Module 17.1 Fundamentals Flashcards
What is the purpose of a propeller?
Convert engine power into thrust
Improved aluminium propeller designs were designed with what?
Thinner aerofoil sections and greater strength
What is the hub?
The central portion of a propeller which carries the blades
What is the blade butt?
The base of the propeller where the root ends
What is a blade station?
A distance measured from the centre of rotation
What is a master reference station?
A distance measured from the centre of rotation where all measurements are taken from
Normally 75% from the centre of rotation on a fixed-pitch propeller and can be 50–75% on a variable pitch propeller.
What is the blade face?
The flat thrust producing side of a propeller blade
What direction does the blade back face?
Direction of flight
What is a blade?
Aerofoil section that is attached to the hub
‘The thickened portion of the blade nearest to the hub’ describes what?
Blade root/shank
What is blade angle measured between?
Chord line and plane of rotation
What angle is the plane of rotation to the engine centreline?
90°
What is the blade chord line?
A line through the blade profile at the points between the face and the back
A propeller mounted on the LE of the wing or nose of an aircraft is called what?
Tractor propeller
A propeller mounted on the TE of the wing or rear of the fuselage is called what?
Pusher propeller
What encase the pitch change mechanism?
The dome assembly
What is the definition of pitch?
Distance advanced in on complete revolution
What can the propeller spinner be described as?
An aerodynamic fairing
What does the spinner house?
The dome assembly
What are the 3 types of pitch?
- Fine (vertical blade angles)
- Coarse (horizontal blade angles)
- Reverse
What purpose does the hub serve?
Attach the blades to the engine
Which of newtons’ laws do propellers utilise?
Newton’s third law
How is thrust produced by a propeller?
Moving large amounts of air rearwards slowly
What surface of the blade is cambered or curved?
Blade back
What is the backwards moving air called?
The slipstream
What is referred to as the propeller’s efficiency?
The ratio between the power developed by the propeller and the power obtained from the aircraft powerplant
What are the energy losses?
Kinetic energy of the slipstream and friction of air flowing over the blade
What does momentum theory assume?
- Propeller is a disc
- Airflow is streamlined
- Air is prefect fluid
What is momentum theory?
The diameter of the air after it is accelerated is less than before it hits the blade
What does blade element theory deal with?
Primarily with the aerodynamic forces acting on the propeller blades
What does momentum theory deal with?
The flow of the air
How does element theory determine the thrust forces and torque along the blade?
By breaking the blade down into several independent sections (blade stations)
What may blade angle be referred to as?
Theta θ
What is it called when a blade is pitched to a fully vertical angle?
It is called ‘Feathered’
What does a fine pitch yield?
Good low speed acceleration (and climb rate)
What does a coarse pitch optimise?
High speed performance and economy
What is blade angle ɑ (Alpha) range?
When you vary the pitch of the blade in flight
What is the angle of attack ɑ between?
The chord line and the Relative Airflow (RAF)
This is relative airflow is different to on a wing
What is the difference between flight fine and flight coarse called?
Alpha range
What is angle of advance also called?
Helix angle
What is the helix angle/angle of advance between?
The rotational plane of the blade and the relative airflow?
What does the angle of advance increase with?
Increasing airspeed
What is blade angle the sum of?
Helix angle + Angle of attack
In reverse pitch what direction is the air pushed?
Towards the front of the aircraft
What does a negative blade angle give?
Negative/reverse thrust
What does reverse pitch result in?
A braking force, reversing and controlling taxi speed
When does the brake moment become very large?
When the blade angle is moved into reverse pitch
What does reverse pitch result in?
Windmilling
What is needed to prevent windmilling?
An increase in power readily available
What is beta β range?
On the ground. Gives ease of starting and reverse thrust
What does the amount of lift produced depend on?
- Aerofoil shape
- RPM
- Angle of Attack
What is done to the blade to ensure a nearly constant angle of pitch?
The propeller is twisted
What is propeller twist called?
Geometric twist
What does geometric twist keep constant?
Constant speed of airflow over the propeller
What happens to the blade angle as it gets further from the centre of rotation?
Blade angle becomes smaller
What effect does making the blade angle smaller towards the tip?
