Transcript Chapter 7 PowerPoint

Propellers
Chapter 7
Aim
To understand principal of operations of
propeller systems
Objectives
1.Review aerodynamic properties of propellers
2.Review propeller performance considerations
3.Discuss principals of operation of constant speed
propellers
4.Describe factors affecting propeller stress and
materials used in construction
1. Aerodynamic properties
Blade Angle
The blade angle is the angle between the chord of the blade and the plane of
rotation
The blade angle is not constant throughout the blade length
For a fixed pitch propeller this is angle is
set at the time of manufacturing
T.E.
Blade back
Blade Angle
Blade face
L.E.
Chord
Line
Plane of
rotation
1. Aerodynamic properties
Propeller Pitch
Propeller pitch is a linear equivalent measure of blade angle
Both blade angle and radius are measured at a standard radius of 75%
blade length
Plane of
rotation
Chord Line
Fine Pitch
Plane of
rotation
Chord Line
Coarse Pitch
1. Aerodynamic properties
Forces acting on a propeller
Total
Reaction
The propeller has two speeds:
1. RPM is produced by the engine
2. TAS is due to the forward movement of
the aircraft
These two speeds result in a helical direction
of travel, opposite the direction of travel is the
relative airflow (RAF)
Thrust
Between the relative airflow and the chord is
the angle of attack, this angle of attack creates
two aerodynamic forces on the propeller:
AoA
1. Propeller torque opposes the engine
torque (RPM) and acts parallel to the
plane of rotation
2. Thrust acts perpendicular to the plane of
Chord Line
rotation
The resultant of these two forces is termed the
RAF
total reaction
Propeller
Torque
RPM
TAS
1. Aerodynamic properties
Centrifugal Twisting Moments
Centrifugal force acts directly away from the
centre of rotation along the full length of the
blade
The centrifugal force attempts to stretch the
tip from the hub
As the centrifugal force does not align with the
pitch change axis, we can break it down into
two forces which act out from the leading and
trailing edge
Pitch Change Axis
X
T.E.
X
L.E.
1. Aerodynamic properties
Centrifugal Twisting Moments
The two forces acting from the leading edge and
trailing edge creates the twisting moment
The twisting moment attempts to change the
pitch of the propeller towards fine pitch
X
T.E.
Pitch Change Axis
L.E.
X
1. Aerodynamic properties
Aerodynamic Twisting Moments
The total reaction force does not
Total
act through the propellers pitch
Reaction
change axis, just like a wing it will
typically act through the centre
of pressure around 1/3 chord
As centre of pressure is forward
of the pitch change axis it tends
to move the blade toward
coarse, counteracting the CTM
Thrust
Aerodynamic twisting moments
are not as strong as centrifugal
twisting moments
Propeller
Torque
1. Aerodynamic properties
Windmilling propeller
If the engine fails the propeller will
windmill (drive the engine)
When the propeller is windmilling the
aerodynamic and centrifugal twisting act
in the same direction, towards fine pitch
This is not what we want to happen as it
will increase the drag produced by the
blade
The drag produced by a windmilling
propeller is equal to the drag produced
by a solid disc of the same radius
Thrust
Propeller
Torque
Total
Reaction
1. Aerodynamic properties
Feathered propeller
Following an engine failure the propeller should be feathered to stop it
from windmilling
The angle of attack of the propeller will be slightly negative to produce zero
thrust as the propeller is cambered
Propeller
Torque
RAF
Total
Reaction
Propeller
Torque
2. Propeller efficiency
Solidity
The blade fitted to an engine must be carefully matched to the power output of
the engine in order to convert power into thrust
Chord
Solidity is the ratio of the propeller disk at a given radius
to the circumference of a solid disk of the same radius
and can be measured by:
Number of blades × Chord at radius
Circumference at radius
Solidity can be increased by:
• Increasing the number of blades
• Increasing the chord of the blades
Although increasing the chord is often the easier option, the
Radius
corresponding decrease in aspect ratio will make the blade less
