Module 17.1 Fundamentals Flashcards
Define the term “hub”.
The central portion of a propeller which carries the blades.
Define the term “blade”.
Aerofoil section that is attached to the hub.
Define the term “blade butt”
The base of the propeller blade where the root ends.
Define the term “blade root/shank”.
The thickened portion of the blade nearest to the hub.
Define the term “blade station”
A distance measured from the centre of rotation (root to tip).
Define the term “master reference station”.
A distance that is measured from the centre of rotation, where all measurements are taken from.
Where is the master reference station on a fixed-pitch propeller?
75% from the centre of rotation.
Where is the master reference station on a variable pitch propeller?
50-75% from the centre of rotation.
What is the propeller face?
The flat thrust producing side of a propeller blade.
What is the propeller back?
The curved side of the propeller blade facing the direction of flight.
What is the blade chord line?
A line through the blade profile between the face and back.
What is the plane of rotation?
The plane in which the propeller rotates.
Where is the plane of rotation?
90° to the engine centreline.
Define the term “blade angle”.
The angle between the blade chord line and the plane of rotation.
Define the term “pitch”.
Distance advanced in one complete revolution.
What does the pitch change mechanism do?
Alters the blade pitch/angle.
Define the term “fine pitch”.
Vertical blade angles, also referred to as “Low pitch”.
Define the term “coarse pitch”.
Horizontal blade angles. Also referred to as “High pitch”.
What is “reverse pitch”?
Turning the propeller blades to a negative angle to produce braking or reversing thrust.
What does the dome mechanism do?
Encases the pitch change mechanism.
What is the “spinner”?
An aerodynamic fairing that covers the centre of the propeller.
What is a tractor propeller?
A propeller mounted in front of the leading edge of the wing or on the nose of the aircraft.
What is a pusher propeller?
A propeller mounted behind the trailing edge of the wing, or at the rear of the fuselage.
What does the propeller consist of?
Two or more blades that are connected by a hub.
What does the hub attach the blades to?
A piston engine, reduction gearbox and sometimes an electric motor drive shaft.
Which one of newtons laws does the propeller work off of?
Newton’s Third Law of Motion and is a result of the amount and speed of the air mass moved.
How does a propeller work?
Rotational energy provided by the engine is converted into forward thrust which is approximately perpendicular to the plane of rotation.
Where does the propeller accelerate the large mass of air?
Slowly rearwards
What is the equation for propeller thrust (F)?
F = pressure jump (delta p) x propeller disk area.
What is the cambered upper surface of the propeller blade known as?
“Blade Back”
What is the flat bottom surface of the propeller blade known as?
“Blade Face”
How does a propeller blade aerofoil generate lift?
By accelerating air over the cambered (blade back) surface.
The high velocity of the air results in lower static pressure in front of the propeller, pulling the aerofoil forward.
For a single revolution of the propeller, what does the amount of air displaced depend on?
Blade angle
What is a horizontal blade angle called?
Coarse (high)
What is a vertical blade angle called?
Fine (low)
When a blade is pitched to a fully horizontal angle, what is this called?
Feathered
What are the three characteristics of a fine (low) blade pitch?
Good low-speed acceleration
Rotates easily without taking a big bite out of the air
Moves forward through the air a short distance with every revolution
What does a fine (low) blade angle allow the engine to do?
Spin easily and operate at a high speed (RPM).
What is the main characteristics of a coarse (high) blade pitch?
It takes a large bite out of the air with every turn.
The propeller moves forward through the air a large distance with every revolution.
What does a coarse (high) blade angle do to the engine?
Limits the speed at which the engine operates.
What does varying in-flight pitch allow for?
An optimum thrust over the maximum amount of the aircraft’s speed range.
What is the α (alpha) range?
When the blade angle is between the fine and coarse positions.
What is the angle of attack?
The angle between the profile chord line and relative airflow.
What is the angle of advance (helix angle) φ?
The angle between plane of rotation and the relative airflow.
When does the angle of advance increase?
When speed increases.
What is relative airflow?
The speed and direction of the air movement past an aerofoil.
What is relative airflow controlled by?
