For climb after takeoff, the power output of the engine
is reduced to climb power by decreasing the manifold
pressure and lowering r.p.m. by increasing the blade
angle. At the higher (climb) airspeed and the higher
blade angle, the propeller is handling a greater mass of
air per second at a lower slipstream velocity. This
reduction in power is offset by the increase in propeller
efficiency. The angle of attack is again kept small by
the increase in the blade angle with an increase
in airspeed.
At cruising altitude, when the airplane is in level flight,
less power is required to produce a higher airspeed
than is used in climb. Consequently, engine power is
again reduced by lowering the manifold pressure and
increasing the blade angle (to decrease r.p.m.). The
higher airspeed and higher blade angle enable the
propeller to handle a still greater mass of air per
second at still smaller slipstream velocity. At normal
cruising speeds, propeller efficiency is at, or near
maximum efficiency. Due to the increase in blade
angle and airspeed, the angle of attack is still small
and efficient.
BLADE ANGLE CONTROL
Once the pilot selects the r.p.m. settings for the
propeller, the propeller governor automatically adjusts
the blade angle to maintain the selected r.p.m. It does
this by using oil pressure. Generally, the oil pressure
used for pitch change comes directly from the engine
lubricating system. When a governor is employed,
engine oil is used and the oil pressure is usually
boosted by a pump, which is integrated with the
governor. The higher pressure provides a quicker blade
angle change. The r.p.m. at which the propeller is to
operate is adjusted in the governor head. The pilot
changes this setting by changing the position of the
governor rack through the cockpit propeller control.
On some constant-speed propellers, changes in pitch
are obtained by the use of an inherent centrifugal
twisting moment of the blades that tends to flatten the
blades toward low pitch, and oil pressure applied to a
hydraulic piston connected to the propeller blades
which moves them toward high pitch. Another type of
constant-speed propeller uses counterweights attached
|
to the blade shanks in the hub. Governor oil pressure
and the blade twisting moment move the blades toward
the low pitch position, and centrifugal force acting on
the counterweights moves them (and the blades)
toward the high pitch position. In the first case above,
governor oil pressure moves the blades towards high
pitch, and in the second case, governor oil pressure and
the blade twisting moment move the blades toward low
pitch. A loss of governor oil pressure, therefore, will
affect each differently.
GOVERNING RANGE
The blade angle range for constant-speed propellers
varies from about 11 1/2 to 40°. The higher the speed
of the airplane, the greater the blade angle range.
[Figure 11-4]
The range of possible blade angles is termed the
propeller's governing range. The governing range is
defined by the limits of the propeller blade's travel
between high and low blade angle pitch stops. As long
as the propeller blade angle is within the governing
range and not against either pitch stop, a constant
engine r.p.m. will be maintained. However, once the
propeller blade reaches its pitch-stop limit, the engine
r.p.m. will increase or decrease with changes in
airspeed and propeller load similar to a fixed-pitch
propeller. For example, once a specific r.p.m. is
selected, if the airspeed decreases enough, the
propeller blades will reduce pitch, in an attempt to
maintain the selected r.p.m., until they contact their
low pitch stops. From that point, any further
reduction in airspeed will cause the engine r.p.m.
to decrease. Conversely, if the airspeed increases,
the propeller blade angle will increase until the
high pitch stop is reached. The engine r.p.m. will
then begin to increase.
CONSTANT-SPEED PROPELLER OPERATION
The engine is started with the propeller control in the
low pitch/high r.p.m. position. This position reduces
the 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.
|
