17.1 Fundamentals Flashcards
1
Q
How does a propeller generate thrust?
A
Pushing a large amount of air back slowly.
2
Q
Define a propellor.
A
A device, with a rotating hub with 2 or more rotating blades to propel the aircraft.
3
Q
Define a hub.
A
The central portion of a propeller which carries the blades.
4
Q
Define a blade.
A
Aerofoil section that is attached to the hub.
5
Q
Define the blade butt.
A
The base of the propeller where the root ends.
6
Q
Define the blade root/shank.
A
The thickened portion of the blade nearest to the hub.
7
Q
Define a blade station.
What’s it measured in?
A
A distance measured from the centre of rotation.
Measured in inches or centimetres.
8
Q
Where the master reference station on a fixed pitch propeller?
A
Normally 75% from the centre of rotation.
9
Q
Where is the master reference station on a variable pitch propeller?
A
50-75% from the centre of rotation.
10
Q
Where are blade measurements take from?
A
Blade master reference station.
11
Q
Define blade face?
A
The flat thrust producing side of a propeller blade.
12
Q
Define the blade back.
A
The curved side of the propeller blade facing the direction of flight.
13
Q
Define the blade chord line.
A
A line through the blade profile at the points between the face and back surfaces.
14
Q
Define the plane of rotation.
What angle?
A
The plane in which the propeller rotates. This is 90° to the engine centreline.
15
Q
Define the blade angle.
A
The angle between the blade chord line and the plane of rotation.
16
Q
Define the pitch?
A
Distance advanced in one complete revolution.
17
Q
Define the pitch change mechanism.
A
Device to alter blade angle.
18
Q
Define a fine/ low pitch.
A
Vertical blade angles.
19
Q
Define a coarse/ high pitch.
A
Horizontal blade angles.
20
Q
Define a reverse pitch.
A
Turning the blades to a negative angle to produce braking or reversing thrust.
21
Q
What does the dome assembly do?
A
Encases the pitch change mechanism.
22
Q
Define a spinner.
A
An aerodynamic fairing that covers the centre of the propeller.
23
Q
Where is a tractor propeller mounted?
A
In front of the wing LE or nose of the aircraft.
24
Q
Where is a pusher propeller mounted?
A
Behind the wing trailing edge or rear of the fuselage.
25
What is the purpose of the propeller?
Move aircraft through the air.
26
What is the purpose of the hub?
It attaches the blades to a piston engine.
27
What does propellor consist of?
2 or more blades.
Hub
Reduction gearbox.
Electric motor drive shaft.
28
What principle does a propeller work of?
Newton’s third law, action and reaction.
29
Why do we feather propellors?
What angle?
To prevent windmilling.
90.
30
What produces thrust? Back or face?
The blade face.
31
What is the slipstream?
The backwards moving air.
32
What is the propellers efficiency?
(Ratio)
Ratio between power developed by propeller and power from aircraft’s power plant.
33
What does the creation of thrust cause?
Loss in kinetic energy.
Friction between the air and blades.
34
What is the momentum theory?
The propeller disc decreases the diameter of air behind the disc.
35
Who came up with the momentum theory?
W.J.M Rankine and R.E. Froud.
36
Where is the diameter greater in the momentum theory?
Infront of the disc.
37
What does element theory deal with?
It deals primarily with aerodynamic forces acting of the propeller blades.
38
What is blade angle called?
Theta
39
What is a feathered blade?
A fully vertical blade angle.
40
What is alpha range?
Varying pitch in flight.
41
What is known as alpha?
Angle of attack.
42
Define the Angle Of Attack.
Angle between the chord line and relative airflow.
43
What is the angle of advance/ helix angle called?
Phi
44
Define the angle of advance/ helix angle?
Angle between propeller rotational plane and relative airflow.
45
How does the angle of advance change with increasing airspeed?
It increases.
46
What is the relationship between the 3 relevant angles?
Helix angle + Angle of attack = Blade angle
47
What is the reverse pitch angle?
Negative.
48
What does a negative/reverse pitch angle produce?
Reverse thrust.
Braking.
49
When is the reverse pitch used?
Backing away from obstacles.
Controlling taxi speed.
Stopping aircraft during landing.
50
What is done to counteract reverse pitch?
An increase in engine power must be readily available to maintain propeller RPM.
51
What does reverse pitch cause?
Propeller windmilling.
Where air is driving the propeller instead of the engine.
52
When is blade angle beta range available?
Only on the ground.
53
What is blade angle beta referred to as?
From flight fine/flat pitch to reverse.
