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CAT B- Mod 17 Propellers > 17.1 Fundamentals > Flashcards

17.1 Fundamentals Flashcards

1
Q

What is a dirigible?

A

Airship

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2
Q

Definition of the propellor?

A

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

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3
Q

Definition of the hub?

A

The central portion of a propeller which carries the blades.

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4
Q

Definition of the blade?

A

Aerofoil section that is attached to the hub.

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5
Q

Definition of the blade butt?

A

The base of the propeller blade where the root ends.

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6
Q

Definition of the blade root/shank?

A

The thickened portion of the blade nearest to the hub.

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7
Q

Definition of the master reference station?

A

A distance is 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 50–75% on a variable pitch propeller.

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8
Q

Definition of the blade face?

A

The flat thrust producing side of a propeller blade.

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9
Q

Definition of the blade chord line?

A

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

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10
Q

Definition of the plane of rotation?

A

Plane in which the propellor rotates, 90 degrees to the engine centreline.

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11
Q

Definition of the blade angle?

A

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

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12
Q

Definition of the pitch?

A

Distance advanced in one complete revolution.

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13
Q

Definition of the pitch change mechanism?

A

Device to alter blade angle.

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14
Q

Definition of fine pitch?

A

Vertical blade angles. Also referred to as “Low pitch”.

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15
Q

Definition of coarse pitch?

A

Horizontal blade angles. Also referred to as “High pitch”.

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16
Q

Definition of reverse pitch?

A

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

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17
Q

Definition of dome assembly?

A

Encases the pitch change mechanism.

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18
Q

Definition of the spinner?

A

An aerodynamic fairing that covers the centre of the propeller.

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19
Q

Definition of the tractor propellor?

A

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

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20
Q

Definition of a pusher propellor?

A

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

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21
Q

Which of newtons laws are propellors based on?

A

Third

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22
Q

What does the propellor do to the air?

A

large mass of air slowly rearwards.

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23
Q

What is the equation for thrust?

A

Delta P x Area

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24
Q

What pulls the aerofoil foward?

A

The high velocity of the air results in lower static pressure in front of the propeller

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25
Q

What is the backward moving air called?

A

Slipstream

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26
Q

What efficiency does a modern prop have?

A

80%

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27
Q

What is the momentum theory?

A

based on the consideration of the momentum and kinetic energy imparted to this mass of air. This theory assumes a propeller to be an advancing disc producing a uniform thrust,

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28
Q

Why is the momentum theory flawed?

A

It is based on all factors being perfect

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29
Q

What is a better theory than momentum theory?

A

Element theory- it deals primarily with the aerodynamic forces acting on the blades.

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30
Q

How does the element theory work?

A

Breaking a blade down into several independent sections along the length and then determining the forces of thrust and torque on each of these small blade elements.

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31
Q

Element theory- what is V0?

A

Axial flow at propeller disc

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32
Q

Element theory- what is V2?

A

Angular flow velocity vector

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33
Q

Element theory- what is V1?

A

Section local flow velocity vector, summation of vectors V₀ and V₂

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34
Q

What is the Greek letter for blade angle?

A

Theta

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35
Q

What is blade angle measured in?

A

degrees

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36
Q

Where is the blade angle taken from?

A

Blade station

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37
Q

Where is the blade station?

A

between 0.5 and 0.75 of the radius of the propeller and is sometimes referred to as the “master station”.

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38
Q

What is the length and effective pitch of a prop designated 74-48?

A

Length 74 inches and effective pitch 48

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39
Q

What is a fully vertical blade?

A

Feathered

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40
Q

What is the alpha range?

A

If blade angle changes are controlled and stay within the ‘fine’ to ‘coarse’ positions, then the propeller is said to be operating in the “α (alpha) range”.`

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41
Q

What is the angle of attack?

A

angle between the profile chord line and the relative airflow towards it.

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42
Q

What two factors produce relative airflow?

A

Airflow velocity and propellor rotational velocity

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43
Q

What is the helix?

A

The path that the prop takes.

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44
Q

What happens to the angle of advance with increasing speed?

A

Increases.

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45
Q

What is the helix angle also known as?

A

Angle of advance.

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46
Q

What governs the relative airflow?

A

both the rotational speed of the propeller and the forward speed of the aircraft.

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47
Q

In normal flight what are the blade angle and angle of attack?

A

Both positive

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48
Q

What kind of torque does windmilling produce?

A

Negative

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49
Q

In which direction does a windmilling propellor turn compared to normal conditions?

A

Same

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50
Q

What is propellor brake moment?

A

the effort which is required to be able to spin the propeller.

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51
Q

At a constant rotational speed, the sum of propeller brake moment and engine torque is?

