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Mod 17 > Fundamentals > Flashcards

Fundamentals Flashcards

1
Q

What is a propeller?

A

A device, consisting of a rotating hub with two or more radiating blades; used to propel an aircraft

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2
Q

What is a hub?

A

The central portion of a propeller which carries the blades

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3
Q

What is a blade?

A

Aerofoil section that is attached to the hub

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4
Q

What is a blade butt?

A

The base of a propeller blade where the root ends

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5
Q

What is a blade root/shank?

A

The thickened portion of the blade nearest the hub

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6
Q

What is a blade station? What is it measured in?

A

A distance measured from the centre of rotation. Inches or cm

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7
Q

What happens to pitch when you increase blade angle?

A

Pitch increases

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8
Q

What principle does a propeller work on?

A

Newtons 3rd law

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9
Q

Why do we feather a propeller?

A

Prevent windmilling

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10
Q

How does a propeller move through the air?

A

Accelerates a large mass of air slowly rearwards

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11
Q

What is the master reference station?

A

A distance measured from centre of rotation

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12
Q

Where are most measurements taken from?

A

Centre of rotation

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13
Q

What % distance is the master reference station from the centre of rotation on a fixed-pitch propeller?

A

75%

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14
Q

What % distance is the master reference station from the centre of rotation on a variable pitch propeller?

A

50-75%

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15
Q

What is the blade face?

A

The flat thrust producing side of a propeller blade

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16
Q

What is the blade back?

A

The curved side of the propeller blade facing the direction of flight

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17
Q

What is the blade chord line?

A

A line through the blade profile at the points between face and back surfaces

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18
Q

What is the plane of rotation?

A

The plane in which the propeller rotates?

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19
Q

What is the angle of the plane of rotation to the engine centreline?

A

90 degrees

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20
Q

What is the blade angle (theta)? Where is blade angle measured from?

A

The angle between the blade chord line and plane of rotation. Measured from master reference station

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21
Q

What is the pitch?

A

Distance advanced in one complete revolution

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22
Q

What is a pitch change mechanism?

A

Device to alter blade angle

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23
Q

What is the fine/low pitch?

A

Vertical blade angles

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24
Q

What is the coarse/high pitch?

A

Horizontal blade angles

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25
Q

Is the diameter of airflow before the propeller disc larger in regards to momentum theory?

A

Yes

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26
Q

What is reverse pitch?

A

Turning the propeller blades to a negative angle to produce braking or reversing thrust

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27
Q

What does momentum theory primarily deal with?

A

Flow of air

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28
Q

What does element theory primarily deal with?

A

The aerodynamic forces acting on propeller blades

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29
Q

How can you predict the overall performance of a propeller in regards to element theory?

A

Breaking down a blade into sections and determining the sum of thrust and torque produced by the section

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30
Q

What theory is useful for determining ideal efficiency?

A

Momentum theory

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31
Q

What is the dome assembly?

A

Encases the pitch change mechanism

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32
Q

What is a spinner?

A

An aerodynamic fairing that covers the centre of the propeller

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33
Q

What is a tractor propeller?

A

A propeller mounted in the front of the leading edge of the wing or on the nose of the aircraft

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34
Q

What is a pusher propeller?

A

A propeller mounted behind the trailing edge of the wing or at the rear of the fuselage

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35
Q

Does a fine or coarse pitch propeller ‘move through the air a larger distance in one revolution’?

A

Coarse pitch propeller

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36
Q

When is blade angle in the alpha range?

A

If a propeller varies pitch (within the fine to coarse positions) during flight

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37
Q

What is the angle of attack (alpha)?

A

The angle between the profile chord and relative airflow

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38
Q

What is the angle of advance or Helix angle (phi)?

A

The angle between the rotational plane of the propeller and the relative airflow

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39
Q

What happens to the angle of advance as speed increases?

A

Angle of advance increases

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40
Q

When is the blade angle in beta range?

A

On ground to provide ease of starting and reverse thrust

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41
Q

What 3 factors affect the amount of lift produced?

A
  • aerofoil shape
  • revolutions per minute
  • Angle of attack
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42
Q

What happens to velocity as the distance of a blade section increases from its rotational axis?

A

Velocity increases

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43
Q

What is pitch distribution? Why is pitch distribution used?

A

Blade angle becomes smaller the further it is from the centre of axis. To maintain a constant angle of attack

44
Q

What is a propellers geometric twist? Why is the propeller twisted?

A

Blade is twisted from root to tip. Maintains a constant angle of pitch

45
Q

What are 3 causes of tip losses? What loss is a result of high rotational speeds?

A
  • tip vortices
  • induced drag (at high rotational speeds)
  • compressibility effects
46
Q

What is blade washout? Why is blade washout used?

A

Leading edge of propeller blade is twisted downwards from root to tip. Maintain a constant angle of attack at differing rotational speeds along blade

47
Q

What is the ratio of propeller efficiency?

A

The ratio between the power developed by the propeller and the power obtained from aircraft’s power plant

48
Q

What is geometric pitch?

A

The theoretical distance a propeller should advance in one revolution

49
Q

What is effective pitch?

A

The actual distance a propeller advances in one revolution

50
Q

Does geometric pitch include propeller slippage in the air?

A

No, effective pitch does

51
Q

What is propeller efficiency (eta)?

A

Defines how well a propeller transmits its rotational force of energy into thrust

52
Q

What are the ranges of normal propeller efficiency?

A

0.8 to 0.9 (80-90%)

53
Q

What is the propeller efficiency measurement for a piston engine?

A

Brake power

54
Q

What is the propeller efficiency measurement for a turboprop aircraft?

A

Shaft power

55
Q

Can you carry out any damage or repair work within the blade root are?