54
What does the amount of lift produced depend on?
Aerofoil shape.
RPM.
AOA.
55
How is a constant angle of pitch achieved?
A propeller geometric twist.
56
Describe pitch distribution.
The blade angle becomes smaller the further away it is from the centre axis.
57
What causes tip losses at high rotational speeds?
Tip vortices and induced drag.
58
What further tip losses are caused by?
Compressibility effects.
59
What is blade washout?
The same thing as pitch distribution and geometric twist.
To maintain a constant AOA at differing rotational speeds.
60
What is root losses?
A thickened root area can withstand high stresses but loses aerodynamic efficiency.
61
What is propeller slip?
The difference between geometric pitch and effective pitch.
62
What does propeller efficiency percentage vary from?
50-90%
63
What is geometric pitch?
The theoretical distance a propeller should advance in 1 regulation.
64
How does effective pitch differ from geometric pitch?
Effective pitch is the distance the propeller actually advances in 1 revolution.
Geometric pitch is theoretical advancement.
65
What does propeller efficiency define?
How well a propeller transmits its rotational force/energy into thrust.
66
What defines a propeller’s efficiency more than speed?
Design and shape of propeller.
67
Generally, what propeller is more efficient?
A propeller with a larger propeller diameter.
(Diameter of disc.)
68
What is propeller efficiency measured in?
Shaft power.
69
What is the normal propeller efficiency range?
80-90%
70
What is the Greek symbol for propeller efficiency?
Eta.
71
What is the greatest static load felt on the propeller?
Centrifugal force.
72
(Static) What is the effect of thrust bending force?
Attempts to bend the propeller blade tips forwards.
73
(Static) What is the effect of centrifugal force?
Attempts to pull the blades out the hub assembly.
74
(Static) What is the effect of torque bending force?
Tends to bend blades against the direction of propeller rotation.
75
(Static) Define Aerodynamic Twisting Moment.
The centre of pressure being forward of blades centre of rotation will try to turn the blade to a higher/coarser angle.
76
(Static) What effect does Centrifugal Twisting Moment have?
Attempts to turn the blade angle to a finer position.
77
CTM and ATM counteract each other, but which is greater?
Centrifugal Twisting Moment.
78
What are the 5 static loads?
Centrifugal,
Thrust bending,
Torque bending,
Aerodynamic twisting moment,
Centrifugal twisting moment.
79
Where on the propeller do static loads act?
At the blade root
80
Can repairs be carried out on the blade root area?
No.
81
What are dynamic load vibrations caused by?
Operating strokes of a piston engine.
Dynamics of propeller reduction gearbox.
82
Where do aerodynamic forces have a greater vibration effect?
At the tip of the blade.
83
How can vibrations be decreased?
Using correct Aerofoil shape and tip design.
84
Where are the highest vibration loads felt?
At 80% of the blade length.
85
What is the area where the highest vibration loads act called?
Outer Nodal Point.
86
What is power absorption?
The propeller must be tailored to the specific needs of the engine it is fitted to.
87
How can the engine be capable of absorbing greater power?
Increasing the blade angle.
Increase the propeller disc diameter.
Increase propeller RPM.
Increase the blade camber.
Increasing the propeller blade chord.
Increasing the number of blades.
88
What’s the problem with increasing the blade angle?
It looses one kind of aerodynamic efficiency.
89
What’s the problem with increasing the blade length?
Inefficient tip speed.
90
What’s the problem with increasing propeller RPM?
Eventually the blade tips would become supersonic, increasing drag.
91
What’s the problem with increasing camber?
Sacrifices one type of aerodynamic efficiency.
92
What are the best options to absorb greater power?
Increasing blade chord (easier).
Increasing number of blades (best).
93
Why is increasing blade chord or the number of blades the best option to absorb more power?
As it increases the propeller disc solidity.
94
What is propeller tip to fuselage clearance?
1 inch.
95
What is the propeller to nose wheel clearance?
1/2 inch.
96
What is the propeller to ground (nose wheel) clearance?
7 inches.
97
What is the propeller to ground (tail wheel in flight attitude) clearance?
9 inches.
98
What is the propeller to water (float plane) clearance?
18 inches.
99
What design is used on high power engines?
Contra-rotating propellors.
100
Describe contra-rotating propellors.
Twin propellers on the same engine rotating in opposite directions.
101
What does torque reaction do?
Tends to make aircraft roll.
102
How is torque reaction compensated for on older aircraft in flight?
Creating more lift of the wing being force down.
103
How is torque section compensated for on modern aircraft in flight?