Rotational speed of the propeller and forward speed of the aircraft.
What is the condition of the blade angle and angle of attack during normal flight?
Both positive, giving positive thrust and torque.
What can lead to a negative angle of attack of the blades?
A change of the relative air flow direction and velocity.
If engine torque to the propeller is reduced, what happens to propeller rotational speed?
It also decreases.
What happens to airspeed as engine torque is reduced?
it decreases much slower than propeller rotational speed.
Airspeed decreasing slower than propeller rotational speed does what?
Decreases propeller angle of attack.
What does the airflow do at a negative propeller angle of attack?
It applies the torque to the propeller and drive shaft.
The propeller will then drive the engine.
What is windmilling?
When the airflow drives the propeller which then drives the engine.
What does windmilling produce?
Negative torque
What direction does windmilling turn the propeller?
The same direction.
Define propeller brake moment.
The effort which is required to spin the propeller.
What happens to AoA if airspeed increases or rotational speed is greatly reduced?
It reduces and becomes negative.
What happens to AoA if airspeed is reduced?
It increases
What does blade angle increasing do to Brake Moment?
Increases it which decreases RPM.
What happens to AoA when RPM is increased and decreased?
Increase in RPM increases AoA. Decrease in RPM decreases AoA.
What does reducing blade angle do to Brake Moment?
Decreases it.
This will increase rotational speed
What happens to Brake Moment when pitch is adjusted to change airspeed?
It is maintained.
This also means rotational speed is maintained?
What does increasing and decreasing Brake Moment do to rotational speed?
Increasing brake moment decreases RPM. Decreasing brake moment increases RPM.
What is reverse angle/pitch?
When the blade pitch is decreased to a negative value.
What direction does the propeller turn in the reverse angle/pitch position?
Same direction as the the normal angle/pitch position.
If the blade angle is positioned to the other side of fine pitch, what is this called?
Negative pitch/angle.
What happens to thrust if the blade angle is reduced to an AoA less than the zero-lift angle of attack?
Thrust acts against the direction of flight (reverse thrust).
What can be selected to provide ease of starting and reverse thrust.
Specific blade angles.
What is the range from “Flight Fine” to ‘Reverse’ called?
Beta β Range.
When is beta range available?
Only on the ground.
What does the amount of lift produced depend on?
Aerofoil shape, RPM and AoA of the blade sections.
What does twisting the blade from root to tip do?
Ensures a nearly constant angle of pitch is maintained.
Define a propellers geometric twist?
When the propellers blade is twisted from the root to the tip.
What is pitch distribution?
When the blades angle of attack becomes smaller the further it is from the centre axis.
Why is pitch distribution needed?
To keep a constant angle of attack
What does the angle of incidence running the length of the blade determine?
The angle of the pitch for optimal distribution of lift.
What happens to velocity the further the blade section is from its rotational axis?
It increases
What is geometric pitch?
The theoretical distance a propeller should advance in one revolution.
What is effective pitch?
The distance the propeller actually advances in one revolution.
Define slip
The difference between geometric and effective pitch.
Define propeller efficiency.
How well a propeller transmits its rotational force/energy into thrust.
The amount of energy it takes to rotate the propeller is almost always greater than what?
Thrust from the propeller.
Reducing this loss is the goal of propeller efficiency.
What is the amount of thrust generated by a propeller controlled by?
The blades angle of attack.
What is the normal propeller efficiency range?
0.8 to 0.9 (80% - 90%)
What is the greatest load felt on the propeller?
Centrifugal forces
What do centrifugal loads do to the blades?
Tries to pull them out of the hub assembly.
What do thrust bending forces do to the blades?
Tries to bed end the propeller blade tips forwards.
What do torque bending forces (bending moments) try to do to the blades?
Bend the blade against the direction of propeller rotation.
What is Aerodynamic Twisting Moment (ATM)?
When the centre of pressure is forward of the blades centre of rotation.
What does Aerodynamic Twisting Moment (ATM) do to the blade?
Tries to turn it to a higher (coarser) blade angle.
In reverse pitch, what does aerodynamic twisting moment do to the blade?