efficient
Increasing the blade length will increase the tip speed, many propeller blades
come close to or exceed the speed of sound. As an object approaches the speed
of sound the shockwaves created increase the drag on the object, kinetic energy
will be turned into sound and heat, reducing efficiency
2. Propeller efficiency
Fixed Pitch Propellers
As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the
same can be said for a propeller blade
The angle of attack of the propeller blade is determined by the RPM and
the TAS
AoA
Chord Line
RPM
RAF
TAS
2. Propeller efficiency
Fixed Pitch Propellers
As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the
same can be said for a propeller blade
The angle of attack of the propeller blade is determined by the RPM and
the TAS
If the RPM increases the AoA will increase
AoA
Chord Line
RPM
RAF
TAS
2. Propeller efficiency
Fixed Pitch Propellers
As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the
same can be said for a propeller blade
The angle of attack of the propeller blade is determined by the RPM and
the TAS
If the RPM increases the AoA will increase
If the TAS reduces the AoA will increase
AoA
Chord Line
RPM
RAF
TAS
2. Propeller efficiency
Fixed Pitch Propellers
As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the
same can be said for a propeller blade
The angle of attack of the propeller blade is determined by the RPM and
the TAS
If the RPM increases the AoA will increase
If the TAS reduces the AoA will increase
If the RPM decrease the AoA will decrease
AoA
Chord Line
RPM
RAF
TAS
2. Propeller efficiency
Fixed Pitch Propellers
As we know a cambered aerofoil will be most efficient around 4⁰ AoA, the
same can be said for a propeller blade
The angle of attack of the propeller blade is determined by the RPM and
the TAS
If the RPM increases the AoA will increase
If the TAS reduces the AoA will increase
If the RPM decrease the AoA will decrease
AoA
If the TAS increases the AoA will decrease
Any decrease in AoA will cause a
corresponding decrease in prop torque,
this will allow the RPM to increase on a
fixed pitch propeller, this can be seen if we
Chord Line
RPM
lower the nose of the aircraft allowing TAS
to increase
RAF
A fixed pitch propeller will be most
TAS
efficient at one RPM and TAS combination,
this is set by the manufacturer and will
typically be set for cruise
2. Propeller efficiency
Variable Pitch Propellers
By varying the pitch of the propeller we are able to operate the propeller
at the optimum AoA over a wider range of RPM and TAS settings
Envelope of max efficiency
100%
Fine Pitch
Coarse Pitch
Efficiency
Blade angle
TAS
3. Constant Speed Propellers
Constant Speed Propellers
In an aircraft fitted with a variable pitch propeller the pitch control lever
controls engine RPM and the throttle controls manifold air pressure
The system utilizes a constant speed unit and a pitch change mechanism to
maintain the RPM set by the pilot
3. Constant Speed Propellers
Constant Speed Units
Constant speed units (sometimes referred to as propeller governors) are
normally fitted to the front of the engine and will incorporate a geared
boost pump to provide the pitch change mechanism with high pressure oil
The CSU utilizes flyweights which are rotated
by the engine and are subject to centrifugal
forces which tends to make them fly
outwards
They are prevented from dong so by the
speeder spring, the pilot sets the tension on
this spring via the pitch control lever
If the system is maintaining the set RPM the
pilot valve will maintain pressure in the
propeller hub, this is known as On-speed
3. Constant Speed Propellers
Constant Speed Units
If the RPM tries to decrease (Under-speed) the flyweights will fly inwards
and open the pilot valve allowing high pressure oil to travel to the pitch
change mechanism decreasing the blade angle.
As the RPM begins to increase back to the
selected RPM the pilot valve will close until the
system returns to the on-speed condition
3. Constant Speed Propellers
Constant Speed Units
If the RPM tries to increase (Over-speed) the flyweights will fly outwards
and raise the pilot valve allowing oil to return to the sump increasing blade
angle.