A

0

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52
Q

With a constant pitch if airspeed increases rapidly or rotational speed is greatly reduced, the angle of attack will?

A

Reduce and become negative, causing it too windmill and then have a braking force.

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53
Q

Reduction in airspeed does what to AoA?

A

increases

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54
Q

RPM is reduced at constant airspeed the AoA is?

A

Reduced

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55
Q

How many degrees past fine/flat is reverse pitch?

A

30 degrees

56
Q

What is the beta range definition?

A

variable pitch propeller, the pilot can select specific blade angles to provide ease of starting and reverse thrust.

57
Q

What is the beta range?

A

Flight Fine” (or sometimes referred to as “flat pitch” and produces no thrust) to ‘Reverse’ and is only available on the ground.

58
Q

How is the lever moved from the alpha to beta range?

A

Lever must be lifted

59
Q

Why are propellors geometrically twisted?

A

To ensure a nearly constant angle of pitch is maintained

60
Q

What is pitch distribution?

A

The blade angle becomes smaller the further it is from the centre axis to keep a nearly constant angle of attack.

61
Q

What are root losses?

A

A thickened root area can withstand high stresses but loses aerodynamic efficiency. Airflow at the root is affected by the engine.

62
Q

What are tip losses?

A

Tip vortices and induced drag cause tip losses at high rotational speeds. Further losses are caused by compressibility effects.

63
Q

What is blade washout?

A

To maintain a constant angle of attack at differing rotational speeds along the blade, the leading edge of the propeller blade is twisted downwards from root to tip.

64
Q

What is the ratio of efficiency?

A

thrust horsepower to brake horsepower

65
Q

What does propeller efficiency vary due to?

A

Slip

66
Q

What is propeller slip?

A

difference between the geometric pitch of the propeller and its effective pitch.

67
Q

What is geometric pitch?

A

theoretical distance a propeller should advance in one revolution

68
Q

if the aircraft moved through the air according to the geometric propeller pitch, the propeller angle of attack would be?

A

zero

69
Q

What is geometric pitch usually expressed in?

A

Pitch inches

70
Q

How is effective pitch calculated?

A

Aircraft speed x duration of one revolution

71
Q

The amount of energy it takes to rotate the propeller is almost always greater or smaller than the thrust from the propeller?

A

Greater

72
Q

What units is piston aircraft efficiency given in?

A

BHP

73
Q

What units is turboprop aircraft efficiency given in?

A

Shaft HP

74
Q

What is the advance ratio?

A

ratio of the aircraft’s airspeed and the propeller speed.

74
Q

What is the greek symbol for efficiency?

A

eta (n)

75
Q

What is the greatest load felt on a prop?

A

Centrifugal

76
Q

How much more load can be created by centrifugal?

A

7500 times

77
Q

What is thrust bending force?

A

attempts to bend the propeller blade tips forwards

78
Q

What is torque bending force?

A

(Braking moment) tends to try and bend the blade against the direction of propeller rotation

79
Q

What is Aerodynamic Twisting Moment(ATM)?

A

The centre of pressure, being forward of the blade’s centre of rotation, will try to turn the blade to a higher (coarser) blade angle.

80
Q

In reverse pitch what will ATM do?

A

turn the blade to a coarser Negative Blade Angle.

81
Q

What is Centrifugal Twisting Moment? (CTM)

A

The mass of the blade is thrown out from the blade’s centre of rotation trying to turn the blade to a lower (finer) blade angle

82
Q

Compared to an ATM what is a CTM?

A

will always oppose ATMs and CTMs are always greater than ATMs.

83
Q

What does CTM depend on and what happens if you increase any of them?

A

Blade chord
Weight
RPM

CTM increases

84
Q

What is the only time ATM works with CTM?

A

Windmilling

85
Q

Where is the point where the greatest stress is felt?

A

Root and therefore hub

86
Q

Is damage and therefore repairs permitted on the root?

A

No

87
Q

Where do aerodynamic forces have greater vibration effect?

A

The tip

88
Q

What are the natural frequency’s of metal and wood props?

A

20Hz Metal
60Hz Wood

89
Q

What is the outer nodal point?

A

80% blade length, the point at which most vibration is felt.

90
Q

In what ways could you design a prop that is capable of higher powers/

A

Increase pitch
Increase diameter
Increase RPM
Increase camber
Increase chord
Increase number of blades

91
Q

What does increasing the RPM for a higher power allow?

A

The same amount of energy is transferred to the air but in a shorter time.

92
Q

What is the minimum clearance of a twin prop aircraft to fuselage?

A

1”

93
Q

What is the minimum clearance of a tail plane prop to ground?

A

9”

94
Q

What is the minimum clearance of a tricycle aircraft prop to ground and to the nose wheel?

