17.1 Flashcards
What is the purpose of the design of turboprop engines?
To push air rearwards to aeorfoil blades that push the air rearwards to create lift to move the aicraft forwards
What is the propellor?
A device consisting of a rotating hub with two or more rotating blades
What is the hub?
The central portion of the propellor that carries the blades
What is the purpose of the blades?
An aerofoil section that is attached to the hub
What is the blade butt?
The base of the propeller where the root ends
What is the blade root/shank?
The thickest part of the blade closest to the hub
What is a blade station?
A distance measured from the centre of rotation (inches or cm)
What is a master reference section?
A distance measured from the centre if rotation where all measurements are taken from
What is the master reference station on a fixed pitch propellor?
Variable pitch propellor?
75%
50%-75%
What is the blade face?
The flat side that produces thrust
What is the blade back?
The curved side of the propellor blade facing the direction of flight
Where does the blade chord line run?
Through the face and back surfaces
What is the plane of rotation?
The line at which the propellor turns. 90 degrees to the engines centreline
What is the blade angle?
The angle between the blade chord line and the plane of rotation
What is the pitch?
The distance advanced in one complete revolution
What is ‘Fine Pitch’?
Vertical blade angles. ‘Low pitch’
What is ‘Coarse Pitch’?
Horizontal blades angles. ‘High pitch’
How do you achieve reverse pitch?
Turning the propellor blades to negative angles to use for braking/reverse thrust
What does the dome assembly encase?
The pitch change mechanism
What is the spinner?
An aerodynamic fairing that overs the centre of the propellor
What is a tractor propellor?
A propellor mounted in front of the wing’s leading edge, also on the nose of the aircraft
What is a pusher propellor?
A propellor mounted behind the wing’s trailing edge, towards thew rear of the aircraft
Which of Newtons Law do propellors work off of and why?
Third law because the rotation of the propellors are converted to forward thrust.
What does the hub attach the blades to?
Piston engine, reduction gearbox and electric motor shaft
How does a propellor distribute air?
Large amounts of air slowly rearwards
What is the slipstream?
The backward moving air
What are the two losses involved in the slipstream?
Kinetic energy and friction between the air and the propellor blades
How do you work out propellor efficiency?
The ratio between the power developed by the propellor (Less due to friction), and the power obtained from the engines power plant
Who created the ‘Momentum Theory’?
W.J.M. Rankine and R.E. Froude
The moment energy describes what two factors that impart air?
Kinetic and momentum
What is the momentum theory?
This theory is an assumed disc to be producing uniform thrust due to a constant pressure difference
What does the moment theory assume about the characteristics of the air?
Incompressible and without viscosity. Also streamlined and continuous in velocity
What does the element theory deal with that momentum doesn’t?
The effect of aerodynamic forces on the propellor blade
What does the element theory involve?
Breaking a blade down into many sections and determining thrust and torque along the different sections.
Using the element theory how can you tell the overall different forces and motions produced over the whole propellor?
The different sections are integrated along the entire blade over one revolution
What is slip?
The difference between geometric pitch and effective pitch (50%-90%)
What are fixed-pitch and ground adjustable propellors designed for?
best efficiency at one rotation and forward speed
What is geometric pitch?
The theoretical distance a propellor should advance in one revolution based on blade angle
What is effective pitch?
The distance it actually travels
In regards to slip when Is your engine most efficient?
With minimal slip
What is propellor efficiency?
It is used to define how well a propellor transmits rotational force or energy into thrust
What is greater, the energy it takes for the propellor to be driven, or the thrust from the propellor?
The amount of energy is takes for the propellor to be driven. So propellor efficiency is the minimising of this
What is the amount of thrust generated by a propellor reliant on?
The angle at which the blades attack the air
What determines the efficiency of the propellor more than the speed at which it turns?
The design an the shape of the propellor
The larger or smaller the prop diameter, means it is more efficient?
Larger
What limit the large size of the propellor?
