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Module 17 > 17.1 Fundamentals > Flashcards

17.1 Fundamentals Flashcards

1
Q

What is a propeller?

A

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

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2
Q

What must all propellers do?

A

It must absorb the power produced by the engine and transmit the power to the airflow passing through the propeller disk.

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3
Q

What is a blade?

A

Aerofoil section attached to the hub

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4
Q

What is the blade root/shank?

A

The thickened portion of the blade nearest to the hub

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5
Q

What is the blade station?

A

A distance measured from the centre of rotation, normally measured in inches or centimetres

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6
Q

What is the master reference section?

A

A distance measured from the centre of rotation where all measurements are taken from. Normally 75% from the centre of rotation on a fixed pitch propeller and can be 50% to 75% on a variable pitch propeller.

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7
Q

What is the hub?

A

The central portion of the propeller which carries the blades

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8
Q

What is the dome assembly?

A

It encases the pitch change mechanism

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9
Q

What is the face?

A

The flat thrust producing side of a propeller blade.

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10
Q

What is the back?

A

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

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11
Q

What is the blade angle?

A

The angle between the blade chord line and the plane of rotation

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12
Q

What is the plane of rotation?

A

The plane in which the propeller rotates. This is 90° to the engine centreline.

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13
Q

What is the pitch?

A

The distance advanced in one complete revolution.

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14
Q

What is the pitch change mechanism?

A

The device to alter propeller blade angle

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15
Q

What is Hi pitch?

A

(Coarse pitch) Large blade angle

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16
Q

What is Lo pitch?

A

(Fine pitch) Small blade angle

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17
Q

What is a tractor propeller?

A

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

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18
Q

What is a pusher propeller?

A

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

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19
Q

What is reverse pitch?

A

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

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20
Q

What is the spinner?

A

An aerodynamic fairing which covers the centre of the propeller

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21
Q

Where are all measurements for a blade taken from?

A

The centre of rotation

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22
Q

On what principle does a propeller work?

A

The reaction principle

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23
Q

What does a propeller convert engine torque into?

A

Thrust

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24
Q

A propeller accelerates a large mass airflow…

A

Slowly rearwards

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25
Q

What speed range is a propeller most efficient?

A

A propeller has a high propulsion efficiency at low to medium airspeeds (Mach 0.5 - 0.6)

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26
Q

How is the blade angle denoted?

A

Angle “β” (beta)

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27
Q

How is the Angle of Attack denoted?

A

Angle “α” (Alpha)

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28
Q

What is the equation for blade angle?

A

Blade angle = helix angle + angle of attack

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29
Q

What is the angle of attack?

A

The angle between the profile chord and the relative airflow towards it

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30
Q

What is RAF?

A

Relative air flow

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31
Q

How is the angle of Advance or Helix Angle denoted?

A

Angle “Ф” (Phi)

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32
Q

What is the helix angle?

A

The angle between the rotational plane of the blade and the RAF.

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33
Q

What happens to the helix angle (angle of advance) with increasing air speeds?

A

It increases

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34
Q

What are the advantages of a adjustable pitch propeller?

A
  • Higher airspeeds can be achieved with the same engine power
  • Good climb performance
  • Towing
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35
Q

What is the disadvantage of a fixed pitch propeller?

A

It is only efficient with a given aircraft speed range

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35
Q

What is the sum of the propeller brake moment and engine torque at a constant rotational speed?

A

Zero

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36
Q

How can the angle of pitch be influenced at a constant rotational speed?

A
  • changes in airspeed

- blade angle

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36
Q

What is the problem if the propeller exceeds its maximum possible rotational speed?

A

The blades can depart the hub or can be torn from the mountings

37
Q

How many forces act on a propeller?

A
  • 5 static

- 1 dynamic

37
Q

What are the 5 static forces acting on a propeller?

A
  • Centrifugal force
  • Thrust bending force
  • Torque bending force
  • Aerodynamic twisting moment
  • Centrifugal twisting moment
38
Q

What two static forces always oppose each other?

A

Centrifugal twisting moment and aerodynamic twisting moment

CTM are always greater than ATM

38
Q

What is the CTM?

A

The centrifugal twisting moment.

This is the mass of the blade being thrown out from the blades centre of rotation

39
Q

What is the ATM?

A

The aerodynamic twisting moment.
This is the centre of pressure being forward of the blades centre of rotation which tries to turn the blade angle to a higher blade angle

39
Q

What is the torque bending force?

A

Known as the “Braking moment”.

Tries to bend the blade against the direction of rotation which creates a resistance to the engine torque

40
Q

Where are all static loads felt?

A

The blade root

42
Q

Where can maintenance repair work not be carried out on a propeller?

A

Within the blade root area

43
Q

Where on a propeller blade, do aerodynamic forces have a greater effect?

A

The tip of the blade

44
Q

When will the maximum dynamic loading occur on a propeller blade?

A

Within its natural frequency range

45
Q

How can vibrations from aerodynamic forces be reduced at the tip of the blade?

A
  • tip design

- correct aerofoil shape

46
Q

What will the natural frequency of the propeller blade depend on?

A
  • length
  • material
  • blade root
  • shape
49
Q

What are the vibration frequency ranges for;

  • Metal?
  • Wood?
A
  • Metal = 20Hz

- Wood = 60Hz

50
Q

What part of the blade would have the point of maximum vibrations?

A

The outer nodal point (At around 80% of the length of the blade)

51
Q

What region of a blade is most susceptible to failure?

