17.1 Fundmentals Flashcards
What is the job of a propeller?
they are used to provide the thrust
What improvements have been made to modern propellers?
They are made with aluminium with thinner aero foil sections and greater strength
What happened when gas turbine engines were developed?
Propeller systems were adapted for these gte
What is a propeller?
A device, consisting of a rotating hub with two or more radiating blades; used to propel an aircraft
What is a hub?
The central portion of a propeller which carries the blades
What is a blade?
Aerofoil section that is attached to the hub
What is a blade butt?
The base of the propeller blade where the root ends
What is a blade root/shank?
The thickened portion of the blade nearest to the hub
What is a blade station?
A distance measured from the centre of rotation, normally measured in inches or centimetres
What is a master reference station?
A distance is 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 is the blade back?
The curved side of the propeller blade facing the direction of flight
What is a blade chord line?
A line through the blade profile at the points between the face and back surfaces
What is a plane of rotation?
The plane in which the propeller rotates. This is 90° to the engine centreline
What is a blade angle?
The angle between the blade chord line and the plane of rotation
What is the pitch?
Distance advanced in one complete revolution
What is pitch change mechanism?
Device to alter blade angle
What is fine pitch?
Vertical blade angles. Also referred to as “Low pitch”
What is 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 is dome assembly?
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 is the set up of a propeller?
The propeller consists of two or more blades that are connected by a hub. The hub serves to attach the blades to a piston engine, a Reduction Gearbox (RGB), or more recently, an electric motor drive shaft
What principle does a propeller work on?
Newton’s third law of motion
How is rotational energy provided?
It’s provided by the engine or motor is converted into forward thrust
How does a propeller make a plane move?
A propeller accelerates a large mass of air slowly rearwards
One surface of the blade is cambered or curved, like the upper surface of an aircraft wing what is this known as?
Blade back
One surface is flat like the bottom surface of a wing what is this known as?
Blade face
What is the backward air called?
Slipstream
What energy does slipstream have?
Kinetic energy due to its motion
What is propeller efficiency?
The ratio between the power developed by the propeller and the power obtained from the aircraft’s power plant
Where can you lose energy?
From friction of airflow over the blade and kinetic energy due to its motion
What does the high velocity on air result in for the propellers?
Lower static pressure in front of the propeller, pulling the aerofoil forward
What is the name of the guy who created momentum theory?
W.J.M. Rankine and R.E. Froude
How does momentum theory work with the propeller disk?
The diameter before the disk is bigger and after passing the disk gets smaller
What are the three things momentum theory assumes?
This theory assumes a propeller to be an advancing disc producing a uniform thrust, because of the pressure difference in front and behind the disc being a constant amount over its area.
It is also assumed that the air is a perfect fluid, incompressible and without viscosity
it is also assumed that the flow of air is streamlined in character and continuous through the propeller so that the axial velocity is the same immediately in front of and immediately behind the disc
What does the element theory deal with?
The aerodynamic forces acting on the propeller blades
What does the element theory involve?
breaking a blade down into several independent sections along the length and then determining the forces of thrust and torque on each of these small blade elements
What is the result of two dimensional list and drag characteristics?
The resulting values of each section’s thrust and torque can be summed to predict the overall performance of the propeller
What is blade angle theta?
The distance between the chord line and plane of propeller rotation
What does the amount of air displaced depend on?
Blade angle
What is blade angle similar to?
Pitch
How do pitch and blade angle correlate?
An increase or decrease in one is usually associated with an increase or decrease in the other
What are the two types of pitch?
Coarse pitch and fine pitch
What is the difference between coarse pitch and fine pitch?
Coarse pitch has a greater horizontal blade angle and fine pitch for a more vertical blade angle
What is fine pitch better for?
Good low speed acceleration
What is coarse pitch better for?
High speed performance and economy
What is blade angle a (alpha) range?
When you change the pitch in flight
Where is angle of attack?
Between chord line and relative airflow
What do variable pitch propellers have?
Variable pitch propellers have varying angles of attack for optimum performance
When is the propeller operating in alpha range?
Between Fine and Coarse
What is the angle of advance or helix angle between?
Between the rotational plane of propeller and the relative airflow
How does the AOA work with airspeed on an angle of advance or helix angle?
Angle of advance increases with increasing airspeed
What does the path the blades follow describe?
A helix
What does reverse pitch allow?
Reverse pitch is a pilot selectable feature of a constant speed or variable pitch propeller which allows the blade pitch to be decreased to a negative value
How does reverse pitch angle work?
the engine/propeller turns in the same direction as in the normal (forward) pitch position, but the propeller blade angle is positioned to the other side of fine pitch (negative pitch)
What does reverse pitch result in?
Braking action
What does reverse pitch get used for?
It is used for backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the aircraft to a stop during the landing roll
How does a windmilling propeller occur?
If the blade angle is reduced to such an extent that the angle of attack is less than the zero-lift angle of attack
How is a reverse pitch made?
When air is pushing the propeller blade it creates drag meaning it won’t go anywhere
How do you overcome reverse pitch?