The speed of the propellor tip
In a jet engine, what is the efficiency of the engine measured as a fraction of?
The potential heat energy of the fuel as it is converted into thrust
What about piston engines?
Brake power
What measures turboprop?
Shaft power (Horsepower)
What is the double drag coefficient that takes a toll on propellor efficiency?
The propellor fights its way through the air ahead of the aircraft
The propellor must fight its way through the air infant of each propeller blade as it makes each revolution
Friction and drag
What is the usual symbol for propellor efficiency?
ETA
What are the ranges of propellor efficiency?
0.8 to 0.9 (80%-90%)
What is the highest static load felt on the propellor?
Centrifugal force
What does the centrifugal force try to do?
Pull the blades out of the hub
What is Thrust bending force an example of?
Static Loads
What is Thrust Bending Force?
It attempts to bend the propeller blade tips forward due to the lift flexing the thin blade section
What is torque bending force also known as?
Braking moment
What is Torque bending force?
It tends to try to bend the blade angle against the direction of the propellor rotation
What does torque bending force create?
A resistance to the torque being produced by the engine
What is Aerodynamic and Centrifugal Twisting force an example of?
Static loads
What is Aerodynamic Twisting moment?
The centre of pressure being forward of the blades centre of rotation, will try to turn the blade to a higher (Coarser) blade angle
In reverse pitch, what will the ATM turn the blade to?
A coarser Negative Blade Angle
What is Centrifugal Twisting moment?
The mass of the blade is thrown out from the blades centre of rotation trying to turn the blade to a lower (finer) blade angle
What wins the opposition, CTM or ATM?
CTMS
Where are Aerodynamic, Centrifugal and thrust loads felt at?
The root so the most stresses will occur in this area and at the hub
During windmilling, what occurs between the ATM’s and CTM’s?
The ATM acts with the CTM to fine off the blades
Where is no damage or repair work permitted?
On the blade root
When will the maximum dynamic loading on a propellor blade occur?
Within its natural frequency range
What are the vibrations the result of on piston engines and propellors?
Operating strokes of a piston engine
Propeller reduction gearbox
What are the dynamic loads induced by?
Aerodynamic and mechanical forces
Where do aerodynamic forces have a greater vibration effect?
On the tip where the effects of the transonic speeds cause buffering and vibration
How can you decrease vibrations?
By using the correct aerofoil shape and tip design
What does the basic frequency ranges range from?
20 Hz to 6Hz
What is the Outer Nodal Point?
The highest vibrational loads are felt about 80% of the blades length making it susceptible to failure
What does the natural frequency of a propellor blade depend on?
Length, shape blade root and material
How can you decrease vibration?
Using correct aerofoil shape and tip design
How can you increase the power of the engine?
Increasing the amount of blades
What must the propellor absorb?
The power produced by the engine and transmit that power to the air flow passing through propellor disc.
What must the propellor be tailored to?
The specific needs of the engine it is fitted to
Increasing the Blade angles to help absorb power increases what?
The AOA allowing the propellors to impart more energy to airflow
What is a disadvantage of increasing the blade angle?
Optimises Aerodynamic efficiency
How does increasing the blade length allow the propellors to optimise more energy?
It makes the blades longer so can transfer more energy by affecting a larger volume of air
What is a drawback of making the blades longer by expanding the diameter of the propeller disc?
Tip blade speed
How does increasing the RPM of the propeller ensure the propellers can absorb more air?
The same amount of energy is transferred to the air but in a shorter time
What is bad about increasing the rpm of the propellor?
Blade tip speed
How does increasing the camber ensure the propellers can absorb more air?
It creates a greater thrust
What is bad about increasing the camber?
Blade structural problems
Optimum aerodynamic efficiency
What does increasing there chord of the propeller blades and the number increase?
The solidity (The outer aerodynamic circle when blades spin) meaning the propeller can transfer more power to the air.
Why is increasing the blade chord (increasing thickness) less efficient?
