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 |
Fixed-pitch and ground-adjustable propellers are designed
for best efficiency at one rotation and forward speed. They
are designed for a given aircraft and engine combination. A
propeller may be used that provides the maximum efficiency
for takeoff, climb, cruise, or high-speed flight. Any change in
these conditions results in lowering the efficiency of both the
propeller and the engine. Since the efficiency of any machine
is the ratio of the useful power output to the actual power
input, propeller efficiency is the ratio of thrust horsepower to
brake horsepower. Propeller efficiency varies from 50 to 87
percent, depending on how much the propeller "slips."
Propeller slip is the difference between the geometric pitch of
the propeller and its effective pitch. [Figure 4-37] Geometric
pitch is the theoretical distance a propeller should advance
in one revolution; effective pitch is the distance it actually
advances. Thus, geometric or theoretical pitch is based on
no slippage, but actual or effective pitch includes propeller
slippage in the air.
Figure 4-37. Propeller slippage.
The reason a propeller is "twisted" is that the outer parts of the
propeller blades, like all things that turn about a central point,
travel faster than the portions near the hub. [Figure 4-38] If
the blades had the same geometric pitch throughout their
lengths, portions near the hub could have negative AOAs
while the propeller tips would be stalled at cruise speed.
Twisting or variations in the geometric pitch of the blades
permits the propeller to operate with a relatively constant
AOA along its length when in cruising flight. Propeller
blades are twisted to change the blade angle in proportion
to the differences in speed of rotation along the length of
the propeller, keeping thrust more nearly equalized along
this length.
Figure 4-38. Propeller tips travel faster than the hub
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The reason a propeller is "twisted" is that the outer parts of the
propeller blades, like all things that turn about a central point,
travel faster than the portions near the hub. [Figure 4-38] If
the blades had the same geometric pitch throughout their
lengths, portions near the hub could have negative AOAs
while the propeller tips would be stalled at cruise speed.
Twisting or variations in the geometric pitch of the blades
permits the propeller to operate with a relatively constant
AOA along its length when in cruising flight Propeller
blades are twisted to change the blade angle in proportion
to the differences in speed of rotation along the length of
the propeller, keeping thrust more nearly equalized along
this length.
Usually 1° to 4° provides the most efficient lift/drag ratio,
but in flight the propeller AOA of a fixed-pitch propeller
varies—normally from 0° to 15°. This variation is caused
by changes in the relative airstream, which in turn results
from changes in aircraft speed. Thus, propeller AOA is the
product of two motions: propeller rotation about its axis and
its forward motion.
A constant-speed propeller automatically keeps the blade
angle adjusted for maximum efficiency for most conditions
encountered in flight During takeoff, when maximum power
and thrust are required, the constant-speed propeller is at
a low propeller blade angle or pitch. The low blade angle
keeps the AOA small and efficient with respect to the relative
wind. At the same time, it allows the propeller to handle a
smaller mass of air per revolution. This light load allows
the engine to turn at high rpm and to convert the maximum
amount of fuel into heat energy in a given time. The high rpm
also creates maximum thrust because, although the mass of
air handled per revolution is small, the rpm and slipstream
velocity are high, and with the low aircraft speed, there is
maximum thrust.
After liftoff, as the speed of the aircraft increases, the constantspeed
propeller automatically changes to a higher angle (or
pitch). Again, the higher blade angle keeps the AOA small
and efficient with respect to the relative wind. The higher
blade angle increases the mass of air handled per revolution.
This decreases the engine rpm, reducing fuel consumption
and engine wear, and keeps thrust at a maximum.
After the takeoff climb is established in an aircraft having
a controllable-pitch propeller, the pilot reduces the power
output of the engine to climb power by first decreasing the
manifold pressure and then increasing the blade angle to
lower the rpm.
At cruising altitude, when the aircraft is in level flight and
less power is required than is used in takeoff or climb, the
pilot again reduces engine power by reducing the manifold
pressure and then increasing the blade angle to decrease the
rpm. Again, this provides a torque requirement to match the
reduced engine power. Although the mass of air handled per
revolution is greater, it is more than offset by a decrease in
slipstream velocity and an increase in airspeed. The AOA is
still small because the blade angle has been increased with
an increase in airspeed. |
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