17.1 Fundamentals

Question 1

propellers made their debut when

Answer 1

late 19th century

Question 2

first propeller designs included

Answer 2

simple wood and fabric paddles to complex multi-bladed wire-braced designs, some of which were used successfully to propel dirigibles.

Question 3

the increase in aircraft size, speed, and engine power required further improvements in propeller design.

at what time period

Answer 3

during world war 1 and the years immediately after

These designs included the four-bladed propeller, aluminium fixed pitch propellers, and the two-position controllable propeller.

Question 4

The advantage of being able to alter the propeller blade angle in flight led to the

Answer 4

acceptance of the two-position propeller and the development of the constant-speed propeller system

Question 5

Propeller -

Answer 5

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

Question 5

Further developments during the period leading up to, and during World War II included the

Answer 5

feathering propeller and the reversing propeller.

Question 6

Hub -

Answer 6

The central portion of a propeller which carries the blades.

Question 7

Blade -

Answer 7

Aerofoil section that is attached to the hub

Question 8

Blade root/shank

Answer 8

The thickened portion of the blade nearest to the hub.

Question 9

Blade station

Answer 9

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

Question 10

Master reference station

Answer 10

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

Question 11

Blade face -

Answer 11

The flat thrust producing side of a propeller blade

Question 12

Blade back

Answer 12

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

Question 13

Blade chord line

Answer 13

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

Question 14

Plane of rotation

Answer 14

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

Question 15

Blade angle

Answer 15

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

Question 16

Pitch

Answer 16

Distance advanced in one complete revolution

Question 17

Pitch change mechanism

Answer 17

Device to alter blade angle

Question 18

Fine pitch

Answer 18

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

Question 19

Coarse pitch

Answer 19

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

Question 20

Reverse pitch

Answer 20

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

Question 21

Dome assembly

Answer 21

Encases the pitch change mechanism.

Question 22

Spinner

Answer 22

An aerodynamic fairing that covers the centre of the propeller

Question 23

Tractor propeller

Answer 23

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

Question 24

Pusher propeller

Answer 24

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

Question 25

a propeller accelerates a large mass of air slowly rearwards.

Answer 25

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Question 26

The propeller consists of two or more blades that are connected by a hub. The hub serves to attach the blades to a

piston engine, a Reduction Gearbox (RGB), or more recently, an electric motor drive shaft

Answer 26

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Question 27

A propeller works on the reaction principle (Newton’s Third Law of Motion)

Answer 27

for every action there is an equal and opposite reaction.

Question 28

cross section of a typical propeller blade is an aerofoil section like that of an aircraft wing

Answer 28

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Question 29

ne surface of the blade is cambered or curved, like the upper surface of an aircraft wing and is known as the

Answer 29

Blade Back

Question 30

the other surface is flat like the bottom surface of a wing and is known as the

Answer 30

Blade Face

Question 31

propellers accelerate airflow over their cambered surfaces. The high velocity of the air results in lower static pressure in front of the propeller, pulling the aerofoil forward.

Answer 31

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Question 32

A typical propeller system has an efficiency of about 80% up to a speed of 800 km/h (497 mph).

Answer 32

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Question 33

propeller’s efficiency is the ratio between what

Answer 33

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

Question 34

The “Momentum Theory” was developed by who

Answer 34

W.J.M. Rankine and R.E. Froude

Question 35

This theory assumes a propeller to be an advancing disc producing a uniform thrust, because of the pressure difference in front and behind the disc being a constant amount over its area. It is also assumed that the air is a perfect fluid, incompressible and without viscosity

Answer 35

it is also assumed that the flow of air is streamlined in character and continuous through the propeller so that the axial velocity is the same immediately in front of and immediately behind the disc. There is no torque imposed on the disc, and no rotation or twist is imparted to the air moving through it.

Question 36

The momentum theory is useful for determining the ideal efficiency, but only provides a basic interpretation of propeller action neglecting things such as torque

Answer 36

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Question 37

the blade element theory deals primarily with the aerodynamic forces acting on the propeller blades.

Answer 37

This theory involves 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. These forces are then integrated along the entire blade over one revolution to obtain the forces and moments produced by the entire propeller.

