17.1 Flashcards
1
Q
What is a propeller
A
- consists of rotating hub with 2 or more radiating blades
2
Q
What is a hub
A
- central portion of propeller which carries the blades
3
Q
What is a blade
A
- aero foil section attached to hub
4
Q
What is a blade butt
A
- base of propeller blade where root ends
5
Q
What is a blade root/shank
A
- thickened portion blade nearest to hub
6
Q
What is a blade station
A
Distance measured from centre rotation (cm/inches)
7
Q
Explain master reference station
A
- distance measured from centre of rotation when all measurements are taken from
- 75% from centre of rotation on fixed pitch propeller
- 50-75% on variable pitch propeller
8
Q
What is blade face
A
- flat thrust producing side of a propeller blade
9
Q
What is a blade back
A
- curved side of propeller blade facing the direction of flight
10
Q
What is a blade chord line
A
- line through blade profile at points between face and back surfaces
11
Q
What is a plane of rotation
A
- plane in which propeller rotates
- 90* to engine centreline
12
Q
What is a blade angle
A
- angle between blade chord line and plane of rotation
13
Q
Explain pitch
A
- distance advanced in one complete revolution
14
Q
What does the ‘pitch change mechanism’ do
A
- device to alter blade angle
15
Q
What is fine pitch
A
- vertical blade angles
- referred as ‘low pitch’
16
Q
What is coarse pitch
A
- horizontal blade angles
- referred as ‘high pitch’
17
Q
What is reverse pitch
A
- turning propeller blade to negative angle to produce braking or reversing thrust
18
Q
What is dome assembly
A
- encases pitch change mechanism
19
Q
What is a spinner
A
- aerodynamic fairing that covers centre of propeller
20
Q
What is a tractor propeller
A
- propeller mounted in front of LE of wing or on nose of aircraft
21
Q
Explain pusher propeller
A
- propeller mounted behind trailing edge of wing or at rear of fuselage
22
Q
Anything moving to 90* does what to the pitch
A
Increase
23
Q
Which newtons law does the propeller work
A
3rd law
24
Q
Propellers accelerate airflow over their what surface
A
Cambered surfaces
25
What face produces thrust in a propeller blade
Blade face
26
What is the backward moving air called
Slipstream
27
Ratio between power developed by propeller and power obtained is referred to as what
Propellers efficiency
28
What is the ‘momentum theory’ based on
Consideration of momentum and kinetic energy imparted to this mass of air
29
Diameter before the disc is greater or smaller
Greater
30
Explain diameter after the disc
- diameter is smaller
- air is compressed and has more kinetic energy
31
What does the element theory involve
Involves breaking a blade down into several independent sections along the length
32
What is the element theory
- aerodynamic forces acting on the propeller blades
33
What is the blade angle (Theta)
Angle between chord of blade and plane of rotation
34
When discussing the blade angle, reference is often made to what
Pitch
35
When a blade is pitched to a fully vertical angle what is it called
‘Feathered’
36
Which pitch allows for large distance with every revolution
Coarse
37
Having a coarse setting limits what
Speed at which engine operates
38
Which pitch gives you short distance with every revolution
- fine pitch
39
What is blade angle (alpha) range
Designed to vary pitch in flight
40
What is angle of attack
Angle between profile chord line and relative airflow
41
As propeller rotates and advances through air, the path the blades follow is described as what
Helix
42
What is the angle of advance (helix angle)(phi)
- angle between rotational plane of propeller and relative airflow
43
The angle of advance increases with what
Increasing airspeed
44
Reverse pitch does what to the blade angle
Blade pitch is decreased to a -ve value
45
What is blade angle (beta) range
- available on ground
46
What is flight fine also known as
‘Flat pitch’ and produces no thrust
47
Explain brake moment at reverse pitch
- air should not drive engine. Therefore more engine power required to maintain propeller speed
48
What are the 3 things that produce lift
- aerofoil shape
- RPM
- AOA
49
The further the blade section is from rotational axis results in greater velocity
True
50
To ensure constant angle of pitch, propeller blade must be twisted from root to tip which is known as
Propellers geometric twist
51
Blade angle becomes smaller the further it is from centre axis to keep constant AOA is known as
Pitch distribution
52
Explain root losses
Can withstand stresses but loses aerodynamic efficiency
53
Explain tip losses
- tip vortices and induced drag cause tip losses at high rotational speeds
- further losses caused by compressibility effects
54
Explain blade washout
Maintain constant AOA at different rotational speeds along the blade
55
How is blade washout constructed
- leading edge of propeller blade is twisted downwards from root to tip
56
What is the ratio for efficiency
Useful power output/actual power input
57
Propeller efficiency varies between
50%-90%
58
What is propeller slip
- difference between geometric pitch of propeller and it’s effective pitch
59
What is geometric pitch
- theoretical distance a propeller should advance in 1 revolution
60
What is effective pitch
Distance it actually advances
61
The larger the prop diameter means what with efficiency
More efficient
62
Usual symbol for propeller efficiency is
Greek symbol (eta)
63
Which load is the greatest felt on the propeller
-centrifugal force
- load created can be >7500 times the weight of the propeller blade
64
What is thrust bending force
Attempts to bend propeller blade tips forward
65
Normal propeller efficient ranges from
80%-90%
66
Explain the torque bending force
- bend the blade against the direction of propeller rotation
67
Explain aerodynamic twisting moment (ATM)
- CP being fWD of blades centre of rotation
- tries to turn blade to higher (coarser) blade angle
68
Explain centrifugal twisting moment (CTM)
- mass of the blade is thrown out from blades centre of rotation trying to turn the blade to a lower (finer) blade angle
69
Where are the greatest static loads felt
At blade root and hub
70
The natural frequency of a propeller blade ranges from
20Hz (metal) - 60Hz (wood)
71
Highest vibrational loads are felt in the area of about ——% of blade length
80
72
80% of blade length region is susceptible to failure. This is also known as ?
