17.01.2009, 12:56
Low pitch refers to low blade angle. Most tech manuals refer to setting the low pitch and high pich stops when setting up the prop for any particular powerplant installation.
Although some older adjustable-pitch propellers could only be adjusted
on the ground, most modern adjustable-pitch propellers are designed so
that you can change the propeller pitch in flight. The first
adjustable-pitch propeller systems provided only two pitch settings?a
low-pitch setting and a high-pitch setting. Today, however, nearly all
adjustable-pitch propeller systems are capable of a range of pitch
settings.
Aconstant-speed propeller is the most common type of adjustable-pitch
propeller. The main advantage of a constant-speed propeller is that it
converts a high percentage of brake horsepower (BHP) into thrust
horsepower (THP) over a wide range of r.p.m. and airspeed
combinations. A constant-speed propeller is more efficient than other
propellers because it allows selection of the most efficient engine
r.p.m. for the given conditions.
An airplane with a constant-speed propeller has two controls-the
throttle and the propeller control. The throttle controls power
output, and the propeller control regulates engine r.p.m. and, in
turn, propeller r.p.m., which is registered on the tachometer.
Once a specific r.p.m. is selected, a governor automatically adjusts
the propeller blade angle as necessary to maintain the selected r.p.m.
For example, after setting the desired r.p.m. during cruising flight,
an increase in airspeed or decrease in propeller load will cause the
propeller blade angle to increase as necessary to maintain the
selected r.p.m. A reduction in airspeed or increase in propeller load
will cause the propeller blade angle to decrease.
The range of possible blade angles for a constant-speed propeller is
the propeller's constant-speed range and is defined by the high and
low pitch stops. As long as the propeller blade angle is within the
constant-speed range and not against either pitch stop, a constant
engine r.p.m. will be maintained. However, once the propeller blades
contact a pitch stop, the engine r.p.m. will increase or decrease as
appropriate, with changes in airspeed and propeller load. For example,
once a specific r.p.m. has been selected, if aircraft speed decreases
enough to rotate the propeller blades until they contact the low pitch
stop, any further decrease in airspeed will cause engine r.p.m. to
decrease the same way as if a fixed-pitch propeller were installed.
The same holds true when an airplane equipped with a constant-speed
propeller accelerates to a faster airspeed. As the aircraft
accelerates, the propeller blade angle increases to maintain the
selected r.p.m. until the high pitch stop is reached. Once this
occurs, the blade angle cannot increase any further and engine r.p.m.
increases.
On airplanes that are equipped with a constant-speed propeller, power
output is controlled by the throttle and indicated by a manifold
pressure gauge. The gauge measures the absolute pressure of the fuel/
air mixture inside the intake manifold and is more correctly a measure
of manifold absolute pressure (MAP). At a constant r.p.m. and
altitude, the amount of power produced is directly related to the fuel/
air flow being delivered to the combustion chamber. As you increase
the throttle setting, more fuel and air is flowing to the engine;
therefore, MAP increases. When the engine is not running, the manifold
pressure gauge indicates ambient air pressure (i.e., 29.92 in. Hg).
When the engine is started, the manifold pressure indication will
decrease to a value less than ambient pressure (i.e., idle at 12 in.
Hg). Correspondingly, engine failure or power loss is indicated on the
manifold gauge as an increase in manifold pressure to a value
corresponding to the ambient air pressure at the altitude where the
failure occurred.
The manifold pressure gauge is color-coded to indicate the engine's
operating range. The face of the manifold pressure gauge contains a
green arc to show the normal operating range, and a red radial line to
indicate the upper limit of manifold pressure.
For any given r.p.m., there is a manifold pressure that should not be
exceeded. If manifold pressure is excessive for a given r.p.m., the
pressure within the cylinders could be exceeded, thus placing undue
stress on the cylinders. If repeated too frequently, this stress could
weaken the cylinder components, and eventually cause engine failure.
You can avoid conditions that could overstress the cylinders by being
constantly aware of the r.p.m., especially when increasing the
manifold pressure. Conform to the manufacturer's recommendations for
power settings of a particular engine so as to maintain the proper
relationship between manifold pressure and r.p.m.