Question 38

Blade element theory (2)

Answer 38

At each section, a force balance is applied involving two-dimensional lift and drag characteristics with the thrust and torque produced by the section. At the same time, a balance of axial and angular momentum is applied. This produces a set of non-linear equations that can be solved by iteration for each blade section. The resulting values of each section’s thrust and torque can be summed to predict the overall performance of the propeller.

Question 39

V₀ = Axial flow at propeller disc

Answer 39

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Question 40

V₂ = Angular flow velocity vector

Answer 40

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Question 41

V₁ = Section local flow velocity vector, summation of vectors V₀ and V₂

Answer 41

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Question 42

Since the propeller blade is set at a given geometric pitch angle (θ) the local velocity vector V₁ creates a flow angle of attack on the section. The lift and drag of the section can be calculated using standard two-dimensional aerofoil properties.

Answer 42

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Question 43

The lift and drag components, normal to and parallel to, the propeller disc can be calculated so that the contribution to thrust and torque of the complete propeller from this single element can be found.

Answer 43

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Question 44

The change of reference line from chord to zero lift line.

Answer 44

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Question 45

for any single revolution of the propeller, the amount of air displaced depends on what

Answer 45

the blade angle, which determines the quantity or amount of mass of air the propeller moves.

Question 46

blade angle, usually measured in what

Answer 46

degrees

Question 47

blade angle is what θ (Theta)

Answer 47

the angle between the chord of the blade and the plane of rotation. It is measured at the master reference station

Question 48

the chord line is often drawn along the face of the propeller blade.

Answer 48

.because most propellers have a “flat face”

Question 49

A pitch is not a blade angle, but because a pitch is largely determined by a blade angle, the two terms are often used interchangeably.

Answer 49

An increase or decrease in one is usually associated with an increase or decrease in the other

Question 50

as the blade angle is not constant over the whole length of the blade, a particular part of the blade, termed the reference “blade station”, is where the blade angle is taken from. This station can be anywhere between 0.5 and 0.75 of the radius of the propeller and is sometimes referred to as the “master station”.

Answer 50

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Question 51

The pitch of a propeller can be designated in inches.

Answer 51

A propeller designated as a “74–48” is 74 inches in length and has an effective pitch of 48 inches

Question 53

A blade pitch acts very much like the gearing of a car. A fine (low) pitch yields good low-speed acceleration (and climb rate in an aircraft), while a course (high) pitch optimises high-speed performance and economy.

Answer 53

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Question 54

fine pitch =

Answer 54

A fine pitch propeller will rotate easily without taking a big bite out of the air and moves forward through the air a short distance with every revolution. This allows the engine to spin easily and operate at a high speed (RPM).

Question 55

coarse pitch =

Answer 55

A coarse pitch propeller takes a large bite out of the air with every turn. The propeller moves forward through the air a large distance with every revolution. However, a coarse pitch setting limits the speed at which the engine operates.

Question 56

Blade Angle α (Alpha) Range

In aircraft, it is quite common for the propeller to be designed to vary pitch in flight. This allows for an optimum thrust over the maximum amount of the aircraft’s speed range, from take-off and climb to cruise.

Answer 56

Often referred to as “Controllable Pitch” and “Constant Pitch” propellers, these propellers can be controlled by the pilot or automatically to ensure that optimum propeller efficiency is maintained throughout the flight profile.

Question 57

If blade angle changes are controlled and stay within the ‘fine’ to ‘coarse’ positions, then the propeller is said to be operating in

Answer 57

the “α (alpha) range”.

Question 58

Angle of Attack α (Alpha)

Answer 58

The angle of attack is the angle between the profile chord line and the relative airflow towards it. With the blade angle running the length of the blade, the desired lift distribution is achieved from the resulting angles of attack.

Question 59

As the propeller moves on a plane that is perpendicular to the forward movement of the aircraft, two velocities, perpendicular to each other define the angle of attack

Answer 59

• The airflow velocity, resulting from aircraft airspeed
• Propeller rotational velocity

Question 60

(RAF) relative airflow is produced from what

Answer 60

• airflow velocity, resuting from aircraft speed
• propeller rotational velocity

Question 61

Variable pitch propellers have varying angles of attack for optimum performance. When operating in the flight range - between ‘Fine’ and ‘Coarse’ the propeller is said to be operating in the α (alpha) range.