Outer nodal point
73
Any vibrations is decreased by
Using correct aerofoil shape and tip design
74
As solidity increases, a propeller can transfer more power to the air
True
75
What is more efficient. Increasing blade chord or increasing number of blades
Increasing number of blades
76
Explain twin propellers
- propellers on same engine rotating in opposite directions
77
What are twin propellers also known as
Contra- rotating propellers
78
The high speed rotation of an aircraft propeller gives what type of rotation to the slipstream
Spiralling
79
Gyroscopes are based upon 2 fundamental properties of gyroscopic action
- rigidity in space and precision
80
Propeller tip to ground for tail wheel
9 inches
81
Propeller tip to ground for nose wheel
7 inches
82
Propeller to tip to fuselage
1 inch
83
Propeller to nose wheel
1/2 inch
84
Vertical stab is mounted obliquely at what angle
1* or 2*
85
To correct gyroscopic action what is required by pilot
- to use elevator and rudder to prevent undesired pitching and yawing
86
Define relative airflow
- defined as speed and direction of air movement past an aerofoil
87
In normal flight which 2 angles are giving positive thrust and positive torque
- blade angle
- AOA
88
If critical engine fails what occurs to flight performance
Usually too low
89
What is the solution of a failing critical engine
Propellers turn opposite direction
90
In a climb does the downward blade create more thrust (T or F)
True
91
Explain windmilling
- airflow driving the propeller which will drive engine and produce negative torque
92
What is propeller brake moment
- effort required to spin propeller
93
Fixed pitch propeller are designed to ensure
- good climb performance
- high cruising speeds
- towing
94
How does the brake moment work when windmilling
Works in the direction of rotation and begins to drive propeller
95
Explain blade shake
Blade is not rotating, there is a certain amount of movement in the mountings
96
What is natural vibration
- body oscillates under action of own gravitational or elastic forces
97
If a vibration occurs at one particular RPM or a range, the vibration is not normally a propeller problem but a problem of poor engine propeller match
True
98
Explain resonant frequency
Natural vibration frequency that occurs if the object is stuck
99
Explain forced vibration
- vibrating body that contacts another body
100
The propeller spinners can be a contributing factor of what condition
Out of balance
101
Propeller noise is made up of
Tonal and a broadband component
102
Explain Broadband noise
- occur as a result of turbulence in the boundary layer of the blade
103
Which component is perceived as disturbance to the human ear
Tonal compartment
104
What is rotating noise
Product of pressure field created by the rotating propeller
105
What is vortex noise
Caused by vortices leaving the blade tip and blade trailing edge
106
Where is the maximum value of vortex noise found
In the plane of rotation of the propeller
107
What is displacement noise
Displacement of air by the propeller blades as they have finite thickness
108
What is the blade vibration noise
- noise that occurs with periodic stalls
109
If diameter of propeller is double at constant peripheral speed, propeller noise is decreased by
decreased by 6db
110
If diameter of propeller and rotational speed is increased what happens to the noise level
Increased
111
Constant RPM with more blades does what to the noise
Reduced
112
Which propeller shape produces least noise
Scimitar-shaped propeller
113
Which materials have favourable vibrational behaviour
Wood or composite
114
How is the critical range indicated on the engine tachometer
Red
115
How do you fix gyroscopic effect
Rudder
Elevator
