17.1 Fundamentals Flashcards
what is the design and function of a propellor?
it takes a large quantity of air and pushes it backwards slowly rather than a small amount quickly like that of a turbojet
what speeds are propellors most efficient at?
lower speeds
what was the initial design of propellors?
originally they were designed as small aerofoils wish merely pushed air back
what improvements have been made to modern propellors?
they are now made of aluminium with thinner aerofoil sections and greater strength
what did the introduction of gas turbine engines allow for?
efficient operation at medium altitudes and speed
what is Propeller?
A device, consisting of a rotating hub with two or more radiating blades; used to propel an aircraft.
what is the Hub?
The central portion of a propeller which carries the blades.
what is the blade?
Aerofoil section that is attached to the hub.
what is the blade butt?
The base of the propeller blade where the root ends.
what is the Blade root/shank?
The thickened portion of the blade nearest to the hub.
what is the 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.
where is a master reference system usually?
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 the blade chord line?
A line through the blade profile at the points between the face and back surfaces.
what is the plane of rotation?
The plane in which the propeller rotates. This is 90° to the engine centreline.
what is blade angle?
The angle between the blade chord line and the plane of rotation.
what is pitch?
Distance advanced in one complete revolution.
what is the function of the pitch change mechanism?
a device to alter blade angle
what does fine pitch mean?
Vertical blade angles. Also referred to as “Low pitch”.
what does coarse pitch mean?
Horizontal blade angles. Also referred to as “High pitch”.
what does reverse pitch mean?
Turning the propeller blades to a negative angle to produce braking or reversing thrust.
what is the dome assembly?
Encases the pitch change mechanism.
what is the spinner?
An aerodynamic fairing that covers the centre of the propeller.
what is ab tractor propellor?
A propeller mounted in front of the leading edge of the wing or on the nose of the aircraft.
what is a pusher propellor?
A propeller mounted behind the trailing edge of the wing, or at the rear of the fuselage.
what is the backward moving air from a propellor called?
slipstream
what is the hubs function?
to attach the blades to a piston engine, reduction gearbox or more recently an electric motor drive shaft
what principle does a propellor function on?
newtons third law
why do we feather propellor blades?
to prevent windmilling
what is the feather angle?
90 degrees
what are the losses associated with a propellor?
-slipstream
-friction between air and propellor blades
what do the losses associated with propellors cause?
the ratio of the power developed by the power plant and the what propellors ability to convert that energy to thrust is lessened
what is the name for propellors ability to take power from the powerplant and convert it into thrust called?
propellor efficiency
what is momentum theory?
the propeller disc decreases the diameter of the air after the disc
who came up with momentum theory?
R.E. Froude and W.J.M Rankine
what does element theory deal with?
aerodynamic forces acting on the propellor blades
how does element theory function?
breaking down the blade into sections and determining how torque and thrust act on these independent sections
how is blade angle expressed?
θ (Theta)
what is pitch likened to?
gearing in a car low pitch for low speed and higher pitch for higher speeds
what does fine pitch allow for in terms of RPM?
high RPM as it does not take a large bite out of the air and can spin easily
what will the finer pitch cause?
a smaller distance traveled in one revolution
what is Blade Angle α (Alpha) Range
varying pitch in flight between fine and coarse
what is α (Alpha)?
angle of attack
what is angle of attack?
the angle between the profile chord line and the relative airflow towards it
what is Angle of Advance or Helix Angle φ (phi)?
the angle between the rotational plane of the propeller and the relative airflow.
what is the path of a propellor described as?
a helix
what changes the angle of advance?
The angle of advance increases with increasing airspeed.
what is caused by reverse angle?
thrust being directed in the opposite direction
what range is reverse angle a part of?
beta
what is another name for reverse pitch angle?
negative pitch angles
what does reverse pitch result in?
braking action
what can reverse angle be used for?
reversing and braking
what is brake moment in reverse thrust?
the air is turning the propellor which is required to be overcome so a torque force is being applied
(also known as a windmilling propellor)
what are the other names for beta range?
flight fine or flat pitch
what does lift depend on in propellors?