Answer 61

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Question 62

Blade angle = Helix angle + Angle of Attack

Answer 62

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Question 63

The angle of advance (helix angle) φ is the angle between the rotational plane of the propeller and the relative airflow. The angle of advance increases with increasing airspeed.

Answer 63

As the propeller rotates and advances through the air, the actual path that the blades follow describes a helix.

Question 64

he propeller has a rotational velocity, vector (A–B), and travels on a circumferential distance equal to 2 in unit time in the plane of rotation.

Answer 64

The rotational velocity is also known as the “tangential velocity”

Question 65

the helix angle is related to the advance per rev, or effective pitch (B–C).

Answer 65

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Question 66

any change in propeller RPM or advance per rev induces a change in the helix angle (AB–AC)

Answer 66

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Question 67

The propeller tip’s helical path is approximately 45° from the vertical and increases towards the blade root.

Answer 67

the blade tip helix angle also varies from zero degrees when the aircraft is stationary through approximately 45° at the design cruise speed, to a greater angle as the aircraft’s speed increases above its design cruise speed.

Question 68

Helix angle tan θ = P/2 πR P over 2 Pi R

Answer 68

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Question 69

Effective Pitch (P) = 2 π R tan θ

Answer 69

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Question 70

Reverse pitch is a pilot selectable feature of a constant speed or variable pitch propeller which allows the blade pitch to be decreased to a negative value

Answer 70

Negative pitch angles result in the thrust generated by the propeller being directed forward against the directional motion of the aircraft.

Question 71

in the ‘reverse’ pitch position, the engine/propeller turns in the same direction as in the normal (forward) pitch position, but the propeller blade angle is positioned to the other side of fine pitch (negative pitch)

Answer 71

In reverse pitch, the air is pushed towards the front of the aircraft creating a reverse thrust.

Question 72

Reverse pitch results in braking action, rather than the forward thrust of the aircraft. It is used for

Answer 72

backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the aircraft to a stop during the landing roll

Question 73

The reverse pitch does not mean reversing the rotation of the engine(s). The engine delivers power just the same, no matter which side of the fine pitch the propeller blades are positioned.

Answer 73

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Question 74

Reverse pitch is achieved by turning the prop blades up to 30 degrees past the fine/flat pitch stop (depending on aircraft type) to a negative angle of attack, this can be referred to as “braking pitch”

Answer 74

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Question 75

When operating in the reverse mode, the propeller blades are at a negative angle of attack and the propeller is less efficient than at normal forward motion pitch settings.

Answer 75

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Question 76

If the blade angle is reduced to such an extent that the angle of attack is less than the zero-lift angle of attack (as seen in a wind-milling propeller), thrust acting against the direction of flight results

Answer 76

The partial torque force acts contrary to the direction of rotation so that the brake moment it causes must be overcome by the drive.

The brake moments, which occur very quickly, become very large when the blade angle is moved into reverse pitch.

Therefore, a corresponding increase in engine power must be readily available to maintain the propeller rotational speed.

Question 77

As the mass of air that is flowing through the propeller plane is not accelerated but decelerated, maximum achievable brake thrust increases with airspeed and can, under certain circumstances, even exceed take-off thrust.

Answer 77

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Question 78

Blade Angle β (Beta) Range

With the development of the variable pitch propeller, the pilot can select specific blade angles to provide ease of starting and reverse thrust. This range is called the Beta β Range and is from

Answer 78

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

Question 79

the beta range of operation consists of power lever positions from flight idle to maximum reverse

Answer 79

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Question 80

Since a propeller blade is a rotating aerofoil, it produces lift by aerodynamic action. The amount of lift produced depends on what

Answer 80

the aerofoil shape, Revolutions Per Minute (RPM), and Angle of Attack (AOA) of the blade sections.

Question 81

a propeller at constant rotational speed, the further that the blade section is from its rotational axis, the greater the distance that the section must cover. Therefore, its velocity is greater.

