Flying Handbook Menu > Transition to Complex Airplanes > Controllable-Pitch Propeller > Constant-Speed Propeller Operation
The engine is started with the propeller control
in the low pitch/high r.p.m. position. This position reducesthe
load or drag of the propeller and the result is easier starting
and warm-up of the engine. During warm-up, the propeller blade
changing mechanism should be operated slowly and smoothly through
a full cycle. This is done by moving the propeller control (with
the manifold pressure set to produce about 1,600 r.p.m.) to
the high pitch/low r.p.m. position, allowing the r.p.m. to stabilize,
and then moving the propeller control back to the low pitch
takeoff position. This should be done for two reasons: to determine
whether the system is operating correctly, and to circulate
fresh warm oil through the propeller governor system. It should
be remembered that the oil has been trapped in the propeller
cylinder since the last time the engine was shut down. There
is a certain amount of leakage from the propeller cylinder,
and the oil tends to congeal, especially if the outside air
temperature is low. Consequently, if the propeller isn’t
exercised before takeoff, there is a possibility that the engine
may overspeed on takeoff.
An airplane equipped with a constant-speed
propeller has better takeoff performance than a similarly powered
airplane equipped with a fixed-pitch propeller. This is because
with a constant-speed propeller, an airplane can develop its
maximum rated horsepower (red line on the tachometer) while
motionless. An airplane with a fixedpitch propeller, on the
other hand, must accelerate down the runway to increase airspeed
and aerodynamically unload the propeller so that r.p.m. and
horsepower can steadily build up to their maximum. With a constantspeed
propeller, the tachometer reading should come up to within 40
r.p.m. of the red line as soon as full power is applied, and
should remain there for the entire takeoff.
Excessive manifold pressure raises the cylinder
compression pressure, resulting in high stresses within the
engine. Excessive pressure also produces high engine temperatures.
A combination of high manifold pressure and low r.p.m. can induce
damaging detonation. In order to avoid these situations, the
following sequence should be followed when making power changes.
• When increasing power, increase the
r.p.m. first, and then the manifold pressure.
• When decreasing power, decrease the manifold pressure
first, and then decrease the r.p.m.
It is a fallacy that (in non-turbocharged engines)
the manifold pressure in inches of mercury (inches Hg) should
never exceed r.p.m. in hundreds for cruise power settings. The
cruise power charts in the AFM/POH should be consulted when
selecting cruise power settings. Whatever the combinations of
r.p.m. and manifold pressure listed in these charts—they
have been flight tested and approved by the airframe and powerplant
engineers for the respective airframe and engine manufacturer.
Therefore, if there are power settings such as 2,100 r.p.m.
and 24 inches manifold pressure in the power chart, they are
approved for use.
With a constant-speed propeller, a power descent
can be made without overspeeding the engine. The system compensates
for the increased airspeed of the descent by increasing the
propeller blade angles. If the descent is too rapid, or is being
made from a high altitude, the maximum blade angle limit of
the blades is not sufficient to hold the r.p.m. constant. When
this occurs, the r.p.m. is responsive to any change in throttle
Some pilots consider it advisable to set the
propeller control for maximum r.p.m. during the approach to
have full horsepower available in case of emergency. If the
governor is set for this higher r.p.m. early in the approach
when the blades have not yet reached their minimum angle stops,
the r.p.m. may increase to unsafe limits. However, if the propeller
control is not readjusted for the takeoff r.p.m. until the approach
is almost completed, the blades will be against, or very near
their minimum angle stops and there will be little if any change
in r.p.m. In case of emergency, both throttle and propeller
controls should be moved to takeoff positions.
Many pilots prefer to feel the airplane respond
immediately when they give short bursts of the throttle during
approach. By making the approach under a little power and having
the propeller control set at or near cruising r.p.m., this result
can be obtained.
Although the governor responds quickly to any
change in throttle setting, a sudden and large increase in the
throttle setting will cause a momentary overspeeding of the
engine until the blades become adjusted to absorb the increased
power. If an emergency demanding full power should arise during
approach, the sudden advancing of the throttle will cause momentary
overspeeding of the engine beyond the r.p.m. for which the governor
is adjusted. This temporary increase in engine speed acts as
an emergency power reserve.
Some important points to remember concerning
constant-speed propeller operation are:
• The red line on the tachometer not
only indicates maximum allowable r.p.m.; it also indicates the
r.p.m. required to obtain the engine’s rated horsepower.
• Amomentary propeller overspeed may occur when the throttle
is advanced rapidly for takeoff. This is usually not serious
if the rated r.p.m. is not exceeded by 10 percent for more than
• The green arc on the tachometer indicates the normal
operating range. When developing power in this range, the engine
drives the propeller. Below the green arc, however, it is usually
the windmilling propeller that powers the engine. Prolonged
operation below the green arc can be detrimental to the engine.
• On takeoffs from low elevation airports, the manifold
pressure in inches of mercury may exceed the r.p.m. This is
normal in most cases. The pilot should consult the AFM/POH for
• All power changes should be made smoothly and slowly
to avoid overboosting and/or overspeeding.