-aerofoil shape
-RPM
-angle of attack
how do we ensure an almost constant angle of pitch?
create geometric twist on the propellor
what is the design of the pitch across the propellor?
the pitch reduces as you move away from the centre axis
why does pitch vary across the blade?
to avoid tip stall at cruise speed
what are the other reasons for shape change across the blade?
static and aerodynamic reasons
what are root losses?
the thicker root has higher aerodynamic losses
what are tip losses?
tip vortices and induced drag cause losses at the tip
what is blade washout?
to maintain a constant angle of attack the blade is twisted downward from root to tip
what is propellor slip?
the difference from geometric pitch and its effective pitch
what is geometric pitch?
where the angle of pitch is equalled by the blade angle, it can only occur if 100% efficiency was achieved
what is effective pitch?
the actual helical path of which the propellor moves through the air, the angle of pitch corresponds to to angle of advance
what is propellor efficiency?
how well the propellor transmits its rotational force into thrust
what is the biggest defining factor of propellor efficiency?
the design and shape of a propellor rather than its RPM
what is the general rule for propellor efficiency and its limitation?
the larger the prop diameter the more efficient the propellor is however, the tip speed limits this
how is the efficiency of the engine measured?
a fraction of the potential heat energy of the fuel as it is converted into thrust energy
what is the normal efficiency range for a propellor?
80-90%
what is the greek symbol for efficiency?
(eta)
what is poor propellor efficiency at least in part reliant on?
the propellor has to fight through the air in front of the aircraft and but also the air in front of each blade caused revolution
what are static loads?
-Centrifugal Force
-Thrust Bending Force
-Torque Bending Force
-Aerodynamic Twisting Moment (ATM)
-Centrifugal Twisting Moment (CTM)
what is the greatest static load?
centrifugal force it can be more than 7500 times the weight of the blade trying to pull the blade out of the hub
what is thrust bending load?
the thrust force created trying to push the blade forward
what is torque bending force?
the force of the blade flexing opposing the direction of spin due to braking forces
what is aerodynamic twisting moment?
the centre of pressure being forward of the centre of rotation trying to force the blade to a coarser pitch
what is centrifugal twisting moment?
the mass of the blade is thrown out from the blades centre of rotation causing the blade to fight for a finer pitch
what is the relationship between aerodynamic twisting moment and centrifugal twisting moment and which is stronger?
they oppose each over and CTM is greater
in reverse pitch what will aerodynamic twisting moment cause?
a coarser negative blade angle
where are static loads felt?
at the root and hub
what damage or repair is permitted at the root?
none
when will the maximum dynamic loads occur?
within the blades natural frequency range
what causes dynamic loads?
vibration from operating strokes of a piston engine or the accessory gear box
what else causes dynamic loads?
aerodynamic or mechanical force
where are aerodynamic force felt more?
the tip of the blade
where are the highest vibration loads felt on the blade?
80% of the blade length
what must a propellor be tailored to?
the specific engine needs its fitted to
what must the propellor be able to do in terms of power absorption?
absorb the power produced by the engine and transmit it to thrust
what does increasing blade angle achieve?
increase the angle of attack which leads to imparting more energy to the airflow
what does increasing the diameter of the propellor disc cause?
the can accept a larger quantity of air transferring more energy
what does increasing the RPM cause?
the same amount of energy is transmitted to the air on a shorter amount of time
what does increasing camber cause?
creates a greater thrust force
what are the two best ways of increasing blade power absorption?
increase chord
increase number of blades
what are the major drawbacks of blade angle change and blade length in terms of power absorption?
blade angle: pitch is set to optimise aerodynamic efficiency changing this is a trade off
blade length: as well as tip speed making larger landing gear etc increases weight
what are the main drawbacks of RPM increase and aerofoil camber increase in terms of power absorption?
RPM: tips can become supersonic increasing drag
Aerofoil camber: it sacrifices aerodynamic qualities
what is the theory behind increasing chord length and number of blades?
they increase the solidity of the propellor disc meaning more of the are is occupied by blades
what is the distance allowed between propellor tip to fuselage?
1 inch
what is the distance allowed between propellor to nose wheel?
1/2 inch
what is the distance allowed between propellor tip to ground nose wheel?
7 inches
what is the distance allowed between propellor tip ground tail wheel?
9 inches
what is the distance allowed between propellor tip to water?
