Transcript Propellers ATC Chapter 2 Aim To review the propeller and its efficiency.

Propellers
ATC Chapter 2
Aim
To review the propeller and its efficiency
Objectives
1.
2.
3.
4.
5.
6.
7.
Define momentum and thrust
State the aerodynamic forces created by the propeller
Define the thrust/torque ratio
State propeller terminology
State the forces acting on the propeller
State the factors effecting propeller efficiency
State the efficiency with the use of a variable pitch
propeller
1. Thrust Production
Thrust Production
• A piston engine produces torque to rotate the engine shaft
• The propeller converts the rotation of the engine shaft to thrust
• Thrust is measured by the change in momentum of the air, as it is accelerated
by the propeller
• Momentum = mass x velocity change
• Thrust
 velocity change
• At low airspeeds an air mass is accelerated through a high velocity change
Thrust = velocity change
Thrust = velocity change
 At low airspeeds thrust is high
 At high airspeeds thrust is lower
2. Propeller Aerodynamic Forces
Wing Vs. Propeller
• A propeller blade is basically a aerofoil that has been turned 90° to the relative
airflow
• The cambered side of the propeller is termed the blade back
• The flat side is the blade face
T.E.
• The chord line is typically
taken as the blade face as the
actual chord varies with the Blade back
Blade Angle
cross sectional area of the
blade
Blade face
L.E.
Chord
Line
Plane of
rotation
2. Propeller Aerodynamic Forces
Relative airflow
• If we consider an aircraft sitting stationary on the ground we can say that the
direction of the airflow across the blade is equal and opposite to the plane of
rotation (rotational velocity or RPM)
• The speed of the airflow will depend on the RPM and radius of rotation. The
further from the propeller hub you go the faster the airflow
2. Propeller Aerodynamic Forces
Relative Airflow
• Once the aeroplane begins to move the propeller will also have a forward
velocity
• When we combine the forward and rotational velocities we can determine the
resultant velocity of a blade section
• Opposite the resultant velocity is the relative airflow
• The angle between the relative airflow
AoA
and the plane of rotation can be termed
helix angle, pitch angle or angle of
advance
Pitch Angle
Chord
RAF (resultant velocity)
RPM
3. Thrust/Torque Ratio
Propeller Terminology
• The total reaction of a propeller is made up of thrust and propeller
torque(drag)
• The forces acting on a wing are:
Lift
Chord
Line
RAF
AoA
Total
Reaction
Drag
Propeller
Torque
Total
Reaction
Thrust
• The forces acting on a propeller blade are: Chord
Line
Blade
Angle
Plane of
rotation
3. Thrust/Torque Ratio
Propeller Terminology
• To produce a plane of rotation, the engine produces power to turn the
propeller shaft – this produces engine torque
Propeller
Torque
Total
Reaction
Blade
Angle
Thrust
Engine
Torque
Chord
Line
Plane of
rotation
4. Propeller Terminology
Blade Angle
• Like a wing the propeller has a chord line
• The propeller unlike a wing has a plane of rotation
• The plane of rotation is perpendicular to propeller shaft
• The angle between the chord line
and plane of rotation is the blade angle
Blade Angle
Chord
Line
Plane of
rotation
4. Propeller Terminology
Propeller Pitch
• Propeller pitch is the linear distance of the blade angle
• This is measured at 75% of the radius of the propeller
• The pitch of the propeller is an expression of the
distance the propeller would move in one revolution
(geometric pitch)
• When the helix angle is equal to
Blade
blade angle
Angle
• The effective pitch of the propeller is the actual
distance the propeller moves forward in one revolution
• The difference between geometric and effective
Chord
is called slip
Line
Plane of
rotation
Propeller Pitch
4. Propeller Terminology
Blade Twist
• The blade angle varies from a large angle at the root to a small angle at the tip
• When quoting blade angle as a measure of blade characteristics, the
radial distance used is 75% of the blade
• As a propeller spins in a circular motion:
• The propeller needs to be investigated in two parts
• Propeller root
• Propeller tip
Greater distance
• Each blade rotates 360° in the same time
• For this to occur the tip travels a greater distance
over the same time
• The tip has a higher velocity to the root
Smaller
Propeller Root
distance
Propeller Tip
4. Propeller Terminology
Blade Twist
• As the propeller works in the same way as a wing we can use the lift formula:
• If the tip travels at a higher velocity than the root we can denote that the tip
will produce more thrust than the root
• To counter this thrust imbalance the angle of
attack at the tip is less than the wing
root (similar to washout)
• Like a wing the propeller is most effective at approximately
4° Angle of attack
4. Propeller Terminology
Blade Angle of Attack
• As a propeller moves forward through the air, the blade angle and angle of
attack are different
• The relative airflow is the resultant of the
rotational and forward speeds
• The twisting of the blade assists keeping a
relatively constant angle of attack
throughout the blade
Angle of
Attack
• It is important to note the relationship between
rotational speed, forward speed and the angle
of attack to give the best thrust/drag ratio
Chord
Line
Blade
Angle
Plane of
rotation
Relative Airflow
4. Propeller Terminology
Helix Angle
• As the propeller rotates and moves through the air it follows a spiral path
• The spiral path is known a helical path
• Each blade section has a different helical path
due to the different rotational velocities
• The angle between the plane of rotation and
the helical path is called the helix angle
Angle of
Attack
Chord
Line
Helix
Angle
Blade
Angle
Plane of
rotation
Relative Airflow
4. Propeller Terminology
Propeller Velocity Vs. Path
• When the propeller rotates it has rotational velocity
• When the aeroplane is in motion the propeller has a forward velocity
• As the propeller rotates the velocity at the tip is greater than at the hub due
to travelling a greater distance in the same period of time
• The change in linear speed and blade twist changes the relative airflow at
each propeller blade section
Blade section
set at tip
Blade section
set at hub
Relative
airflow at hub
Forward velocity in
half a revolution
Relative
airflow at tip
4. Propeller Terminology
Propeller Angle of Attack
• The optimum lift/drag ratio for a wing is at approximately 4°
• For a fixed pitch propeller, the optimum angle of attack can only be achieved
at one rpm/TAS combination
Constant
RPM
Angle of
Attack
Low
TAS
Constant
RPM
Angle of
Attack
Medium
TAS
Constant
RPM
Angle of
Attack
Constant Blade Angle /Pitch
High
TAS
5. Forces Acting on the Propeller
Twisting Moments
• Propellers that can change the blade angle are called variable pitch propellers
• When the pitch of the propeller is changed, forces are applied for and against
the blade when changing angle
• These forces are twisting moments and are made up of:
• Centrifugal forces
• Aerodynamic forces
5. Forces Acting on the Propeller
Centrifugal Twisting Moment (CTM)
• Centrifugal force acts directly away from the centre of rotation
• The centrifugal force attempts to stretch the tip from the hub
• As the centrifugal force does not align with the pitch change axis, it produces
two forces which act out from the leading and trailing edge
Tip
Direction of
CF
CF Rotation
x
x
T.E.