Answer 81

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Question 82

To ensure a nearly constant angle of pitch is maintained, the propeller blade must be twisted from the root to the tip. This is known as the

Answer 82

propellers geometric twist

Question 83

The geometric twist is the angle between the blade chord and its plane of rotation that varies along the blade’s length

Answer 83

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Question 84

the actual blade twist is designed to provide the correct angle of attack at the design cruise speed.

Answer 84

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Question 85

If the blades had the same geometric pitch throughout their lengths, portions near the hub could have a negative angle of attack, while the propeller tips would be stalled at cruise speed.

Question 86

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

Answer 86

pitch distribution

Question 87

in addition to the angle of incidence, the profile shape also changes for what reasons

Answer 87

static and aerodynamic reasons

Question 88

Root Losses

Answer 88

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

Question 89

Tip Losses

Answer 89

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

Question 90

Blade Washout

Answer 90

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

Question 91

Propeller efficiency varies from 50% - 90%, depending on how much the propeller ‘slips’. Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch

Answer 91

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Question 92

Geometric pitch is

Answer 92

the theoretical distance a propeller should advance in one revolution

Question 93

effective pitch is

Answer 93

the distance it actually advances; this takes into account that air is compressible and the effects of drag.

Question 94

geometric, or theoretical pitch is based on no slippage, but actual or effective pitch includes propeller slippage in the air

Answer 94

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Question 95

If the propeller were to spiral through the air on a course, where the angle of pitch equalled the blade angle, the propeller would, in one rotation, have moved forward axially by the geometric pitch.

Answer 95

In this case, if the aircraft moved through the air according to the geometric propeller pitch, the propeller angle of attack would be zero. This is the theoretical, or design pitch and will only occur if the propeller was 100% efficient.

Question 96

Effective Pitch

the actual helical path on which the propeller moves through the air has an angle of pitch which corresponds to the angle of advance

Answer 96

This means that one revolution of the propeller will move the aircraft forward by the effective pitch. All propellers will lose a certain amount of efficiency due to aerodynamic and compressibility losses.

Question 97

Geometric pitch is usually expressed in pitch inches and calculated by using the following formula:

Answer 97

GP = 2 x πR x Tangent of blade angle at 75 percent station

Question 97

to calculate the effective pitch:

Answer 97

effective pitch = Aircraft speed X duration of a Revolution

Question 98

Propeller efficiency is used to define how well a propeller transmits its rotational force or energy into thrust.

Answer 98

the amount of thrust generated by a propeller is controlled by the angle at which its blades attack the air

Question 99

it is the design and shape of a propeller that defines its efficiency more than the speed at which it turns

Answer 99

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Question 100

In general, the larger the prop diameter, the more efficient it will be. However, the maximum useful propeller diameter will be limited by the speed of the propeller tip.

Answer 100

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Question 101

Propeller efficiency is measured as

Answer 101

brake power for piston engine aircraft and shaft power for turboprop aircraft, and not thrust.

Question 102

Normal propeller efficiency ranges are in the region of 0.8 to 0.9 (80% - 90%). The usual symbol for propeller efficiency is the Greek symbol (eta). η

Answer 102

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Question 103

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

Answer 103

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Question 104

As a propeller rotates, various forces interact to cause torsion, tension, bending, and compression loads which the propeller must be designed to withstand. These can be sub-divided into six separate loadings - five static and one dynamic.

Answer 104

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Question 105

static loads = (5 of them)

Answer 105

• Centrifugal Force
• Thrust bending force
• Torque bending force
• Aerodynamic Twist Moment
• Centrifugal Twist Moment

Question 106

Centrifugal Force is the greatest load felt on the propeller, trying to pull the blades out of the hub assembly. The amount of load created can be more than 7500 times the weight of the propeller blade.

Answer 106

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Question 107

Thrust Bending Force attempts to bend the propeller blade tips forwards. This is due to the lift (thrust) flexing the thin blade section.

