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Amt 113: AIRCRAFT PROPELLER
Introduction
Propeller Definition
- a rotating shaft with blades which driven byeither a turbine or reciprocating engine, moves
an aircraft through the air.
Propeller Function- the primary purpose of a propeller is to convert
engine power to thrust.
Nomenclature
All modern propellers consist of at least two blades that
are connected to a central hub. The portion of a
propeller blade that is nearest the hub is referred to as
the blade shank whereas the portion furthest from the
hub is called the blade tip. The propeller hub, or hub
assembly, is bored out to create a hub bore whichpermits a propeller to be mounted on the engine
crankshaft or to a reduction gear assembly.
In addition to the basic nomenclature just discussed,
propeller blades share much of the same nomenclature
as aircraft wings. For example, all propeller blades have
a leading edge, a trailing edge, and a chord line. If you
recall from your study of airfoils, a chord line is an
imaginary line drawn through an airfoil from the leading
edge to the trailing edge. The curved, or cambered side
of a propeller blade is called the blade back and the flat
side is called the blade face. A propeller's blade angle is
the acute angle formed by a propeller's plane of
rotation and the blade's chord line. A propeller's plane
of rotation is always perpendicular to the enginecrankshaft.
Propeller Theory
The amount of thrust produced depends on several
factors including, the angle of attack of the propeller
blades, the speed the blades move through the air, and
the shape of the airfoil. The angle of attack of a
propeller blade is the angle formed by the chord line of
the blade and the relative wind. The direction of the
relative wind is determined by the speed an aircraft
moves through the air and the rotational motion of the
propeller.
With no forward velocity, the relative wind is directly
opposite the movement of the propeller blade. In this
case, a propeller's angle of attack is the same as its
blade angle.
In forward flight, the airplane moves from point A to
point B while the propeller moves from point C to point
D. In this case, the propeller's trailing edge follows the
path from C to D which represents the resultant relative
wind. This results in an angle of attack that is less than
the blade angle.
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If the forward velocity of the aircraft remains constant,
but a propeller's rotational speed increases, the
propeller's trailing edge will move a greater distance for
a given amount of forward movement. This increases
the angle at which the relative wind strikesthe
propeller blade which, in turn, increases the angle of
attack.
As a propeller blade rotates at a fixed rpm, each blade
segment moves through the air at a different velocity.
The variation in airfoil shape and blade angle along the
length of a propeller blade compensates for differences
in rotational speed and allows for a more even
distribution of thrust along the blade.
Forces Acting on the Propeller
Centrifugal force
Can best be described as the force which tries to pull
the blades out of the hub. The amount of stress created
by centrifugal force can be greater than 7,500 times the
weight of the propeller blade.
Thrust bending force
Attempts to bend the propeller blades forward at the
tips. This occurs because propeller blades are typically
thinner near the tip and this allows the thrust produced
at the tip to flex the blade forward. Thrust bending
force opposes centrifugal force to some degree.
Torque bending forces
Occur as air resistance opposes the rotational motion of
the propeller blades. This force tends to bend the
blades opposite the direction of rotation.
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Aerodynamic twisting force
Tends to increase a propeller's blade angle. In some
cases, aerodynamic twisting force is used to help
change the blade angle on a propeller.
Centrifugal twisting force
Opposes aerodynamic twisting force in that it attempts
to decrease a propeller's blade angle. When a propeller
rotates, centrifugal force tries to align the propeller's
center of mass with its center of rotation.
Propeller Pitch
- is the theoretical distance a propeller advanceslongitudinally in one revolution.
Geometric pitch
-is defined as the distance, in inches, that apropeller will move forward in one revolution if
it were moving through a solid medium and did
not encounter any loss of efficiency.
Effective Pitch
- is the actual amount a propeller moves forwardin one revolution.
Propeller Classification
Propellers are typically classified according to their
position on the aircraft.
Tractor Propellers are mounted on the front of an
engine and pull an aircraft through the air.
Pusher-type Propellers - are mounted on the aft end ofan aircraft and push an airplane through the air.
Types of Propeller
Fixed Propeller Ground Adjustable Propeller Controllable Pitch Propeller Constant Speed Propeller
Fixed type Propeller
Simplest type of propeller is a fixed-pitch propeller. As
its name implies, the blade angle on a fixed-pitch
propeller is fixed and cannot easily be changed. Because
of this, fixed-pitch propellers achieve their optimum
efficiency at a specific rotational and forward speed.
Ground Adjustable Propeller
This propeller is constructed in a way that allows the
blade angle to be changed on the ground. This type of
propeller is found mostly on aircraft built between the
1920s and 1940s.
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Controllable Pitch Changing Propeller
The blade angle may be changed while the propeller is
rotating. This allows the propeller to assume a blade
angle that provides the best performance for a
particular flight condition.
Constant Speed Propeller
Sometimes referred to as automatic propellers, are
unique in that once a pilot selects an operating rpm, the
propeller blades automatically adjust to maintain the
selected rpm. With this type of propeller, pitch control
is provided by a controlling device known as a governor.
Propeller Construction
WOOD
Wood was the most reliable material for fabrication ofpropellers for many years. Hardwoods such as birch,maple, and several others possess the flexibility andstrength required for a propeller used on lowhorsepower engines of small aircraft. The molecularstructure of wood allows it to absorb engine vibration toa large degree and does not support resonant vibrations.However, unless wood materials are coated with atough protective layer of resin or other material, they
are susceptible to damage from gravel and debris duringground operations.
ALUMINUM ALLOY
Today, the vast majority of propellers used are con-structed of an aluminum alloy. Aluminum is moredesirable than wood because it allows thinner, moreefficient airfoils to be constructed without sacrificingstructural strength. In addition, the airfoil sections onan aluminum propeller typically extend close to the
hub providing better airflow for engine cooling.Furthermore, aluminum propellers require much lessmaintenance than wood propellers, thereby reducingthe operating cost.
STEEL
Steel propellers and blades are found primarily on
antique and older generation transport aircraft.Because steel is a heavy metal, steel blades are normally
hollow consisting of steel sheets attached to a rib
structure. The hollow area is then filled wida a foam
material to help absorb vibration and maintain a rigid
structure.
COMPOSITE
Composite propeller blades are slowly gaining inpopularity. Some advantages of composite propellers
include the fact that they are lightweight and extremelydurable. In addition, composites absorb vibration andare resilient, making them resistant to damage andcorrosion.
FIXED PITCH PROPELLER
A typical fixed-pitch propeller installed on a light aircraft
has a diameter between 67 and 76 inches and a pitch
between 53 and 68 inches. The exact diameter andpitch required for a specific airplane is specified by the
aircraft manufacturer.
Metal Tipping
Metal tipping is applied to propeller blade tips and
leading edges to help prevent erosion damage. Three
small holes drilled in the tip of each blade release
moisture and allow the wood to breathe.
Monel, brass, or stainless steel tipping is applied to the
leading edge and tip of most wooden propellers to
prevent damage from small stones.
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Aluminum Alloy Propeller
Wood has given way to aluminum as the most often
used material for fixed-pitch propeller fabrication. As
mentioned earlier, propeller blades can be made
thinner and more efficient without sacrificing structural
strength when using aluminum instead of wood. In
addition, aluminum has the strength and flexibility to
accommodate the high horsepower engines available in
today's small aircraft.