A

At 80% of the blade length (outer normal point)

53
Q

What section of the vibration graph shows the part to be avoided due to high vibration loads?

A

The red area

55
Q

What is the maximum number of propeller blades on a propeller?

A

8

56
Q

As engine power increases, what can be done to efficiently absorb the power?

A
  • increase blade angle
  • increase diameter of propeller disc
  • increase RPM of propeller
  • increase the camber of the blade
  • increase the chord
  • increase the number of blades
57
Q

What is the minimum propeller tip clearance from:

  • tip to fuselage?
  • tip to nose wheel?
  • tip to ground (nose wheel)?
  • tip to ground (tail wheel in flight attitude)?
  • tip to water?
A
  • fuselage = 1 inch (25.4mm)
  • nose = 0.5 inch (13.7mm)
  • ground (nose) = 7 inches (17.78cm)
  • ground (tail) = 9 inches (22.86cm)
  • water = 18 inches (45.72cm)
58
Q

What is the best option to increase efficiency of a propeller?

A

Increasing the number of blades

59
Q

What does the amount of lift produced from a propeller blade depend on?

A
  • shape
  • RPM
  • angle of attack of the blade sections
60
Q

What is pitch distribution?

A

Where the angle of attack becomes smaller, with the distance from the root

61
Q

What are root losses?

A

The thickened root area is less efficient as it is less aerodynamic due to having to withstand high stresses

62
Q

What are tip losses?

A

Tip vortices and induced drag occur at higher RPM speeds

63
Q

What is blade washout?

A

The leading edge of the blade is twisted downwards from the root to the tip

64
Q

What is the geometric pitch of a propeller?

A

The theoretical, or design pitch that would occur if the propeller was 100% efficient (moving forwards with no other movement)

65
Q

What is the effective pitch?

A

The actual helical path which the propeller advances through the air (one revolution of the propeller)

66
Q

What is the slip?

A

The difference between geometric and effective pitch (efficiency losses of the propeller)

67
Q

What can the efficiencies of slip be, for a propeller?

A

Between 50% and 90% depending on the amount of slip

68
Q

How can propeller efficiency be calculated?

A

By dividing effective pitch by geometric pitch

69
Q

What are the normal propeller efficiency ranges?

A

In the region of 0.8 to 0.9 (80% - 90%)

70
Q

What is done to a wind-milling propeller to reduce drag caused by the blades?

A

It is put into the feathering position

- the leading edge is presented into the airflow

71
Q

What can maximum achievable thrust with airspeed exceed?

A

The take-off thrust

72
Q

Which Newton law of motion is associated with torque?

A

Newton’s 3rd Law

Every action has an equal and opposite reaction

73
Q

How are modern propeller aircraft designed to reduce the torque effect?

A

The engine is offset

74
Q

What can be used to compensate torque forces at different speeds on a propeller aircraft?

A

Aileron trim tabs and rudder trim

75
Q

How is the yawing moment of the propeller engine corrected on the take-off roll?

A

The pilot uses rudder or rudder trim

76
Q

What is done to the vertical stabiliser to compensate for the spiralling slipstream of air generated by the propeller?

A

It is mounted obliquely 1° or 2° to the aircrafts longitudinal axis

77
Q

What is gyroscopic procession?

A

The resultant action when a force is applied to a gyroscope. This force acts 90° to its rotation

78
Q

What type of aircraft is the gyroscopic effect mor prominent?

A

Tail wheel aircraft (usually during takeoff roll)

79
Q

What is asymmetric loading?

A

(P-factor)

When one engine fails, the aircraft isn’t symmetrical and yaws to one side

80
Q

What is rotation noise?

A

The rotating pressure field of the propeller produces a rotation noise at mach numbers of the blade tips between 0.5 and 0.85

81
Q

What is vortex noise?

A

Noise caused by vortices leaving the blade tips

82
Q

What is displacement noise?

A

Noise from the displacement of the air by the propeller blades. It is critical at higher Mach numbers
Above Mach 0.9 it becomes equal to the rotation noise

83
Q

What is blade vibration noise?

A

This occurs with periodic stalls (usually heard with helicopter rotors)

84
Q

If the power was doubled to the propeller, what would happen to the noise?

A

It would increase the noise level by approximately 5dB

85
Q

What would happen to the noise if the propeller diameter was doubled?

A

If comparing at a constant peripheral speed, the noise would be reduced by 6dB

86
Q

What would happen if the number of propeller blades increases from 2 to 3?

A

The noise is reduced by about 1.1dB per each added blade

87
Q

How much heavier is a propeller with 4 blades instead of 3?

A

The 4 bladed propeller weighs 35% more

88
Q

How much heavier is a propeller with 3 blades instead of 2?

A

50% heavier

89
Q

What type of propeller produces the least noise?

A

A scimitar-shaped propeller

Straight tips = the most noise

90
Q

Why are wooden or composite propellers more favourable?

A

They have better self-dampening functions which reduces noise

91
Q

What can influence the propeller blade noise, in regards to the shape?

A
  • tip shape = 3-6dB
  • profile type = 2-3dB
  • blade contour 1-2 dB
  • blade twist = 1-2dB
  • profile camber = 1-2 dB
  • profile section ratio = 1-2dB
92
Q

How much force can centrifugal force create in relation to the weight of a propeller blade?

A

7500 times

93
Q

How much force can centrifugal force create in relation to the weight of a propeller blade?

A

7500 times

Module 17 (6 decks)

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