A

7”
0.5”

95
Q

What is the minimum clearance of a water plane prop to water?

A

18”

96
Q

What is the best option to design a prop to handle higher power?

A

Increase the number of blades

97
Q

What is solidity?

A

ratio of total blade area to total prop disc area

98
Q

What are the four elements of torque?

A

Torque reaction from engine and propeller
Twist effect of the propeller wash
Gyroscopic action of the propeller
Asymmetric loading of the propeller (P-factor)

99
Q

Due to the fact that engine components spin in one direction what does the aircraft do?

A

Trying to rotate in the opposite direction.

100
Q

On modern aircraft what is used to counteract the torque motion?

A

Aileron trim tabs

101
Q

What is the twist effect of propeller wash?

A

At high propeller speeds and low forward speeds (as in take-off, approaches), this spiralling rotation is very compact and exerts a strong sidewards force on the aircraft’s vertical tail surface.

102
Q

How is twist effect of prop wash compensated against?

A

vertical stabiliser is mounted obliquely 1° or 2° to the aircraft’s longitudinal axis

103
Q

What is the p factor?

A

Asymmetric loading

104
Q

What is asymmetric loading?

A

Yawing moment caused by a nose up attitude that causes the downward blade to have a greater AoA moving the centre of thrust away from the middle

105
Q

What is the critical engine?

A

The engine which would produce the smallest yaw moment, should the other engine fail

106
Q

In the case of small twin-engine aircraft, the flight performance is usually too low if the critical engine fails. This situation can be somewhat improved by having the propellers turn in?

A

Opposite directions (inboard downwards)

107
Q

What is resonant frequency?

A

Many objects because of their shape and material have a natural vibration frequency that occurs if the object is struck. This natural frequency is the resonant frequency

108
Q

What is forced vibration?

A

If a vibrating body is brought into contact with another body, the second body will begin to vibrate and will continue to do so until the source is removed

109
Q

If a vibration occurs only at one particular RPM or within a limited RPM range (e.g., 2200 – 2350 RPM), the vibration is not normally?

A

A prop but a bad engine-prop match

110
Q

If a propeller vibration is suspected but cannot be positively determined, the ideal troubleshooting method is to?

A

temporarily replace the propeller with one known to be airworthy and then test fly

111
Q

How can cabin vibration be improved?

A

reindexing the propeller to the crankshaft.

112
Q

What is reindexing?

A

The propeller can be removed, rotated 180°, and reinstalled.

113
Q

What happens when the forced vibration frequency matches the resonant frequency of the body?

A

the amplitude of the vibration will increase dramatically

114
Q

What is broadband noise?

A

occurs as a result of turbulence in the boundary layer of the blade, and the interaction of the blade with turbulence in the air.

115
Q

When does rotation noise exceed all other noise?

A

With the blade tips travelling at Mach numbers of between M 0.5 – M 0.85 and an undisturbed flow of air to the blade

116
Q

What is vortex noise?

A

caused by the vortices leaving the blade tip and blade trailing edge

117
Q

What is displacement noise?

A

displacement of the air by the propeller blades as they have a finite thickness.

118
Q

When does displacement noise equal rotation noise?

A

At blade tip numbers above Mach 0.9 this noise source equals that of rotation noise.

119
Q

What is blade vibration noise?

A

This noise occurs with periodic stalls

120
Q

What is the influence of power on blade noise?

A

double it and it causes a 5db increase

121
Q

What is the influence of diameter on blade noise?

A

propeller diameter is doubled, propeller noise is reduced by 6 dB

122
Q

What is the influence of no. of blades on blade noise?

A

if the number of blades is increased from 2 to 3, noise is reduced by about 1.1 dB (A

123
Q

How much does a 4 blade prop weigh than a 3 blade?

A

35%

124
Q

What is the influence of blade tip mach number on blade noise?

A

increase in the airflow Mach number of the propeller blade tips from M 0.63–0.87 increases noise by 16db

125
Q

Which blade shape produces the least noise?

A

Scimitar

126
Q

Which blade shape produces the most noise?

A

Straight tip

127
Q

How much can blade tip shape vary db?

A

3-6

128
Q

How much can profile shape vary db?

A

2-3

129
Q

How much can blade contour and twist vary db?

A

1-2

130
Q

How much can profile camber and section ratio vary db?

A

1-2

131
Q

When does constructive interference happen?

A

when two waves overlap in such a way that they combine to create a larger wave

132
Q

How are beat frequencies removed?

A

Synchronising

133
Q

What is the purpose of blade position phasing?

A

So that tips never meet and create noise.

134
Q

What is the critical range?

A

Stoke pulsations from the engine transmitting to the prop.

135
Q
A

CAT B- Mod 17 Propellers (7 decks)

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