17.1 Fundamentals Flashcards
What is a dirigible?
Airship
Definition of the propellor?
A device, consisting of a rotating hub with two or more radiating blades; used to propel an aircraft.
Definition of the hub?
The central portion of a propeller which carries the blades.
Definition of the blade?
Aerofoil section that is attached to the hub.
Definition of the blade butt?
The base of the propeller blade where the root ends.
Definition of the blade root/shank?
The thickened portion of the blade nearest to the hub.
Definition of the 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.
Definition of the blade face?
The flat thrust producing side of a propeller blade.
Definition of the blade chord line?
A line through the blade profile at the points between the face and back surfaces.
Definition of the plane of rotation?
Plane in which the propellor rotates, 90 degrees to the engine centreline.
Definition of the blade angle?
The angle between the blade chord line and the plane of rotation.
Definition of the pitch?
Distance advanced in one complete revolution.
Definition of the pitch change mechanism?
Device to alter blade angle.
Definition of fine pitch?
Vertical blade angles. Also referred to as “Low pitch”.
Definition of coarse pitch?
Horizontal blade angles. Also referred to as “High pitch”.
Definition of reverse pitch?
Turning the propeller blades to a negative angle to produce braking or reversing thrust.
Definition of dome assembly?
Encases the pitch change mechanism.
Definition of the spinner?
An aerodynamic fairing that covers the centre of the propeller.
Definition of the tractor propellor?
A propeller mounted in front of the leading edge of the wing or on the nose of the aircraft.
Definition of a pusher propellor?
A propeller mounted behind the trailing edge of the wing, or at the rear of the fuselage.
Which of newtons laws are propellors based on?
Third
What does the propellor do to the air?
large mass of air slowly rearwards.
What is the equation for thrust?
Delta P x Area
What pulls the aerofoil foward?
The high velocity of the air results in lower static pressure in front of the propeller
What is the backward moving air called?
Slipstream
What efficiency does a modern prop have?
80%
What is the momentum theory?
based on the consideration of the momentum and kinetic energy imparted to this mass of air. This theory assumes a propeller to be an advancing disc producing a uniform thrust,
Why is the momentum theory flawed?
It is based on all factors being perfect
What is a better theory than momentum theory?
Element theory- it deals primarily with the aerodynamic forces acting on the blades.
How does the element theory work?
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.
Element theory- what is V0?
Axial flow at propeller disc
Element theory- what is V2?
Angular flow velocity vector
Element theory- what is V1?
Section local flow velocity vector, summation of vectors V₀ and V₂
What is the Greek letter for blade angle?
Theta
What is blade angle measured in?
degrees
Where is the blade angle taken from?
Blade station
Where is the blade station?
between 0.5 and 0.75 of the radius of the propeller and is sometimes referred to as the “master station”.
What is the length and effective pitch of a prop designated 74-48?
Length 74 inches and effective pitch 48
What is a fully vertical blade?
Feathered
What is the alpha range?
If blade angle changes are controlled and stay within the ‘fine’ to ‘coarse’ positions, then the propeller is said to be operating in the “α (alpha) range”.`
What is the angle of attack?
angle between the profile chord line and the relative airflow towards it.
What two factors produce relative airflow?
Airflow velocity and propellor rotational velocity
What is the helix?
The path that the prop takes.
What happens to the angle of advance with increasing speed?
Increases.
What is the helix angle also known as?
Angle of advance.
What governs the relative airflow?
both the rotational speed of the propeller and the forward speed of the aircraft.
In normal flight what are the blade angle and angle of attack?
Both positive
What kind of torque does windmilling produce?
Negative
In which direction does a windmilling propellor turn compared to normal conditions?
Same
What is propellor brake moment?
the effort which is required to be able to spin the propeller.
At a constant rotational speed, the sum of propeller brake moment and engine torque is?
0
With a constant pitch if airspeed increases rapidly or rotational speed is greatly reduced, the angle of attack will?
Reduce and become negative, causing it too windmill and then have a braking force.
Reduction in airspeed does what to AoA?
increases
RPM is reduced at constant airspeed the AoA is?
Reduced
How many degrees past fine/flat is reverse pitch?
30 degrees
What is the beta range definition?
variable pitch propeller, the pilot can select specific blade angles to provide ease of starting and reverse thrust.
What is the beta range?
Flight Fine” (or sometimes referred to as “flat pitch” and produces no thrust) to ‘Reverse’ and is only available on the ground.
How is the lever moved from the alpha to beta range?
Lever must be lifted
Why are propellors geometrically twisted?
To ensure a nearly constant angle of pitch is maintained
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.
What are 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 is the ratio of efficiency?
thrust horsepower to brake horsepower
What does propeller efficiency vary due to?
Slip
What is propeller slip?
difference between the geometric pitch of the propeller and its effective pitch.
What is geometric pitch?
theoretical distance a propeller should advance in one revolution
if the aircraft moved through the air according to the geometric propeller pitch, the propeller angle of attack would be?
zero
What is geometric pitch usually expressed in?
Pitch inches
How is effective pitch calculated?
Aircraft speed x duration of one revolution
The amount of energy it takes to rotate the propeller is almost always greater or smaller than the thrust from the propeller?
Greater
What units is piston aircraft efficiency given in?
BHP
What units is turboprop aircraft efficiency given in?
Shaft HP
What is the advance ratio?
ratio of the aircraft’s airspeed and the propeller speed.
What is the greek symbol for efficiency?
eta (n)
What is the greatest load felt on a prop?