When both manifold pressure and r.p.m. need to be changed, avoid
engine overstress by making power adjustments in the proper order:
When power settings are being decreased, reduce manifold pressure
before reducing r.p.m. If r.p.m. is reduced before manifold pressure,
manifold pressure will automatically increase and possibly exceed the
manufacturer's tolerances.
When power settings are being increased, reverse the order-increase
r.p.m. first, then manifold pressure.
To prevent damage to radial engines, operating time at maximum r.p.m.
and manifold pressure must be held to a minimum, and operation at
maximum r.p.m. and low manifold pressure must be avoided.
Under normal operating conditions, the most severe wear, fatigue, and
damage to high performance reciprocating engines occurs at high r.p.m.
and low manifold pressure.
In actuality most aircraft throttle quadrants are probably marked increase/decrease RPM which is what it acually does by changing the tension on the speeder spring inside the prop govenor. The speeder holds tension on the flyweights that sense rpm,s by the cent force they feel at any given rpm. This aspect is also adjusted when installing props. Though I have seen some WWII quadrants marked as low pitch high rpm/ high pitch low rpm. Aviation sure is fun.
During run up you normally cycle the prop control several times to check its operation and to get warm oil up into the prop dome. It is left in the high rpm setting for take off. As the throttle is advanced you monitor the manifold pressure so as not to overboost. The operating manual will tell you the maximum allowable manifold pressure . For take off it might say 2250 rpms at 35" . If the manual says 2000 RPM,s and 30" manifold pressure for climbout you adjust prop control and throttle to acheive these settings. When leveling out for cruise the manual might say 1900 rpms at 28" . Inches refers to inches of mercury . This is most WWII aircraft flown by the USA. Most of this information is gleamed from the TO,s or Technical Orders that come with the aircraft. This is a series of tech manuals that cover everything from the erection and maintenance to operations. The erection and maintenance portion will cover things like control surface installation. Control rod settings. Control cable tension settinga at given temps. Covering methods for fabric covered control surfaces etc.
Although some older adjustable-pitch propellers could only be adjusted
on the ground, most modern adjustable-pitch propellers are designed so
that you can change the propeller pitch in flight. The first
adjustable-pitch propeller systems provided only two pitch settings?a
low-pitch setting and a high-pitch setting. Today, however, nearly all
adjustable-pitch propeller systems are capable of a range of pitch
settings.
Aconstant-speed propeller is the most common type of adjustable-pitch
propeller. The main advantage of a constant-speed propeller is that it
converts a high percentage of brake horsepower (BHP) into thrust
horsepower (THP) over a wide range of r.p.m. and airspeed
combinations. A constant-speed propeller is more efficient than other
propellers because it allows selection of the most efficient engine
r.p.m. for the given conditions.
An airplane with a constant-speed propeller has two controls-the
throttle and the propeller control. The throttle controls power
output, and the propeller control regulates engine r.p.m. and, in
turn, propeller r.p.m., which is registered on the tachometer.
Once a specific r.p.m. is selected, a governor automatically adjusts
the propeller blade angle as necessary to maintain the selected r.p.m.
For example, after setting the desired r.p.m. during cruising flight,
an increase in airspeed or decrease in propeller load will cause the
propeller blade angle to increase as necessary to maintain the
selected r.p.m. A reduction in airspeed or increase in propeller load
will cause the propeller blade angle to decrease.
The range of possible blade angles for a constant-speed propeller is
the propeller's constant-speed range and is defined by the high and
low pitch stops. As long as the propeller blade angle is within the
constant-speed range and not against either pitch stop, a constant
engine r.p.m. will be maintained. However, once the propeller blades
contact a pitch stop, the engine r.p.m. will increase or decrease as
appropriate, with changes in airspeed and propeller load. For example,
once a specific r.p.m. has been selected, if aircraft speed decreases
enough to rotate the propeller blades until they contact the low pitch
stop, any further decrease in airspeed will cause engine r.p.m. to
decrease the same way as if a fixed-pitch propeller were installed.