An offset engine.
104
What effect does torque reaction have on ground?
Yaw moment.
105
How is torque reaction counteracted on ground?
Pilot rudder use.
106
What is the twist effect of propeller wash.
Propeller rotation causes Spiralling rotation of the slipstream.
107
How is propeller wash compensated for?
Vertical stabiliser is mounted obliquely 1 or 2 degrees to the longitudinal axis.
108
What effect does the gyroscopic effect cause?
Yaw movement.
109
How is gyroscopic effect corrected?
Using the elevator and rudder to prevent undesired pitching and yawing.
110
What is asymmetric loading (P factor)?
The downwards facing blade has a greater AOA causing a yawing moment.
111
What is the critical engine?
The engine which would produce the smallest yaw moment, should the other engine fail.
112
How can asymmetric loading be improved?
Having the propellers turn in opposite directions.
113
What is relative airflow?
Speed and direction of the air movement past an Aerofoil.
114
What is windmilling?
At a negative AOA, incoming air will apply enough torque to turn the propeller.
Propeller then drives the engine.
115
What 3 factors are considered when choosing a fixed pitch propeller angle?
Good climbing performance,
High cruising speeds,
Towing.
116
What is propeller brake moment?
The effort which is required to spin the propeller.
117
What effect does airspeed have on AOA?
Airspeed increase = AOA decreases.
Airspeed decreases = AOA increases.
Inversely proportional.
118
What effect does RPM have on AOA?
RPM increase = AOA increases.
RPM decreases = AOA increases.
Directly proportional.
119
What are the sources of vibration?
Mechanical and aerodynamic.
120
What is the first category of vibrations?
Static and dynamic balance of a propeller.
121
What is the second category of vibrations related to?
Even distribution of thrust and torque loads acting on the propeller blades.
122
Why is vibration a problem?
Because it typically vibrates throughout the entire RPM range.
123
What does it mean if a vibration occurs only at one particular RPM?
Poor propeller-engine match.
124
What is blade shake?
When the blade is not rotating, there is a certain amount of movement in the mountings.
125
How can vibration sometimes be corrected?
Reindexing.
Removing the propeller, rotated 180 degrees and re installed.
126
How can the spinner be identified as a factor to an out-of-balance condition?
Noticeable spinner wobble.
127
How can vibration be identified?
Observing the propeller hub, dome or spinner while the engine is running between 1200 to 1500 RPM.
128
What are the 3 causes for excessive propeller vibration?
Blade imbalance,
Blades not tracking,
Variation in blade angle settings.
129
What is propellor noise made up off?
Tonal and broadband component.
130
What is the tonal component of vibration?
The aerodynamic interaction with surrounding structures.
131
What is the broadband noise of vibration?
Turbulent air in the boundary layer of the blade and interaction of the blade with turbulence in the air.
132
What is rotation noise?
Undisturbed flow of air to the blade.
133
What causes vortex noise?
By vortices leaving the blade tip and trailing edge.
134
What is displacement noise?
Displacement of air by the propeller blades.
135
When does blade vibration noise occur?
With periodic stalls.
136
When does displacement noise equal rotation noise?
At blade tip numbers above Mach 0.9.
137
What influence does power have on noise?
Causes an increase in noise level.
138
What influence does propeller diameter have on noise?
If diameter is doubled, propeller noise is reduced.
(Rotational speed will decrease)
If speed remains constant, noise will increase.
139
What influence does number of blades have on noise?
Increasing blade number reduces noise.
140
What is the Influence of blade tips Mach number?
Increasing blade tip speed increases noise.
141
What propeller blade shape produces the least noise?
A scimitar shape.
142
What blade shape produces the most noise?
Straight tip blades.
143
What material has more favourable vibration behaviour?
Wood or composite construction.
144
What blade material produces the least noise?
Composite construction.
145
What are beats caused by?
2 engines frequencies tend to cross over each others value in a rhythmic fashion.
146
What is constructive interference?
2 waves overlap to create a larger wave.
147
What is destructive interference?
2 blades overall and cancel each other out.
148
When does blade position phasing occur?
Only on multi engine aircraft where 2 propellers operate on the same wing in close proximity.
149
What creates a pulse?
Blade tips of adjoining blades passing near each other.
150
What causes a standing wave vibration?
Power stroke pulsations from a piston engine transmitting into the propeller.
151
What is the red line on the engine tachometer indicating?
The critical range of standing wave RPM.
152
What makes up Monel?
Nickel and copper.
153
What makes up terneplate?
Steel, tin, lead.