Centrifugal
How much more load can be created by centrifugal?
7500 times
What is thrust bending force?
attempts to bend the propeller blade tips forwards
What is torque bending force?
(Braking moment) tends to try and bend the blade against the direction of propeller rotation
What is Aerodynamic Twisting Moment(ATM)?
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.
In reverse pitch what will ATM do?
turn the blade to a coarser Negative Blade Angle.
What is Centrifugal Twisting Moment? (CTM)
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
Compared to an ATM what is a CTM?
will always oppose ATMs and CTMs are always greater than ATMs.
What does CTM depend on and what happens if you increase any of them?
Blade chord
Weight
RPM
CTM increases
What is the only time ATM works with CTM?
Windmilling
Where is the point where the greatest stress is felt?
Root and therefore hub
Is damage and therefore repairs permitted on the root?
No
Where do aerodynamic forces have greater vibration effect?
The tip
What are the natural frequency’s of metal and wood props?
20Hz Metal
60Hz Wood
What is the outer nodal point?
80% blade length, the point at which most vibration is felt.
In what ways could you design a prop that is capable of higher powers/
Increase pitch
Increase diameter
Increase RPM
Increase camber
Increase chord
Increase number of blades
What does increasing the RPM for a higher power allow?
The same amount of energy is transferred to the air but in a shorter time.
What is the minimum clearance of a twin prop aircraft to fuselage?
1”
What is the minimum clearance of a tail plane prop to ground?
9”
What is the minimum clearance of a tricycle aircraft prop to ground and to the nose wheel?
7”
0.5”
What is the minimum clearance of a water plane prop to water?
18”
What is the best option to design a prop to handle higher power?
Increase the number of blades
What is solidity?
ratio of total blade area to total prop disc area
What are the four elements of torque?
Torque reaction from engine and propeller
Twist effect of the propeller wash
Gyroscopic action of the propeller
Asymmetric loading of the propeller (P-factor)
Due to the fact that engine components spin in one direction what does the aircraft do?
Trying to rotate in the opposite direction.
On modern aircraft what is used to counteract the torque motion?
Aileron trim tabs
What is the twist effect of propeller wash?
At high propeller speeds and low forward speeds (as in take-off, approaches), this spiralling rotation is very compact and exerts a strong sidewards force on the aircraft’s vertical tail surface.
How is twist effect of prop wash compensated against?
vertical stabiliser is mounted obliquely 1° or 2° to the aircraft’s longitudinal axis
What is the p factor?
Asymmetric loading
What is asymmetric loading?
Yawing moment caused by a nose up attitude that causes the downward blade to have a greater AoA moving the centre of thrust away from the middle
What is the critical engine?
The engine which would produce the smallest yaw moment, should the other engine fail
In the case of small twin-engine aircraft, the flight performance is usually too low if the critical engine fails. This situation can be somewhat improved by having the propellers turn in?
Opposite directions (inboard downwards)
What is resonant frequency?
Many objects because of their shape and material have a natural vibration frequency that occurs if the object is struck. This natural frequency is the resonant frequency
What is forced vibration?
If a vibrating body is brought into contact with another body, the second body will begin to vibrate and will continue to do so until the source is removed
If a vibration occurs only at one particular RPM or within a limited RPM range (e.g., 2200 – 2350 RPM), the vibration is not normally?
A prop but a bad engine-prop match
If a propeller vibration is suspected but cannot be positively determined, the ideal troubleshooting method is to?
temporarily replace the propeller with one known to be airworthy and then test fly
How can cabin vibration be improved?
reindexing the propeller to the crankshaft.
What is reindexing?
The propeller can be removed, rotated 180°, and reinstalled.
What happens when the forced vibration frequency matches the resonant frequency of the body?
the amplitude of the vibration will increase dramatically
What is broadband noise?
occurs as a result of turbulence in the boundary layer of the blade, and the interaction of the blade with turbulence in the air.
When does rotation noise exceed all other noise?
With the blade tips travelling at Mach numbers of between M 0.5 – M 0.85 and an undisturbed flow of air to the blade
What is vortex noise?
caused by the vortices leaving the blade tip and blade trailing edge
What is displacement noise?
displacement of the air by the propeller blades as they have a finite thickness.
When does displacement noise equal rotation noise?
At blade tip numbers above Mach 0.9 this noise source equals that of rotation noise.
What is blade vibration noise?
This noise occurs with periodic stalls
What is the influence of power on blade noise?
double it and it causes a 5db increase
What is the influence of diameter on blade noise?
propeller diameter is doubled, propeller noise is reduced by 6 dB
What is the influence of no. of blades on blade noise?
if the number of blades is increased from 2 to 3, noise is reduced by about 1.1 dB (A
How much does a 4 blade prop weigh than a 3 blade?
35%
What is the influence of blade tip mach number on blade noise?
increase in the airflow Mach number of the propeller blade tips from M 0.63–0.87 increases noise by 16db
Which blade shape produces the least noise?
Scimitar
Which blade shape produces the most noise?
Straight tip
How much can blade tip shape vary db?
3-6
How much can profile shape vary db?
2-3
How much can blade contour and twist vary db?
1-2
How much can profile camber and section ratio vary db?
1-2
When does constructive interference happen?
when two waves overlap in such a way that they combine to create a larger wave
How are beat frequencies removed?
Synchronising
What is the purpose of blade position phasing?
So that tips never meet and create noise.
What is the critical range?
Stoke pulsations from the engine transmitting to the prop.