Tries to turn it to a coarser negative blade angle.
What is Centrifugal Twisting Moment (CTM)?
Where the mass of the blade is thrown out from its centre of rotation.
What does Centrifugal Twisting Moment (CTM) do to the blade?
Tries to turn the blade to a lower (finer) blade angle.
What does Centrifugal Twisting Moment (CTM) appose?
Aerodynamic Twisting Moment (ATM).
Is Centrifugal Twisting Moment (CTM) or Aerodynamic Twisting Moment (ATM) greater?
Centrifugal Twisting Moment (CTM).
Where do Centrifugal Twisting Moments (CTMs) and Aerodynamic Twisting Moments (ATMs) occur?
At the blades root.
Where do all the greatest stresses occur?
At the blade root and on the hub.
Where is no damage or repair work is permitted
Within the blade root area.
When does maximum dynamic loading on a propeller blade occur?
Within its natural frequency range.
What causes dynamic loads (vibrations)?
The operating strokes of a piston engine or the dynamics of the propeller reduction gearbox.
They will also be caused by aerodynamic and mechanical forces felt on the propeller blades.
Where on the blade do aerodynamic forces have a greatest vibration effect?
At the tip, where the effects of transonic speeds cause buffeting and vibration.
What does the basic frequency range from?
20 Hz (metal) to 60 Hz (wood).
The highest vibrational loads are felt where?
In the area of about 80% of the blade length.
Define the outer nodal point.
The area where the highest vibrational loads are felt.
What must the propeller do to the power produced by the engine?
Absorb it and transmit that power to the air flow passing through the propeller disc.
What two things have an effect of increasing the solidity of the propeller disc?
Blade chord or the number of blades.
Define propeller disc solidity.
The area of the propeller disc occupied by the blades in relation to area open to the air flow.
What does increasing solidity allow for the propeller to do?
Transfer more power to the air.
What is the best way to increase propeller disc solidity
Increasing the number of blades
Torque reaction involves which one of Newton’s laws?
Newton’s Third Law of Motion.
When airborne, where does torque reaction act?
What does this do?
On the longitudinal axis, making the aircraft roll.
When the wheels are on the ground, what does the torque reaction do?
Creates a turning moment around the vertical axis (yaw).
What is used to correct yawing moment on the take-off roll?
The rudder
What does the high-speed rotation of an aircraft propeller do?
Gives a spiralling rotation to the slipstream, exerting a strong sidewards force on the aircraft’s vertical tail surface.
What happens when the spiralling slipstream strikes the vertical fin?
It causes a turning moment about the aircraft’s vertical axis (yaw).
What is done to compensate for the spiralling slipstream that strikes the vertical fin?
The vertical stabiliser is mounted obliquely (slantwise) 1° or 2° to the aircraft’s longitudinal axis.
What happens when force is applied to deflect the propeller out of its plane of rotation?
The resulting force is 90° ahead, in the direction of rotation and application
What type of moment does the gyroscopic precession effect cause?
Pitching and yawing moment.
Where does the resultant force act when pitch is increased for take-off?
What does this cause?
90° ahead, causing a yawing moment to the left around the vertical axis.
When air flows towards the propeller during horizontal flight, where is the centre of balance?
In the middle of the propeller.
What direction does airflow act in a climbing altitude?
Obliquely (slanted) from below.
Which blade has a greater angle of attack during climbing attitude?
Downward moving blade.
Where does centre of total thrust move during climbing altitude?
What does this cause?
Towards the blade with greater angle of attack. This causes a yawing moment.
What is the critical engine?
The engine which would produce the smallest yaw moment, should the other engine fail.
What can lack of force symmetry cause if one engine fails?
Yaw moments of differing amounts, depending on which engine fails.
How can different yaw moment amounts be improved?
Have the propellers turn in opposite directions (inboard downwards).
What shaped propeller produces the least noise?
A scimitar-shaped propeller.
Which shaped propeller produces the most noise?
Straight tip shaped propeller.
When does blade position phasing occur?
On multi-engine aircraft where two propellers are operating on the same wing in close proximity to one another.