Nearly constant angle of attack
What is geometric twist the same as?
Pitch distribution and blade washout
What is airflow at the root affected by? (Root losses)
The engine
What can a thickened blade root withstand and what is the drawback of this?
High stresses but loses aerodynamic efficiency
What causes tip losses?
Tip vortices and induced drag at high RPM
Further losses caused by compressibility effects
What is slip?
Difference between geometric and effective pitch
What is the effective pitch?
Actual movement in one revolution
What is the geometric pitch?
Theoretical movement in one revolution based on your blade angle
Propeller efficiency varies depending on what?
The slip
Minimising slip ensures what?
Maximum efficiency
Does propeller shape affect the efficiency of the propeller?
Yes
The larger the propeller diameter, will it be more or less efficient?
More
What does normal propeller efficiency range between?
80-90%
What is the greatest static load?
Centrifugal force, can be 7500 times the weight of the propeller blade
Thrust bending forces attempt to bend the blade tip what way?
Forwards, due to the lift flexing the thin blade section
Torque bending force is also known as?
Braking moment
The torque bending force tends to try and bend the blade what way?
Against the direction of propeller rotation
What does ATM stand for?
Aerodynamic twisting moment
What type of angle does ATM force the blade to go to?
Coarser
What does CTM stand for?
Centrifugal Twisting Moment
What type of angle does CTM force the blade to go to?
Finer
What can be said about CTM and ATM?
They oppose each other
Which is always bigger CTM or ATM?
CTM
What are all 5 static loads?
- Centrifugal force
- Thrust bending force
- Torque bending force
- Aerodynamic twisting moment
- Centrifugal twisting moment
Where are all of these static loads felt?
At the blade root
Note: No damage or repair work is permitted
When will the maximum dynamic loading on a propeller blade occur?
Within its natural frequency
What are dynamic loads from?
Mechanically and aerodynamically
Aerodynamic forces have a greater vibration effect at what part of the blade?
The tip
Where are the highest vibrational loads felt?
Outer nodal point, area about 80% of the blade length
What is the basic frequency range?
20Hz (metal) to 60Hz (wood)
A propeller must be tailored to what?
Specific needs of the engine
What is power absorption?
Matching the propeller blade size to the engine output
If the propeller and engine are not properly matched what occurs?
Becomes inefficient
What are the methods of improving power absorption?
- Increase blade angle (more drag)
- Increase the diameter of the propeller disc (worried about tip speed)
- Increase the RPM (worried about tip speed)
- Increase the camber
- Increase the chord of the blades (wider)
- Increase the no. of blades
What are the two most effective ways of increasing the power absorption?
- Increase the chord length
- Increase the number of blades (the most effective)
What does increasing the chord length
and increasing the number of blades improve?
Propeller disc solidity
Twin propellers on the same engine rotating in opposite directions are called what?
Contra-rotating propellers
Propeller Tip to Fuselage clearance?
1 inch
Propeller to Nose wheel clearance?
1/2 inch
Propeller Tip to Ground (Nose wheel) clearance?
7 inches
Propeller Tip to Ground (Tail wheel in flight attitude) clearance?
9 inches
Propeller Tip to Water (Float-plane) clearance?
18 inches
What aircraft are susceptible to torque reaction in flight?
Nose mounted propeller
What are the ways of counteracting the torque reaction in flight?
More lift on the wing being forced downwards or engine offset in more modern aircraft
What is the way of counteracting the torque reaction on the ground?
Yaw, rudder input from the pilot
The magnitude of the torque moment is dependent on what variables?
- Size and horsepower of engine
- Size of propeller and the RPM
- Size of the aircraft
- Condition of the ground surface
What is the twist effect?
Spiral slipstream from the propeller hits against the vertical stabiliser and gives a yawing moment
What are the methods of correcting the twist effect?
- Pilot input
- Obliquely/offset mounted vertical stabiliser
Due to the gyroscopic effect what occurs after a force is applied to deflect the propeller out of its plane of rotation?
Causes a pitching or yawing moment or a combination of the two
How is the gyroscopic effect counteracted?
Rudder or elevator deflection or both
In level flight do both the up moving and down moving blade provide the same lift (thrust)?