A

No

56
Q

What is the greatest load felt on a propeller? How much times the weight of a propeller blade can the load create?

A

Centrifugal force. Over 7500x the weight of the propeller blade

57
Q

How does centrifugal force act on the propeller? Is it a static or dynamic load?

A

Tries to pull blades out of hub assembly. Static load

58
Q

How does thrust bending force act on the propeller? Is it a static or dynamic load?

A

Attempts to bend the propeller blade tips forwards. Static load

59
Q

How does a torque bending force (braking moment) act on the propeller? Is it a static or dynamic load?

A

Tried to bend the blade against the direction of propeller rotation. Static load

60
Q

How does an aerodynamic twisting moment (ATM) act on the propeller? Is it a static or dynamic load?

A

The centre of pressure will try to turn the blade to a coarser (higher) blade angle. Static load

61
Q

How does a centrifugal twisting moment (CTM) act on the propeller? Is it a static or dynamic load?

A

The mass will try to turn the blade to a finer (lower) blade angle. Static load

62
Q

Is the centrifugal twisting moment or aerodynamic twisting moment always greater?

A

Centrifugal twisting moment is always greater than

63
Q

What % of the blade length are the highest vibrational loads felt? What is this area known as?

A

80% of the blade length. Outer Nodal point

64
Q

What is the best method a propeller can use to absorb engine power?

A

Increasing the number of blades

65
Q

What are the 6 methods a propeller can use to absorb engine power?

A
  • 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
66
Q

What does torque reaction cause? How do you compensate for torque reaction on older a/c and modern a/c?

A

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

67
Q

What does twist effect of propeller wash cause? How can you compensate for twist effect? When does twist effect occur?

A

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).

68
Q

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?

A

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

69
Q

What does gyroscopic effect cause? How can you compensate for gyroscopic effect?

A

A force deflects the propeller out of its plane of rotation causing a pitching and/or yawing moment. Proper use of elevator and rudder

70
Q

What is relative airflow (RAF)?

A

The speed and direction of the air movement past an aerofoil

71
Q

Does a windmilling propeller turn the same way?

A

Yes

72
Q

What is propeller brake moment?

A

The effort which is required to spin the propeller

73
Q

What 2 conditions can cause windmilling if aircraft has a constant pitch?

A
  • rapid velocity increase
  • rotational speed greatly reduced
74
Q

What happens to angle of attack, brake moment and RPM if speed increases?

A

Angle of attack decreases, brake moment decreases and RPM increases

75
Q

What happens to angle of attack if RPM is reduced at a constant airspeed?

A

Angle of attack decreases

76
Q

What happens to the brake moment when the blade angle is reduced?

A

Brake moment is reduced

77
Q

What is natural vibration?

A

When a body oscillates under the action of its own gravitational or elastic forces, with no external forces present e.g. spring

78
Q

What is resonant frequency?

A

The natural vibration frequency that occurs if certain objects are struck e.g. church bells

79
Q

What is forced vibration?

A

If a vibrating body is bought into contact with another body, the second body will vibrate until the source is removed

80
Q

What are the 2 main sources of propeller vibration?

A

Mechanical and aerodynamic

81
Q

What is usually the cause of vibration at one RPM with limited RPM range only?

A

Poor engine-propeller match

82
Q

What is blade shake? Is blade shake a source of vibration?

A

Certain amount of movement in the mountings when the blade is not rotating. No

83
Q

How can cabin vibration be improved?

A

Reindexing the propeller to the crankshaft

84
Q

What would be an indication of an out of balance propeller spinner condition? What is this caused by?

A

Spinner wobble whilst engine is running. Inadequate shimming of the spinner front support or a cracked/deformed spinner

85
Q

If the propeller hub does not rotate in orbit then is this a propeller or engine vibration issue?

A

Engine

86
Q

What is resonance?

A

If a body is set into forced vibration it will vibrate with the frequency of that vibration

87
Q

What two components is propeller noise made up off?

A

Tonal and Broadband

88
Q

How is broadband noise caused?

A

Turbulence in boundary layer of the blade

89
Q

How is tonal noise caused?

A

Displacement of air caused by blade motion

90
Q

What type of noise occurs at Mach 0.5-0.85?

A

Rotation noise

91
Q

What type of noise exceeds all other noise components?

A

Rotation noise

92
Q

What is rotation noise caused by?

A

Pressure field created by the rotating propeller

93
Q

What is vortex noise caused by?

A

Vortices leaving the blade tip and blade trailing edge

94
Q

At what Mach numbers does displacement noise equal rotation noise?

A

Above mach 0.9

95
Q

What is the cause of blade vibration noise?

A

Periodic stalls

96
Q

If you double the power how many decimals will the noise level increase to?

A

5dB

97
Q

If the propeller diameter is doubled at a constant peripheral speed then how many decibals is the propeller noise reduced by?

A

6dB

98
Q

What happens if the propeller diameter increases e.g. doubles, with constant rotational speed?

A

Noise increases

99
Q

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?

A

1.1dB

100
Q

If the blade tip Mach number is increased from Mach 0.63-0.87, how many decibals with the noise level increase by?

A

16dB

101
Q

Does a scimitar or straight tip propeller create the most noise?

A

Straight tip

102
Q

Do wood and composite blades or metal blades have better vibrational behaviours?

A

Wood and composite

103
Q

When does constructive interference occur?

A

When 2 RPM frequency waves overlap and combine to create a larger wave

104
Q

When does destructive interference occur?

A

When 2 RPM frequency waves overlap and cancel each other out

105
Q

What vibration cause occurs only on multi engine a/c with 2 propellers operating on same wing in close proximity?

A

Blade position phasing

Mod 17 (7 decks)

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