Answer 107

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Question 108

Torque Bending Force (Braking moment) tends to try and bend the blade against the direction of propeller rotation. This creates a resistance to the torque being produced by the engine

Answer 108

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Question 109

Aerodynamic Twisting Moment (ATM): 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.

Answer 109

In reverse pitch, the ATM will turn the blade to a coarser Negative Blade Angle.

Question 110

Centrifugal Twisting Moment (CTM): 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. CTMs will always oppose ATMs and CTMs are always greater than ATMs

Answer 110

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Question 111

The magnitude of CTM depends on blade chord, weight and RPM. Any increase of the three mentioned parameters will increase CTM.

Answer 111

The ATM acts with the CTM to fine off the blades only when the propeller is Windmilling.

Question 112

All these static loads are all felt at the blade root. Therefore, the greatest stresses will occur in this area as well as on the hub.

Answer 112

No damage or repair work is permitted within the blade root area

Question 113

Dynamic loads

Answer 113

The maximum dynamic loading on a propeller blade will occur within its natural frequency range. These vibrations are the result of the operating strokes of a piston engine or the dynamics of the propeller reduction gearbox. Additionally, they will be induced by aerodynamic and mechanical forces felt on the propeller blades

Question 114

if the aerodynamic and mechanical effects are not compensated for in the design, then excessive flexing and work hardening of the metal will lead to structural failure during operation

Answer 114

Aerodynamic forces have a greater vibration effect at the tip of the blade where the effects of transonic speeds cause buffeting and vibration

Question 115

Aero-elastic flutter is speculated to be a dominant mechanism causing rapid fatigue failure near a tip when insufficient tip stiffness exists

Answer 115

any vibrations may be decreased using the correct aerofoil shape and tip design.

Question 116

more powerful engines require more propeller blades.

Answer 116

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Question 117

As the power of engines has increased over the years, aircraft designers have adopted increasingly more propeller blades. Once they ran out of room on the propeller hub, they adopted twin propellers on the same engine rotating in opposite directions, called contra-rotating propellers.

Answer 117

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Question 118

Blade angle: The pitch of the blade is set by the angle that optimizes the aerodynamic efficiency of the blade. If this angle is changed, one kind of efficiency is lost in order to gain another. This trade-off makes changing the blade angle a very unattractive alternative

Answer 118

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Question 119

Blade length: While increasing tip speed is a significant issue, size constraints are usually the biggest problem with this option. As the propeller size increases, the landing gear must become longer to avoid scraping the blade tips on the runway. This change has a domino effect on a number of other structural and weight issues.

Answer 119

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Question 120

Any increase in diameter, therefore, is restricted by the need to maintain adequate ground and fuselage clearance. Typical minimum clearances are (Certification Specification (CS) 25.925 refers):

Answer 120

Propeller Tip to Fuselage 1 inch (25.4 mm)
Propeller to Nose wheel 1/2 inch (13.7 mm)
Propeller Tip to Ground (Nose wheel) 7 inches (17.78 cm)
Propeller Tip to Ground (Tail wheel in flight attitude) 9 inches (22.86 cm)
Propeller Tip to Water (Float-plane) 18 inches (45.72 cm)

Question 121

Revolutions per minute: For the same propeller diameter, the blade tips travel faster and faster as the rotational speed increases. Eventually the blade tips become supersonic where shock waves form, drag increases substantially, and efficiency plummets

Answer 121

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Question 122

Aerofoil camber: The blade aerofoil is chosen for optimum aerodynamic efficiency. By changing sections, one kind of efficiency is again sacrificed for another. Increasing camber may also result in blade structural problems.

Answer 122

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Question 123

Both the blade chord or the number of blades will have the effect of increasing the solidity of the propeller disc. Solidity simply means the area of the propeller disc occupied by the blades in relation to area open to the air flow. As solidity increases, a propeller can transfer more power to the air.

Answer 123

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Question 124

Solidity is the ratio of total blade area to total prop disc area, which can be found from the following formula:

Answer 124

Solidity = S/ π r²

S = total blade area

π r² = prop disc area

Question 125

While increasing the blade chord is the easier option, it is less efficient because the aspect ratio of the blades is decreased resulting in some loss of aerodynamic efficiency. Therefore, increasing the number of blades is the most attractive option.

