limit on maintaining a
constant speed propeller
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Beyond these limits, throttle has a direct control on the
RPM’s
Within these limits, the propeller pitch adjusts for any
given throttle setting to maintain the RPM’s
In general, maintain propeller setting (RPM) higher
than throttle setting (manifold pressure)
Technical (Backup)
Propeller Efficiency
Efficiency (nu)
(1.0 = 100%) is a
function of the
advanced ratio
(J), which is
dependent on
velocity (V),
propeller speed
(n), and propeller
diameter (D)
Thus, nu =
f(V/nD)
Source: http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion7/UnifiedPropulsion7.htm