The same holds true when an airplane equipped with a constant-speed
propeller accelerates to a faster airspeed. As the aircraft
accelerates, the propeller blade angle increases to maintain the
selected r.p.m. until the high pitch stop is reached. Once this
occurs, the blade angle cannot increase any further and engine r.p.m.
increases.
On airplanes that are equipped with a constant-speed propeller, power
output is controlled by the throttle and indicated by a manifold
pressure gauge. The gauge measures the absolute pressure of the fuel/
air mixture inside the intake manifold and is more correctly a measure
of manifold absolute pressure (MAP). At a constant r.p.m. and
altitude, the amount of power produced is directly related to the fuel/
air flow being delivered to the combustion chamber. As you increase
the throttle setting, more fuel and air is flowing to the engine;
therefore, MAP increases. When the engine is not running, the manifold
pressure gauge indicates ambient air pressure (i.e., 29.92 in. Hg).
When the engine is started, the manifold pressure indication will
decrease to a value less than ambient pressure (i.e., idle at 12 in.
Hg). Correspondingly, engine failure or power loss is indicated on the
manifold gauge as an increase in manifold pressure to a value
corresponding to the ambient air pressure at the altitude where the
failure occurred.
The manifold pressure gauge is color-coded to indicate the engine's
operating range. The face of the manifold pressure gauge contains a
green arc to show the normal operating range, and a red radial line to
indicate the upper limit of manifold pressure.
For any given r.p.m., there is a manifold pressure that should not be
exceeded. If manifold pressure is excessive for a given r.p.m., the
pressure within the cylinders could be exceeded, thus placing undue
stress on the cylinders. If repeated too frequently, this stress could
weaken the cylinder components, and eventually cause engine failure.
You can avoid conditions that could overstress the cylinders by being
constantly aware of the r.p.m., especially when increasing the
manifold pressure. Conform to the manufacturer's recommendations for
power settings of a particular engine so as to maintain the proper
relationship between manifold pressure and r.p.m.
When both manifold pressure and r.p.m. need to be changed, avoid
engine overstress by making power adjustments in the proper order:
When power settings are being decreased, reduce manifold pressure
before reducing r.p.m. If r.p.m. is reduced before manifold pressure,
manifold pressure will automatically increase and possibly exceed the
manufacturer's tolerances.
When power settings are being increased, reverse the order-increase
r.p.m. first, then manifold pressure.
To prevent damage to radial engines, operating time at maximum r.p.m.
and manifold pressure must be held to a minimum, and operation at
maximum r.p.m. and low manifold pressure must be avoided.
Under normal operating conditions, the most severe wear, fatigue, and
damage to high performance reciprocating engines occurs at high r.p.m.
and low manifold pressure.
In actuality most aircraft throttle quadrants are probably marked increase/decrease RPM which is what it acually does by changing the tension on the speeder spring inside the prop govenor. The speeder holds tension on the flyweights that sense rpm,s by the cent force they feel at any given rpm. This aspect is also adjusted when installing props. Though I have seen some WWII quadrants marked as low pitch high rpm/ high pitch low rpm. Aviation sure is fun.
During run up you normally cycle the prop control several times to check its operation and to get warm oil up into the prop dome. It is left in the high rpm setting for take off. As the throttle is advanced you monitor the manifold pressure so as not to overboost. The operating manual will tell you the maximum allowable manifold pressure . For take off it might say 2250 rpms at 35" . If the manual says 2000 RPM,s and 30" manifold pressure for climbout you adjust prop control and throttle to acheive these settings. When leveling out for cruise the manual might say 1900 rpms at 28" . Inches refers to inches of mercury . This is most WWII aircraft flown by the USA. Most of this information is gleamed from the TO,s or Technical Orders that come with the aircraft. This is a series of tech manuals that cover everything from the erection and maintenance to operations. The erection and maintenance portion will cover things like control surface installation. Control rod settings. Control cable tension settinga at given temps. Covering methods for fabric covered control surfaces etc.