Fundamentals Flashcards
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 a propeller blade where the root ends
What is a blade root/shank?
The thickened portion of the blade nearest the hub
What is a blade station? What is it measured in?
A distance measured from the centre of rotation. Inches or cm
What happens to pitch when you increase blade angle?
Pitch increases
What principle does a propeller work on?
Newtons 3rd law
Why do we feather a propeller?
Prevent windmilling
How does a propeller move through the air?
Accelerates a large mass of air slowly rearwards
What is the master reference station?
A distance measured from centre of rotation
Where are most measurements taken from?
Centre of rotation
What % distance is the master reference station from the centre of rotation on a fixed-pitch propeller?
75%
What % distance is the master reference station from the centre of rotation on a variable pitch propeller?
50-75%
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 face and back surfaces
What is the plane of rotation?
The plane in which the propeller rotates?
What is the angle of the plane of rotation to the engine centreline?
90 degrees
What is the blade angle (theta)? Where is blade angle measured from?
The angle between the blade chord line and plane of rotation. Measured from master reference station
What is the pitch?
Distance advanced in one complete revolution
What is a pitch change mechanism?
Device to alter blade angle
What is the fine/low pitch?
Vertical blade angles
What is the coarse/high pitch?
Horizontal blade angles
Is the diameter of airflow before the propeller disc larger in regards to momentum theory?
Yes
What is reverse pitch?
Turning the propeller blades to a negative angle to produce braking or reversing thrust
What does momentum theory primarily deal with?
Flow of air
What does element theory primarily deal with?
The aerodynamic forces acting on propeller blades
How can you predict the overall performance of a propeller in regards to element theory?
Breaking down a blade into sections and determining the sum of thrust and torque produced by the section
What theory is useful for determining ideal efficiency?
Momentum theory
What is the dome assembly?
Encases the pitch change mechanism
What is a spinner?
An aerodynamic fairing that covers the centre of the propeller
What is a tractor propeller?
A propeller mounted in the 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
Does a fine or coarse pitch propeller ‘move through the air a larger distance in one revolution’?
Coarse pitch propeller
When is blade angle in the alpha range?
If a propeller varies pitch (within the fine to coarse positions) during flight
What is the angle of attack (alpha)?
The angle between the profile chord and relative airflow
What is the angle of advance or Helix angle (phi)?
The angle between the rotational plane of the propeller and the relative airflow
What happens to the angle of advance as speed increases?
Angle of advance increases
When is the blade angle in beta range?
On ground to provide ease of starting and reverse thrust
What 3 factors affect the amount of lift produced?
- aerofoil shape
- revolutions per minute
- Angle of attack
What happens to velocity as the distance of a blade section increases from its rotational axis?
Velocity increases
What is pitch distribution? Why is pitch distribution used?
Blade angle becomes smaller the further it is from the centre of axis. To maintain a constant angle of attack
What is a propellers geometric twist? Why is the propeller twisted?
Blade is twisted from root to tip. Maintains a constant angle of pitch
What are 3 causes of tip losses? What loss is a result of high rotational speeds?
- tip vortices
- induced drag (at high rotational speeds)
- compressibility effects
What is blade washout? Why is blade washout used?
Leading edge of propeller blade is twisted downwards from root to tip. Maintain a constant angle of attack at differing rotational speeds along blade
What is the ratio of propeller efficiency?
The ratio between the power developed by the propeller and the power obtained from aircraft’s power plant
What is geometric pitch?
The theoretical distance a propeller should advance in one revolution
What is effective pitch?
The actual distance a propeller advances in one revolution
Does geometric pitch include propeller slippage in the air?
No, effective pitch does
What is propeller efficiency (eta)?
Defines how well a propeller transmits its rotational force of energy into thrust
What are the ranges of normal propeller efficiency?
0.8 to 0.9 (80-90%)
What is the propeller efficiency measurement for a piston engine?
Brake power
What is the propeller efficiency measurement for a turboprop aircraft?
Shaft power
Can you carry out any damage or repair work within the blade root are?
No
What is the greatest load felt on a propeller? How much times the weight of a propeller blade can the load create?
Centrifugal force. Over 7500x the weight of the propeller blade
How does centrifugal force act on the propeller? Is it a static or dynamic load?
Tries to pull blades out of hub assembly. Static load
How does thrust bending force act on the propeller? Is it a static or dynamic load?
Attempts to bend the propeller blade tips forwards. Static load
How does a torque bending force (braking moment) act on the propeller? Is it a static or dynamic load?
Tried to bend the blade against the direction of propeller rotation. Static load
How does an aerodynamic twisting moment (ATM) act on the propeller? Is it a static or dynamic load?
The centre of pressure will try to turn the blade to a coarser (higher) blade angle. Static load
How does a centrifugal twisting moment (CTM) act on the propeller? Is it a static or dynamic load?
