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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.

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