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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems
Powerplant

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

When operating altitude increases, the tachometer may not
show correct power output of the engine. For example, 2,300
rpm at 5,000 feet produces less horsepower than 2,300 rpm
at sea level because power output depends on air density. Air
density decreases as altitude increases and a decrease in air
density (higher density altitude) decreases the power output
of the engine. As altitude changes, the position of the throttle
must be changed to maintain the same rpm. As altitude is
increased, the throttle must be opened further to indicate the
same rpm as at a lower altitude.

Adjustable-Pitch Propeller
The adjustable-pitch propeller was the forerunner of the
constant-speed propeller. It is a propeller with blades whose
pitch can be adjusted on the ground with the engine not
running, but which cannot be adjusted in flight It is also
referred to as a ground adjustable propeller. By the 1930s,
pioneer aviation inventors were laying the ground work for
automatic pitch-change mechanisms, which is why the term
sometimes refers to modern constant-speed propellers that
are adjustable in flight.

The first adjustable-pitch propeller systems provided only
two pitch settings: low and high. Today, most adjustable-pitch
propeller systems are capable of a range of pitch settings.
A constant-speed propeller is a controllable-pitch propeller
whose pitch is automatically varied in flight by a governor
maintaining constant rpm despite varying air loads. It 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 rpm and airspeed
combinations. A constant-speed propeller is more efficient
than other propellers because it allows selection of the most
efficient engine rpm for the given conditions.

An aircraft 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 rpm.
This in turn regulates propeller rpm which is registered on
the tachometer.

Once a specific rpm is selected, a governor automatically
adjusts the propeller blade angle as necessary to maintain
the selected rpm. For example, after setting the desired rpm
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 rpm. A reduction in
airspeed or increase in propeller load will cause the propeller
blade angle to decrease.

The propeller's constant-speed range, defined by the high
and low pitch stops, is the range of possible blade angles for
a constant-speed propeller. As long as the propeller blade
angle is within the constant-speed range and not against
either pitch stop, a constant engine rpm will be maintained.
If the propeller blades contact a pitch stop, the engine rpm
will increase or decrease as appropriate, with changes in
airspeed and propeller load. For example, once a specific
rpm 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 rpm
to decrease the same way as if a fixed-pitch propeller were
installed. The same holds true when an aircraft equipped with
a constant-speed propeller accelerates to a faster airspeed. As
the aircraft accelerates, the propeller blade angle increases to
maintain the selected rpm until the high pitch stop is reached.
Once this occurs, the blade angle cannot increase any further
and engine rpm increases.

On aircraft 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 rpm and altitude, the amount of power produced
is directly related to the fuel/airflow being delivered to the
combustion chamber. As the throttle setting is increased,
more fuel and air flows to the engine and MAP increases.
When the engine is not running, the manifold pressure gauge
indicates ambient air pressure (i.e., 29.92 inches mercury
(29.92 "Hg)). When the engine is started, the manifold
pressure indication will decrease to a value less than ambient
pressure (i.e., idle at 12 "Hg). 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. [Figure 6-9]

Engine power output is indicated on the manifold pressure gauge.
Figure 6-9. Engine power output is indicated on the manifold
pressure gauge.

 

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