Yes they do
In a climbing attitude what blade produces more lift (thrust)?
Downward blade
What is this difference in thrust in the up and down moving propeller called in a climbing attitude?
Asymmetric loading, P-factor
What effect does Asymmetric loading result in?
Left yawing movement
What engine is referred to as the critical engine in P-factor?
The one that produces the smallest yaw moment if the other engine fails
What is a method to eliminate P-factor on twin prop aircraft?
Spin the propeller in opposite directions
What is Relative Airflow (RAF) defined as?
The speed and direction of the air movement past an aerofoil
The RAF is governed by what?
- Propeller RPM
- Forward speed of the aircraft
In normal flight conditions the blade angle and the angle of attack are both positive giving positive thrust and positive torque. What does this show?
The engine is driving the propeller
What is windmilling?
Propeller drives the engine
What is windmilling caused by?
A negative angle of attack
In fixed pitch propellers what are the 3 parameters the angle should meet?
- Good climb performance
- High cruising speeds
- Towing
Propeller brake moment can be considered to be what?
The effort required to to be able to spin the propeller
Coarser angle = Higher brake moment
Finer angle = Lower brake moment
At a constant rotational propeller speed, what is the sum of the propellor brake moment and the engine torque?
0
With a constant pitch if airspeed increases rapidly or rotational speed is greatly reduced, what can happen to the angle of attack?
Reduce and can become negative
With a constant pitch if airspeed increases rapidly or rotational speed is greatly reduced, the braking moment will do what?
Assist the propeller in windmilling the engine
If airspeed is increased what occurs to the angle of attack?
Decrease in AoA
If airspeed is decreased what occurs to the angle of attack?
Increase in AoA
If RPM is increased what occurs to the angle of attack?
Increase in AoA
If RPM is decreased what occurs to the angle of attack?
Decrease in AoA
What are the two categories vibration causes?
First - Mechanical balancing
Second - Aerodynamic, balancing the forces acting on the blade
If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates when?
Throughout the entire RPM range
If a vibration occurs only at one particular RPM or within a limited RPM range occurs what does this mean?
A poor engine-propeller match
What is blade shake?
When the engine is not rotating, there is a certain amount of movement in the mountings
How can cabin vibration sometimes be improved?
Reindexing the propeller to the crankshaft. Propeller can be removed and rotated 180 and reinstalled
What can be contributing factor to an out-of-balance condition?
Propeller spinner
What is this out-of-balance condition shown by?
Spinner wobble when the engine is running
How can the cause of vibration be determined?
By observing the propeller hub, dome, or spinner while the engine is running within a 1200-to-1500-RPM range
When propeller vibration is the reason for excessive vibration, the difficulty is usually caused by what?
- Propeller blade imbalance
- Propeller blades not tracking
- Variation in propeller blade angle settings
Propeller noise is the result of what?
Motion between the propeller blades and the surrounding air
What are the two components of propeller noise?
- Tonal (interaction with surrounding structures)
- Broadband (turbulent air over propeller)
What are the remaining noise causes?
- Rotation noise
- Vortex noise
- Displacement noise
- Blade vibration (periodic stalls)
- Noise caused by inconsistent airflow
At blade tip speed of above mach 0.9 displacement noise is equal to what?
Rotation noise
Doubling of the power causes an increase in the noise level by approximately how many decibels?
5 dB
If the propeller diameter is doubled at constant peripheral speed, propeller noise is reduced by how many decibels? (RPM reduced)
6 dB
What effect does increasing the number of blades have on sound produced?
Reduces the noise
What effect does an increase in blade tip mach number have on the noise levels?
Increases the noise
What shaped propellers produce the least amount of noise?
Scimitar
What shaped propeller blades produce the most noise?
Straight tip
What materials have favourable vibration characteristics over metal?
Wood or composite
Beat frequencies are a result of what types of waves interference?
- Constructive
- Destructive
Blade position phasing is required on what type of aircraft?
Multi engine aircraft where two propellers are operating on the same wing in close proximity to each other
What occurs in the ‘critical range’?
Power stroke pulsations from a piston engine can transmit into a propeller causing a standing wave that can be detrimental.
Only at a certain RPM range
How is the critical range indicated?
Red region on the tachometer