L.E.
Pitch
Change Axis
Hub
5. Forces Acting on the Propeller
Centrifugal Twisting Moment (CTM)
• 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
• The CTM attempts to create a finer pitch
Pitch Change Axis
x
x
L.E.
CTM
T.E.
5. Forces Acting on the Propeller
Aerodynamic Twisting Moment (ATM)
• The total reaction force does not act through the propellers pitch change axis
• Therefore if:
• The total reaction force is behind the pitch change axis the ATM is
attempting to decrease the blade pitch
• The total reaction force is ahead of the pitch change axis the ATM is
attempting to increase the blade pitch
• This can assist in counteracting the CTM
ATM
ATM
Total
Reaction
Total
Reaction
5. Forces Acting on the Propeller
Thrust Bending Force
• As the propeller is thinner at the tip compared with the hub, thrust attempts
to bend the propeller blade forward
• Thrust bending force opposes centrifugal reaction (hub to tip)
CR
Thrust
Thrust
CR
5. Forces Acting on the Propeller
Windmilling
• Usually the propeller is driven by the engine
• In the event of an engine failure, the propeller will be driving the engine
• This is known as windmilling
• When the propeller is windmilling:
• The ATM act is the same direction of the CTM
• This causes the propeller to attempt to flatten out (fine pitch)
5. Forces Acting on the Propeller
Propeller Sections
• The propeller does not create uniform thrust from the hub to tip
• At the hub the blade is thick and a high amount of interference to the
airflow due to the engine and associated structures
• At the tip air will flow from the area of high pressure on the back of the
blade to the area of low pressure on the front of the blade.
• This has the same effect as wingtip vortices
• Only a small part of the blade is effective in producing thrust
• This is between 60% and 90% of the tip radius
• The greatest useful thrust is produced at approximately 75% f the tip radius
98%
75%
50%
25%
6. Propeller Efficiency
Variation in Propeller Efficiency
• At a constant RPM
• The direction of the relative airflow and the angle of attack will determine the
forward speed
• Any increase in the speed will cause the angle of attack to decrease
• Resulting is reduced performance
Constant
RPM
Angle of Medium
Attack
TAS
Constant
RPM
Angle of
Attack
Higher
TAS
6. Propeller Efficiency
Variation in Propeller Efficiency
• For a given RPM, only one airspeed will result in the propeller to operate at
the best thrust/drag ratio (approximately 4° angle of attack)
• A designer would choose a fixed-pitch propeller airspeed/RPM combination
for the aeroplanes intended purpose
Constant
RPM
Angle of Medium
Attack <4°
TAS
Angle of
Attack = 4°
Constant
RPM
Higher
TAS
6. Propeller Efficiency
Variation in Propeller Efficiency
• To expand the flight envelope for maximum propeller efficiency we would
have to:
• Physically change to propeller
• Adjust the blade angle on the ground
• Adjust the blade angle in flight
• Have the propeller automatically adjust the blade angle for the given
airspeed/RPM setting
Blade
Angle
Chord
Line
Blade
Angle
Chord
Line
Plane of
Plane of
rotation
rotation
Propeller Pitch
Fine Pitch
Blade
Angle
Chord
Line
Course Pitch
Plane of
rotation
7. Variable Pitch Propeller
Variable Pitch Propeller
• At low Airspeeds the blade must be small to have the best angle of attack
• i.e. a climb
• As the airspeed increases the blade angle must increase (coarsen) to maintain
the optimal angle of attack
Fine Pitch
Course Pitch
Constant
RPM
Angle of
Attack
Slow
TAS
Constant
RPM
Angle of
Attack
Medium
TAS
CONSTANT ANGLE OF ATTACK
Constant
RPM
Angle of
Attack
High
TAS
7. Variable Pitch Propeller
Variable Pitch Propeller
• A variable pitch propeller adjusts to the most efficient angle of attack
• The pilot uses the propeller leaver to set rpm
• When the throttle is advanced the blade angle automatically increases to
absorb the increase in engine power, maintaining the most efficient angle of
attack
• This maintains rpm and creates more thrust
• The opposite occurs when reducing the power
• More on this in aircraft general knowledge
Throttle
Propeller Pitch
Mixture
Questions?