Answer 125

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Question 126

‘Torque’ is made up of four elements that cause or produce a twisting or rotating motion around at least one of the aircraft’s three axes. These four elements are

Answer 126

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

Question 127

Torque reaction involves Newton’s Third Law of Motion; for every action, there is an equal and opposite reaction

Answer 127

this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction. When airborne, this force is acting around the longitudinal axis, tending to make the aircraft roll

Question 128

to compensate torque reaction inflight what was done

Answer 128

some of the older aircraft are rigged in such a manner as to create more lift on the wing that is being forced downward, while more modern aircraft are designed with the engine offset to counteract this torque.

aileron trim tabs permit further adjustment for other speeds.

Question 129

When the wheels are on the ground during taxi and the take-off roll, an additional turning moment around the vertical axis is induced by this torque reaction. If the left side of the aircraft is being forced down, more weight is placed on the left main landing gear. This results in increased ground friction or drag on the left tyre than on the right, causing a further turning moment to the left. The magnitude of this moment is dependent on many variables, including:

Answer 129

Size and horsepower of engine
Size of propeller and the RPM
Size of the aircraft
Condition of the ground surface

Question 130

The high-speed rotation of an aircraft propeller gives a spiralling rotation to the slipstream. This leads to a loss in propeller efficiency of

Answer 130

just under 2%

Question 131

(in relation to propeller wash)

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

Answer 131

When this spiralling slipstream strikes the vertical fin, it causes a turning moment about the aircraft’s vertical axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective.

Question 132

to compensate effect of propeller wash the vertical stabiliser is mounted obliquely 1° or 2° to the aircraft’s longitudinal axis. This aerodynamic compensation is only ideal for one operational regime, normally cruise.

Answer 132

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Question 133

The corkscrew flow of the slipstream also causes a rolling moment around the longitudinal axis in the opposite direction to the one caused by torque reaction.

Answer 133

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Question 134

two fundamental properties of gyroscopic action: rigidity in space and precession

Answer 134

the one of interest with regards to propellers is precession.

Question 135

Precession is the resultant action, or deflection, of a spinning rotor when a deflecting force is applied to its rim.

Answer 135

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Question 136

precession occurs when a force is applied to any point on the rim of the propeller’s plane of rotation; the resultant force will still be 90° from the point of application in the direction of rotation.

Answer 136

Depending on where the force is applied, the aircraft will yaw left or right, pitch up or down, or in a combination of both

Question 137

Asymmetric loading (P-factor)

When air flows towards the propeller during horizontal flight, it is the same for all blades and the centre of balance of thrust lies in the middle of the propeller.

Answer 137

In a climbing attitude, however, the direction of airflow is obliquely from below. Thus, the downward moving blade has a greater angle of attack than in horizontal flight, while the upward moving blade has a smaller one. In this situation the centre of total thrust is no longer in the middle of the propeller. It moves away from the middle towards the blade with the larger angle of attack. This causes a yawing moment around the vertical axis

Question 138

The lack of symmetry is no problem when both engines are operating. If, however, one engine fails, yaw moments of differing amounts arise, depending on which engine fails. The engine which would produce the smallest yaw moment, should the other engine fail, is known as the critical engine

Answer 138

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 opposite directions (inboard downwards). In this way the yaw moments will be smaller if one engine fails and therefore flight performance will be improved.

Question 139

A change in airspeed or a change in rotational velocity will result in a change of the relative air flow direction and velocity. These changing parameters can lead to a negative Angle of Attack (AoA).

(windmilling)

Answer 139

At a negative AoA, the incoming air flow will apply the necessary torque for turning the propeller. This means that the airflow will now drive the propeller which will then drive the engine (Windmilling) and produce a negative torque.

Note: A Windmilling propeller turns the same way.

Question 140

windmilling when the propeller drives the engine

Answer 140

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Question 141

fixed pitch propellers are designed for a particular type of flight regime. For example:

Answer 141

Good climb performance
High cruising speeds
Towing

Question 142

Propeller Brake Moment

Answer 142

Propeller brake moment can be considered to be the effort which is required to be able to spin the propeller.