As the RPM decreases back to selected RPM the pilot
valve will close until the system returns to the onspeed condition
3. Constant Speed Propellers
Pitch Change Mechanism
There are a number of different systems utilized by aircraft manufacturers to
adjust the pitch of the propeller, the most common is the use of hydraulic
pistons
Centrifugal twisting moments are used to assist in moving the blade to fine
pitch, this is often augmented by the use of springs, counter weights or
air/nitrogen charges
Typically engine oil is used as the hydraulic fluid as is boosted to the required
pressure via a pump
In this system when an increase in
pitch is required the CSU pumps high
pressure oil into the cylinder and the
piston coarsens the blade via
mechanical linkage.
When a decrease in pitch is required
pressure is released by the CSU and oil
is allowed to return to the sump as the
centrifugal twisting moments move
the blade to the fine position
3. Constant Speed Propellers
Pitch Change Mechanism
Here the piston is fixed and the moveable
dome coarsens the blade pitch, again to
fine the blade the CSU will release the oil
pressure and the blade will fine via the
centrifugal twisting moments
This system can be seen as the opposite of
the pervious two, a counterweight is
introduced which is offset from the pitch
change axis, this counterweight is used to
coarsen the pitch. The hydraulic system is
used to move the blade to fine
3. Constant Speed Propellers
Feathering Systems
As previously mentioned, a windmilling propeller will produce drag equal to a
disk of the same radius
In a twin engine aircraft this excess drag will cause a yaw and rolling moment
towards the failed engine, depending on the airspeed and power produced by
the live engine the rudder may not have enough authority to overcome this
The type of feathering system depends on the type of pitch change mechanism
used, a large number of aircraft incorporate an auto feather system
In some aircraft the aircraft must be
feathered before the RPM drops below
a certain value, beyond this the
feathering system will not be able to
overcome the combination of
centrifugal and aerodynamic twisting
moments
4. Propeller Stress
Propeller Failure
Propeller stress or failure can be caused by a number of factors including:
• Over Speeding – Increased centrifugal forces
resulting in excess stress on the propeller hub
• Lightning Damage – Can result in burn damage
• Vibration – When the propeller is producing
thrust aerodynamic and mechanical forces
cause the blade to vibrate. High tip speeds
cause excess drag on the blade tips, especially
on high powered aircraft where the blade tips
can exceed the speed of sound. Power pulses
from piston engines have the potential to set
up standing waves in the propeller. The
resulting bending loads may cause blade
sections to shear off.
4. Propeller Stress
Propeller Failure
• Nicks and fatigue cracks – Chips in the
propeller may lead to a fatigue crack and
eventual failure. The crack will develop in a
chord wise direction and will be visible from
the back of the blade, it may not be visible
from the front of the blade until just before
failure due to thrust bending loads. Any nicks
should be filed back by a qualified LAME,
typically an equivalent amount will be filed
from both blades to reduce vibration
• Fatigue of the hub – It can be hard to detect
fatigue in the hub due to its size. Caution
must be taken when selecting the cleaning
products used on aircraft as some general
purpose cleaners promote fatigue and
corrosion in the propeller, especially in the
hub
4. Propeller Stress
Propeller Materials
Propellers on older aircraft were typically made of wood, as engines became
more powerful metal propellers were used
The disadvantages of metal propellers include:
• Heavier than wooden propellers
• Harder to manufacture
• Increased tendency to vibrate
• Cost more
The advantages of metal propellers include:
• Ease of maintenance should the prop be chipped
• Resistance to weathering
• Low drag
• Low service requirements
• Ease of storage
4. Propeller Stress
Propeller Materials
Most modern aircraft propeller blades are
constructed from aluminium
Wood may still be used on small aircraft, such
as the jabiru, where low power engines are
used
A number of composite materials have been
developed, including Kevlar composite,
however these are not yet widely used
Questions?