Turn your engine faster or apply more power
What is the starting and reverse thrust range?
Beta range and is from flight fine to reverse
How does a propeller produce lift?
Since it’s a rotating aerofoil it produces lift by aerodynamic action
What does the amount of lift produced depend on?
The amount of lift produced depends on the aerofoil shape, Revolutions Per Minute (RPM), and Angle of Attack (AOA) of the blade sections
How do you ensure there is a constant angle of pitch?
You twist the blade this is known as propellers geometric twist
What does geometric twist ensure?
That the blade is always moving at the same speed even tho parts have to travel different distances and that there is even airflow along the length of the blade
What is pitch distribution?
The blade angle becomes smaller the further it is from the centre axis to keep a nearly constant angle of attack also known as geometric twist
What is root losses?
A thickened root area can withstand high stresses but loses aerodynamic efficiency. Airflow at the root is affected by the engine
What are tip losses?
Tip vortices and induced drag cause tip losses at high rotational speeds. Further losses are caused by compressibility effects
What is blade washout?
To maintain a constant angle of attack at differing rotational speeds along the blade, the leading edge of the propeller blade is twisted downwards from root to tip
What does propeller efficiency vary between?
From 50% to 90%
What is effective pitch?
The amount you actually travel
What is geometric pitch?
The perfect distance it could travel
What is propeller efficiency?
How well a propeller transmits its rotational force one energy into thrust
What does the amount of thrust generated depend on?
Angle at which its blades attack the air
In general the larger the prop diameter means what?
The more efficient it will be
How do you find the efficient in a jet engine?
Measured as a fraction of the potential heat energy of the fuel as it is converted into thrust energy
What is normal propeller efficiency range?
0.8-0.9 or 80% - 90%
What is the greatest load felt on the propeller?
Centrifugal forces
What is centrifugal force doing to the propeller?
It is trying to pull the blades out of the hub assembly
What does the thrust bending force do and why?
Attempts to bend the propeller blade tips forwards. This is due to the lift (thrust) flexing the thin blade section
What does the torque bending force do?
Tends to try and bend the blade against the direction of propeller rotation
Shat is aerodynamic twisting moment?
The centre of pressure, being forward of the blade’s centre of rotation, will try to turn the blade to a higher (coarser) blade angle
What is centrifugal twisting moment?
The mass of the blade is thrown out from the blade’s centre of rotation trying to turn the blade to a lower (finer) blade angle. CTMs will always oppose ATMs and CTMs are always greater than ATMs
Where are all static stresses felt?
At the blade root
when is no damage or repair work permitted?
Within the blade root area
Where will the maximum dynamic loading on a propeller blade occur?
Within its natural frequency range
Where do these vibrations come from?
The result of operating strokes of a piston engine or the dynamics of the propeller reduction gearbox
Why do the aerodynamic and mechanical effects need to be compensated in design?
Because the excessive flexing and work hardening of the metal will lead to structural failure during operation
Where do aerodynamic forces have a greater vibration effect?
At the tip of the blade
How can vibrations be decreased?
Using the correct aerofoil shape and tip design
Where is the highest vibrational load felt?
About 80% of the blade length
Why does propeller design vary?
How Mitch power needs to be produced
What must a propeller be?
Tailored to the specific needs of the engine it is fitted to
What are the options when engine power increases to design a propeller capable of efficiently absorbing power?
Increase the blade angle
Increase the diameter of the propeller disc
Increase the rpm of the propeller
Increase the camber if the blade
Increase the chord of the propeller blade
Increase the number of blades
What is the problem of increasing the blade angle?
The pitch of the blade is set by the angle that optimizes the aerodynamic efficiency of the blade, if this is changed then it is trading one negative off for another
What is the problem of increasing blade length?
The problem with this is that they became to big and then they could scrap the floor meaning you have to increase landing gear size
What is the problem for increasing the rpm?
The blade tips travel faster and faster as the rotational speed increase, meaning they will eventually be some supersonic
What is the problem for increasing the aerofoil camber?
Because the blade aerofoil is chosen for optimum aerodynamic efficiency if you change one negative it creates another
What are the two best options for propeller power absorption?
Increasing blade chord or the number of blades
What is the best possible option for propeller power?
Adding more blades
What are the 4 elements torque are made up of that cause a twisting it rotating motion?
Torque reaction from engine and propeller
Twist effect of the propeller wash
Gyroscopic action of the propeller
Asymmetric loading of the propeller (P-factor)
What is torque reaction?
There is an equal and opposite reaction for every action, Newton’s third law
How do you compensate for torque reaction?
some of the older aircraft are rigged in such a manner as to create more lift on the wing that is being forced downward, while more modern aircraft are designed with the engine offset to counteract this torque
When are the compensating factors in place?
Permanently
What is a twist effect of the propeller wash?
The high-speed rotation of an aircraft propeller gives a spiralling rotation to the slipstream. When this hits the vertical fin it causes a turning motion about the aircraft vertical axis
How do you compensate for twist effect?