18 inches
what is a drawback of increasing blade chord?
the aspect ratio is effected causing aerodynamic loss
what is the most attractive option for power absorption?
increasing number of blades
what has increase in engine power allowed for in terms of power absorption?
contra rotation propellors
what is torque reaction?
due to newtons third law the spinning of the propellor causing the aircraft to rotate in the opposing direction
what is done on older aircraft with a nose propeller to counteract torque reaction?
have one wing create more lift
what can account for torque reaction at lower speeds in older aircraft?
aileron trim tabs
what is done on modern engines to counteract torque reaction?
engine offset
what is caused by torque reaction in the take off roll?
yawing corrected by the rudder
what is caused by propellor wash?
twist effect
what is propellor wash?
where the propellor causes air to swirl in the slip stream
what is twist effect?
where swirling slip streams impact the rudder causing yaw
how can twist effect by corrected for?
either deflecting the rudder 1-2 degrees or deflecting the rudder manually
what does gyroscopic effect cause?
yaw
how can gyroscopic effect be corrected for?
input of the rudder and elevator
what is P factor?
in climb the angle of attack on the downward moving blade and upward angle of attack is smaller causing a yawing movement
if an aircraft has two engines what occurs to p factor if both blades move in the same direction?
the force is slightly asymmetrical
if one engine fails what will occur in terms of P factor if the engines spin in the same direction?
a yawing motion, the engine which fails that will cause the smallest yaw motion if failed is known as the critical engine
how can P factor be corrected for?
have the engines spin in opposing directions for dual engine aircraft
what is relative airflow?
the speed and direction of air movement past an aerofoil
what effects relative airflow?
rotational speed of the propellor and forward speed of the aircraft
what are the blade angle and angle of attack in normal flight?
positive giving positive thrust and torque
what is windmilling?
-change in airspeed or rotational velocity can lead to a negative angle of attack
-at a negative angle of attack the airflow will drive the propellor and therefore the engine giving negative torque
what are the operating flight phases that fixed propellor aircraft are suited to because of the inability to change the angle of attack?
-good climb performance
-high cruising speeds
-towing
what is propellor brake moment?
the energy required to be able to spin the propellor
what is the sum of propellor brake moment and engine torque when at a constant rotational speed?
zero
what occurs to brake moment when windmilling occurs?
the brake moment works in the direction of the propellor and drives the propellor
what is the effect of airspeed on the angle of attack?
decreased airspeed increases angle of attack and vice versa
what effect does angle of attack have on braking moment?
the higher the angle of attack the higher the braking moment
what can decreased airspeed lead on the basis of higher angle of attack?
decreased RPM
what does change of RPM lead to?
high RPM leads to high angle of attack and vice versa
what does a decrease of blade angle cause in term of brake moment?
a decrease in brake moment and vice versa
what is natural vibration?
when a body oscillates naturally under its own gravitational or elastic forces
what is resonant frequency?
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 of the object.
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
what is the largest cause of vibration?
-the propellor rotating
what are the two categories of vibration causes?
mechanical and aerodynamic
what are the methods of preventing vibration caused by mechanical vibration?
static and dynamic balance
what is the best methods of preventing vibration caused by aerodynamics?
evenly distribute the thrust and torque loads acting on each blade
what is blade shake?
when a propellor is not moving there is a certain amount of in the mounting it is not a cause of vibration
what can sometimes improve cabin vibration?
reindex the propellor to the crankshaft
what can the propellor spinner be a contributing factor towards?
out of balance condition called wobble which is noticeable from viewing the propellor spinner whilst it is running
how can it be deciphered if vibration is coming from the engine or propellors?
by observing the propellor hub, dome or spinner while the engine is running at 1200 - 1500 RPM range
what are the two components of propellor noise
tonal and broad band
what is tonal component of noise?
the most audible section, caused by the displacement of the air caused by blade movement and unsteady periodic loading on the blades
what is the broadband component of noise?
a result of turbulence in the boundary layer of the blade, and the interaction of the blade with turbulence in the air.
what is rotation noise?
caused by a pressure field created by the rotating propellor between mach 0.5 - 0.85 at the tips
this noise is the loudest
what is vortex noise?
caused by vortices leaving blade tip and trailing edge
what is displacement noise?
displacement of air by propellor blades at mach 0.9 it equals rotation noise
what is blade displacement noise?
noise occurs with periodic stalls, for example when the stall limit of the blade is alternately exceeded and fallen below
what effect does propellor diameter have on noise?
if diameter is increased noise level decreases
what effect does the number of propellor blades have on noise?
increasing the number of blades decreases noise
which propellor shape produces the least noise?
scimitar-shaped
what influence does material have on noise?
woods and composites absorb more energy and naturally dampen
what are beat frequencies?
when two propellors are running at similar RPM, frequencies tend to cross over each other in a rythmic fashion
what are the two types of beat frequencies?
constructive - where the combined frequencies build together and increase
destructive - where the opposite occurs
what is the significance of blade positioning phases?
As the propellers rotate it can occur where the phase of rotation of each propeller results in the tips of propeller blades pass near each other. This creates an aerodynamic interaction or ‘pulse’ in the air gap