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

Question 143

With a constant pitch if airspeed increases rapidly or rotational speed is greatly reduced, the angle of attack will reduce and can become negative. The flow of air to the propeller now causes the propeller to windmill. In this case the brake moment works in the direction of rotation and begins to drive the propeller.

Answer 143

As the thrust is relatively large in this situation and directed against the direction of flight, the aircraft drag is considerably increased by the wind-milling propeller.

This increased drag can be greatly reduced if the propeller is driven into the feathered position so that the leading edge is presented into the oncoming airflow.

Question 144

as airspeed decreases, AoA increases resulting in an increase in Brake Moment which would decrease RPM (u) for a given power setting

Answer 144

increasing airspeed the AoA decreases resulting in a reduction of Brake Moment which, for a given power setting would increase RPM

This scenario can be considered dangerous if uncontrolled as the propeller could exceed its maximum permissible rotational speed.

Question 145

Effect of Changing RPM

Answer 145

increasing RPM for a propeller with a given pitch at a constant airspeed the AoA increases and similarly when RPM is reduced at a constant airspeed the AoA is reduced.

Question 146

A reduction in blade angle leads to a reduction of Brake Moment. With constant motive power the rotational speed will increase.

Answer 146

An increase in pitch has the opposite effect. If the pitch is adjusted to a changing airspeed, the magnitude of Brake Moment can be maintained. This leads to a constant rotational speed without changing engine power and to almost constant propeller thrust.

In this way propeller efficiency improves for the whole of the aircraft’s speed range. Thus, with the same engine power higher airspeeds can be achieved than in the case of a fixed propeller.

Question 147

Natural Vibration

Answer 147

When a body oscillates under the action of its own gravitational or elastic forces, with no external forces being present, it is described as having a free or natural vibration. Examples of these are pendulums and springs

Question 148

Resonant Frequency

Answer 148

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 of the object. Church bells or crystal glassware, being struck for example.

Question 149

Forced Vibration

Answer 149

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. A good example is a tuning fork. If it is struck it will vibrate at its resonant frequency. If the fork is then pressed onto the top of a table, the table will begin to vibrate at the same frequency

Question 150

Causes of Vibration

The sources of vibration in a propeller may be grouped into two main categories. These are mechanical and aerodynamic.

Answer 150

In the first category the static and dynamic balance of the propeller will, if not correct, give rise to a once per revolution cycle of disturbance whose amplitude will be related to the amount of imbalance and the speed of rotation.

The second category is related to the even distribution of the thrust and torque loads acting on each of the propeller blades. Another consideration in this second category is the effect of the propellers speed and blade position or phase when related to those of neighbouring, operating propellers.

Question 151

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 propeller problem but a problem of a poor engine-propeller match.

Answer 151

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Question 152

If a propeller vibration is suspected but cannot be positively determined, the ideal troubleshooting method is to temporarily replace the propeller with one known to be airworthy and then test fly the aircraft, if possible..

Answer 152

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Question 153

When the blade is not rotating there is a certain amount of movement in the mountings, this is called

Answer 153

Blade Shake

Question 154

Once the engine is running, centrifugal force holds the blades firmly (approximately 30 000 – 40 000 pounds) against blade bearings.

Answer 154

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Question 155

Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180°, and reinstalled.

Answer 155

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Question 156

In most cases, the cause of the vibration can be determined by observing the propeller hub, dome, or spinner while the engine is running within a 1200-to-1500-RPM range and determining whether or not the propeller hub rotates on an absolutely horizontal plane. If the propeller hub appears to swing in a slight orbit, the vibration is usually caused by the propeller. If the propeller hub does not appear to rotate in an orbit, the difficulty is probably caused by engine vibration.

Answer 156

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Question 157

Resonance

Answer 157

If a body is set into forced vibration it will vibrate with the frequency of that vibration. The amplitude will be proportional to the disturbing forces. If, however, the forced vibration frequency matches the resonant frequency of the body, the amplitude of the vibration will increase dramatically.