The vertical stabiliser is mounted obliquely 1° or 2° to the aircraft’s longitudinal axis
What are all practical applications of the gyroscope are based upon?
Two fundamentals rigidity in space and precession
How is the gyroscopic action fought against?
For the pilot to properly use elevator and rudder to prevent undesired pitching and yaw
How much air do all blades push during horizontal flight?
The same for all blades and the centre of balance of thrust lies in the middle of the propeller
In a climbing attitude where does more airflow come from?
The direction of airflow is from below therefore the downward moving blades have a greater angle of attack
During climb attitude where is the centre of total thrust and what does this cause?
It moves away from the middle towards the blade with the larger angle of attack. This causes a yawing moment around the vertical axis
What does it depend on for the shift of the aircraft to go right or left?
Depending on the direction of rotation
What happens if both engines are spinning in the same direction?
The force acting in the aircraft is slightly asymmetrical
How do you prevent p factor?
By spinning the engines in opposite directions
What is relative airflow?
defined as the speed and direction of the air movement past an aerofoil
What does the relative airflow acting in a propeller have?
Has both direction and velocity and is governed by both rotational speed of the propeller
What are the blade angle and AOA in normal flight conditions?
They are both positive giving positive thrust and torque
What can lead to a negative angle of attack?
A change in airspeed or a change in rotational velocity will result in a change of the relative air flow direction and velocity
What will a negative AOA cause?
Windmilling where the wing is turning the propeller
What do you have to do for fixed propellers?
Pick somewhere in between fine and coarse pitch because you can’t change it
What is propeller brake moment?
Propeller brake moment can be considered to be the effort which is required to be able to spin the propeller
With a constant pitch what can cause AOA to reduce and potentially become negative?
If airspeed increases rapidly or rotational speed is greatly reduced
If air causes the propeller to spin what can you do?
Brake moment works in the direction of rotation and begins to drive the propeller
What happens if airspeed increases?
AOA decreases and RPM increases
What happens if airspeed decreases?
AOA increases and RPM decreases
What are the two main categories of vibration?
Mechanical and aerodynamic
What are the effects of mechanical vibration?
Static and dynamic balance of the propeller will, if not correct, give rise to a once per revolution cycle of disturbance
What is aerodynamic vibration related to?
Thrust and torque loads acting on each of the propellers
What does it mean if you get vibration throughout a certain rev range (e.g 2200 - 2350 rpm)?
Poor engine to propeller match
There is a certain amount of movement in the mountings what are they called?
It’s called blade shake
How can the propeller spinner show an out of balance condition?
Through noticeable spinner wobble while the engine is running
How can you tell if it is engine or propeller vibration?
By observing the propeller hub, dome, or spinner while the engine is running within a 1200-to-1500-RPM range and determining whether or not the propeller hub rotates on an absolutely horizontal plane
When propeller vibration is the problem for excessive vibration what is it caused by?
Caused by propeller blade imbalance, propeller blades not tracking or variation in propeller blade angle settings
What is propeller noise caused by?
Propeller noise is caused by their special aerodynamic features and is the result of the relative motion between the propeller blades and the surrounding air
What is broadband noise?
Broadband noise, on the other hand, occurs as a result of turbulence in the boundary layer of the blade, and the interaction of the blade with turbulence in the air
What is tonal noise?
Air that is interacting with near by structures
What is rotation noise?
Rotation noise is the product of a pressure field created by the rotating propeller
What is vortex noise?
This noise is caused by the vortices leaving the blade tip and blade trailing edge. Its maximum value is found in the plane of rotation of the propeller
What is displacement noise?
The origin of this noise is the displacement of the air by the propeller blades as they have a finite thickness
What is blade vibration noise?
This noise occurs with periodic stalls, for example when the stall limit of the blade is alternately exceeded and fallen below
What is noise caused by inconsistent airflow?
with variable pitch propellers when the angle of pitch is negative, and the propeller has zero thrust then the vortices of the preceding blade hit the leading edge of the following blade, resulting in noise
What is influence of power?
The more power you put in the more noise it will make
What is the influence of the propeller diameter?
If a blade gets longer but stays at the same constant speed then the noise will be reduced
What is the influence of the number of propellers?
The more propellers you have the less noise your propeller makes
What is the influence of blade tip Mach number?
The faster the top speed goes the louder the noise from the propeller
What is the influence of propeller blade shape?
The different shape of the blade means they make different amounts of noise
What blades make the least and most amount of noise?
A scimitar-shaped propeller produces the least noise, and a propeller with straight tips produces the most
What gives the least amount of vibration?
Wood and composite give off less noise than metal blades
What blade material makes the least amount of noise?
Composite
What is beat frequencies?
As the sound waves cross, the intensity of the noise and vibration increases dramatically and then dies away only to repeat on the next cross. These are the ‘beats’ that are a result of what is called constructive and destructive interference
What is blade position phasing?
Making them out of sync so they don’t make more noise
What is the critical range?
These power stroke pulsations from a piston engine can transmit into the propeller causing ‘standing wave’ vibration in the blades that can be particularly detrimental within a given RPM range
