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speed, the torque (power) exerted by the engine at the
propeller shaft must equal the opposing load provided
by the resistance of the air. The r.p.m. is controlled by
regulating the torque absorbed by the propeller—in
other words by increasing or decreasing the
resistance offered by the air to the propeller. In the
case of a fixed-pitch propeller, the torque absorbed
by the propeller is a function of speed, or r.p.m. If the
power output of the engine is changed, the engine will
accelerate or decelerate until an r.p.m. is reached at
which the power delivered is equal to the power
absorbed. In the case of a constant-speed propeller,
the power absorbed is independent of the r.p.m., for
by varying the pitch of the blades, the air resistance
and hence the torque or load, can be changed without
reference to propeller speed. This is accomplished
with a constant-speed propeller by means of a
governor. The governor, in most cases, is geared to
the engine crankshaft and thus is sensitive to changes
in engine r.p.m.
The pilot controls the engine r.p.m. indirectly by means
of a propeller control in the cockpit, which is
connected to the governor. For maximum takeoff
power, the propeller control is moved all the way
forward to the low pitch/high r.p.m. position, and the
throttle is moved forward to the maximum allowable
manifold pressure position. To reduce power for climb
or cruise, manifold pressure is reduced to the desired
value with the throttle, and the engine r.p.m. is reduced
by moving the propeller control back toward the high
pitch/low r.p.m. position until the desired r.p.m. is
observed on the tachometer. Pulling back on the
propeller control causes the propeller blades to move
to a higher angle. Increasing the propeller blade angle
(of attack) results in an increase in the resistance of the
air. This puts a load on the engine so it slows down. In
other words, the resistance of the air at the higher blade
angle is greater than the torque, or power, delivered to
the propeller by the engine, so it slows down to a point
where the two forces are in balance.
When an airplane is nosed up into a climb from level
flight, the engine will tend to slow down. Since the
governor is sensitive to small changes in engine r.p.m.,
it will decrease the blade angle just enough to keep the
engine speed from falling off. If the airplane is nosed
down into a dive, the governor will increase the blade
angle enough to prevent the engine from overspeeding.
This allows the engine to maintain a constant r.p.m.,
and thus maintain the power output. Changes in
airspeed and power can be obtained by changing
r.p.m. at a constant manifold pressure; by changing
the manifold pressure at a constant r.p.m.; or by
changing both r.p.m. and manifold pressure. Thus
the constant-speed propeller makes it possible to
obtain an infinite number of power settings.
TAKEOFF, CLIMB, AND CRUISE
During takeoff, when the forward motion of the
airplane is at low speeds and when maximum power
and thrust are required, the constant-speed propeller
sets up a low propeller blade angle (pitch). The low
blade angle keeps the angle of attack, with respect to
the relative wind, small and efficient at the low speed.
[Figure 11-3]
At the same time, it allows the propeller to “slice it
thin” and handle a smaller mass of air per revolution.
This light load allows the engine to turn at maximum
r.p.m. and develop maximum power. Although the
mass of air per revolution is small, the number of
revolutions per minute is high. Thrust is maximum at
the beginning of the takeoff and then decreases as the
airplane gains speed and the airplane drag increases.
Due to the high slipstream velocity during takeoff,
the effective lift of the wing behind the propeller(s)
is increased.
As the airspeed increases after lift-off, the load on the
engine is lightened because of the small blade angle.
The governor senses this and increases the blade angle
slightly. Again, the higher blade angle, with the higher
speeds, keeps the angle of attack with respect to the
relative wind small and efficient.
Angle of
Attack
Chord
Line
Plane of
Propeller
Rotation
Angle of
Attack
Chord Line
(Blade Face)
STATIONARY FORWARD MOTION
Plane of
Propeller
Rotation
Forward
Airspeed
Relative
Wind
Relative
Wind
Figure 11-3. Propeller blade angle.
Ch 11.qxd 5/7/04 8:50 AM Page 11-4
11-5
For climb after takeoff, the power output of the engine
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AIRPLANE FLYING HANDBOOK 飞机飞行手册下(12)