The mass will try to turn the blade to a finer (lower) blade angle. Static load
Is the centrifugal twisting moment or aerodynamic twisting moment always greater?
Centrifugal twisting moment is always greater than
What % of the blade length are the highest vibrational loads felt? What is this area known as?
80% of the blade length. Outer Nodal point
What is the best method a propeller can use to absorb engine power?
Increasing the number of blades
What are the 6 methods a propeller can use to absorb engine power?
- increase blade angle
- increase diameter of the propeller disc
- increase RPM
- increase the camber of the blade
- increase the chord of the blade
- increase the number of blades
What does torque reaction cause? How do you compensate for torque reaction on older a/c and modern a/c?
Equal force tries to rotate a/c in opposite direction to propeller causing the aircraft roll. Older a/c - generate more lift on downwards wing, Modern a/c - engine offset
What does twist effect of propeller wash cause? How can you compensate for twist effect? When does twist effect occur?
Spiral slipstream caused by propeller turning hits vertical fin and causes a/c to yaw. Offset vertical stab by 1 or 2 degrees. At high propeller speeds (take off) and low forward speeds (approaches).
What does asymmetric loading (p-factor) cause? How do you compensate for asymmetric loading in small a/c? What phase of flight does this occur in?
Downwards moving blade has a greater angle of attack so total thrust is not balanced causing a yawing moment on vertical axis. Have propellers turn in opposite direction. During climb
What does gyroscopic effect cause? How can you compensate for gyroscopic effect?
A force deflects the propeller out of its plane of rotation causing a pitching and/or yawing moment. Proper use of elevator and rudder
What is relative airflow (RAF)?
The speed and direction of the air movement past an aerofoil
Does a windmilling propeller turn the same way?
Yes
What is propeller brake moment?
The effort which is required to spin the propeller
What 2 conditions can cause windmilling if aircraft has a constant pitch?
- rapid velocity increase
- rotational speed greatly reduced
What happens to angle of attack, brake moment and RPM if speed increases?
Angle of attack decreases, brake moment decreases and RPM increases
What happens to angle of attack if RPM is reduced at a constant airspeed?
Angle of attack decreases
What happens to the brake moment when the blade angle is reduced?
Brake moment is reduced
What is natural vibration?
When a body oscillates under the action of its own gravitational or elastic forces, with no external forces present e.g. spring
What is resonant frequency?
The natural vibration frequency that occurs if certain objects are struck e.g. church bells
What is forced vibration?
If a vibrating body is bought into contact with another body, the second body will vibrate until the source is removed
What are the 2 main sources of propeller vibration?
Mechanical and aerodynamic
What is usually the cause of vibration at one RPM with limited RPM range only?
Poor engine-propeller match
What is blade shake? Is blade shake a source of vibration?
Certain amount of movement in the mountings when the blade is not rotating. No
How can cabin vibration be improved?
Reindexing the propeller to the crankshaft
What would be an indication of an out of balance propeller spinner condition? What is this caused by?
Spinner wobble whilst engine is running. Inadequate shimming of the spinner front support or a cracked/deformed spinner
If the propeller hub does not rotate in orbit then is this a propeller or engine vibration issue?
Engine
What is resonance?
If a body is set into forced vibration it will vibrate with the frequency of that vibration
What two components is propeller noise made up off?
Tonal and Broadband
How is broadband noise caused?
Turbulence in boundary layer of the blade
How is tonal noise caused?
Displacement of air caused by blade motion
What type of noise occurs at Mach 0.5-0.85?
Rotation noise
What type of noise exceeds all other noise components?
Rotation noise
What is rotation noise caused by?
Pressure field created by the rotating propeller
What is vortex noise caused by?
Vortices leaving the blade tip and blade trailing edge
At what Mach numbers does displacement noise equal rotation noise?
Above mach 0.9
What is the cause of blade vibration noise?
Periodic stalls
If you double the power how many decimals will the noise level increase to?
5dB
If the propeller diameter is doubled at a constant peripheral speed then how many decibals is the propeller noise reduced by?
6dB
What happens if the propeller diameter increases e.g. doubles, with constant rotational speed?
Noise increases
If a propeller blade is added at constant RPM, same power and with same propeller diameter then the noise would be reduced by how many decibels?
1.1dB
If the blade tip Mach number is increased from Mach 0.63-0.87, how many decibals with the noise level increase by?
16dB
Does a scimitar or straight tip propeller create the most noise?
Straight tip
Do wood and composite blades or metal blades have better vibrational behaviours?
Wood and composite
When does constructive interference occur?
When 2 RPM frequency waves overlap and combine to create a larger wave
When does destructive interference occur?
When 2 RPM frequency waves overlap and cancel each other out
What vibration cause occurs only on multi engine a/c with 2 propellers operating on same wing in close proximity?
Blade position phasing