The aspect ratio is decreased so less aerodynamic efficiency
What is the most attractive option?
Increases the number of blades
Because they ran out of room on the propeller hub, what did they adopt to compensate for this?
Twin propellers on the same engine rotating in opposite directions (Contra-rotating propellers)
What is the propeller tip to fuselage?
1 inch
Propeller to nose wheel?
1/2 inches
What is the propeller tip to the ground (Nose wheel)?
7 inches
Propeller tip to ground (Tail wheel in flight attitude) (When the tail wheel has come off the ground due to the take off configuration)?
9 inches
Propeller Tip to Water (Float-Plane)?
18 Inches
What is torque reacting?
More lift is created on one wing so is being pulled downward
How do you correct torque reaction?
An offset in the engine (modern)
More lift in the downward wing (old)
What are the compensating factors permanently set for what phase of flight?
Cruise because this is where the aircraft is mostly
What do aileron trim tabs do?
Permit further adjustments for other speeds
When the wheels are on the ground during take off rolls and taxing, what occurs?
Yaw (A turning moment around the vertical axis) induced by a torque reaction
If the left side of the aircraft is being forced down, where is more weight placed?
On the left main landing gear
What does putting more weight on the left landing gear increase?
Ground friction or drag on the right tyre causing a further movement to the left
How do you correct yawing moment on the take off roll?
Rudder or Rudder trim
What is the twist effect?
The high speed of rotation got a propeller gives a spiralling rotation to the slipstream (exerts a sidewards force on vertical tail)
How can you correct the twist effect?
Move the vertical stabiliser 1 to 2 degrees along longitudinal axis
Pilot input on rudder
What is moving the vertical stabiliser 1 to 2 degrees along the longitudinal axis only good for?
Cruise
What does the corkscrew flow of the slipstream causes what?
A rolling moment on longitudinal axis in the opposite direction to the one caused by torque reaction.
What is gyroscopic effect?
When you put an input in, there is a resultant force at 90 degrees trying to oppose it
What does precession occur?
When a force is applied to any point on the rim of the propellors plane of rotation
When is the gyroscopic effect more likely?
During take-off role when the tail is being raised In tail wheel aircraft
What does gyroscopic effects rely on?
Rigidity and precession (More of interest)
What is precession?
The resultant action or deflection of a spinning rotor when a deflecting force is applied to its rim
How do you fix the gyroscopic effect?
Elevators and Rudder to prevent undesired yawing and pitching
When is Asymmetric loading (P-factor) a problem?
During climbing
What does Asymmetric loading (P-factor cause)
Yaw
The angle of the blade going down does what?
Creates more lift (bigger angle of attack)
The angle of the upgoing blade creates more or less lift ?
Creates less lift (smaller angle of attack)
What does the difference in lift cause?
A yaw towards the side giving less thrust (more thrust takes over)
What is the critical engine?
The engine that if it fails will create the highest yaw
What is relative airflow?
The speed (velocity) and direction of airflow moving past an airfoil
What two things have an effect on the relative airflow?
Rotational speed and forward speed
In normal flight, what is the state of the angle of attack and the blade angle?
Positive
What is windmilling?
A change in airspeed or rotational velocity resulting in a change of airflow over the blade leading to a negative angle of attack
If you have a fixed pitch propellor how do you prevent windmilling?
By choosing a blade angle that
need to be good for;
Good climbing performance
High cruising speed
Towing
What is Propeller Brake Moment?
The effort which is required to be able to spin the propeller
What happens to the brake moment if you have a higher angle of attack?
Higher blade angle is more brake moment
What is the brake moment on windmilling?
If you go into a negative angle of attack, meaning the air pushes the propeller to turn and then the propeller drives the engine. The rake moment works in the direction of rotation and begins to drive the propellor
How does the angle of attack change in regards to air speed?
If airspeed increases, angle of attack decrease and the other way round
How does the angle of attack change in regards to RPM?
If RPM increases, the angle of attack increases and the other way around
