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325
111/2°
15°
20°
30°
40°
101/2°
11°
14°
10°
10/15°
22°
26°
34°
40°
50/55°
Figure 11-4. Blade angle range (values are approximate).
Ch 11.qxd 5/7/04 8:50 AM Page 11-5
11-6
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 setting.
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
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