Question 158

If propeller noise is analysed, it can be broken down into the following components:

Answer 158

Rotation noise
Vortex Noise
Diseplacement Noise
Blade Vibration Noise
Noise caused by inconsistent airflow

Question 159

vortex noise =

Answer 159

This noise is caused by the vortices leaving the blade tip and blade trailing edge. Its maximum value is found in the plane of rotation of the propeller.

Question 160

Displacement Noise

Answer 160

The origin of this noise is the displacement of the air by the propeller blades as they have a finite thickness. It first becomes critical at higher Mach numbers at the propeller tips. At blade tip numbers above Mach 0.9 this noise source equals that of rotation noise.

Question 161

Blade Vibration Noise =

Answer 161

This noise occurs with periodic stalls, for example when the stall limit of the blade is alternately exceeded and fallen below. The rotors of helicopters are a good example of this phenomenon.

Question 162

Noise Caused by Inconsistent Airflow =

Answer 162

Normally the vortices leave the trailing edge and blade tips in such a way that they do not affect the following propeller blade. The latter can then work in an undisturbed airflow. This is not the case with variable pitch propellers when the angle of pitch is negative, and the propeller has zero thrust then the vortices of the preceding blade hit the leading edge of the following blade, resulting in noise. A similar occurrence is possible if the airflow on the preceding blade stalls as a result of excessive load.

Question 163

Influences on the Level of Noise

Answer 163

Influence of power
influence of the propeller diameter
influence of number of propeller blades
influence of blade tip mach number
influence of propeller blade shape
influence of material

Question 164

If the propeller diameter is doubled at constant peripheral speed, propeller noise is reduced by 6 dB. But if the propeller diameter is increased with rotational speed remaining constant, the noise level increases.

Answer 164

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Question 165

With constant RPM, the same power and the same propeller diameter, ifthe number of blades is increased from 2 to 3, noise is reduced by about 1.1 dB (A). Increasing the number of blades from 3 to 4 or from 4 to 6 has the same result. Apart from the higher manufacturing costs, the additional weight of a propeller with more blades, must also be taken into account. For example, a 4-blade propeller weighs about 35% more than a 3-blade propeller, and the difference between a 2 and 3-blade propeller is as much as 50%.

Answer 165

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Question 166

An increase in the airflow Mach number of the propeller blade tips from M 0.63–0.87 with propeller data and power remaining constant leads to an increase in noise level of about 16 dB. The figure shows how the noise level is dependent on the Mach number for 2, 3 and 4-blade propellers.

Answer 166

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Question 167

With the same power, blade area, profile type, camber, profile section, ratio and diameter, a scimitar-shaped propeller produces the least noise, and a propeller with straight tips produces the most. This favourable effect of the sabre-shape is due to the increasing outward sweep of the propeller blade as the locally occurring effective Mach number is reduced by the factor ( = angle of sweep).

Answer 167

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Question 168

The following list shows by about how much propeller noise can be changed by various influencing factors:

Answer 168

Blade tip shape: 3–6 dB
Profile type: 2–3 dB
Blade contour and twist: 1–2 dB
Profile camber and section ratio: 1–2 dB

Question 169

If the blades are made of wood or composite construction, they have a more favourable vibrational behaviour than metal blades. This is due to better self-damping properties and therefore the noise caused by blade vibrations is lower in the case of such blades. Also, by using composite construction more aerodynamic and low-noise blade shapes can be produced without problems regarding strength and stiffness occurring.

Answer 169

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Question 170

Beat Frequencies

Answer 170

When two engine driven propellers are running at almost the same RPM on the same aircraft the frequencies of the noise from each power plant may be very close to one another in value. Engine RPM very rarely remains constant,

frequencies tend to keep crossing over each other’s value in an almost rhythmic fashion. As the sound waves cross, the intensity of the noise and vibration increases dramatically and then dies away only to repeat on the next cross. These are the ‘beats’ that are a result of what is called constructive and destructive interference

Question 171

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

Answer 171

Destructive interference happens when two waves overlap in such a way that they cancel each other out.

Question 172

only way to remove Beat frequencies is to

Answer 172

to guarantee the removal of this problem is to use a synchronising system.

Question 173

Blade Position Phasing

Answer 173

This is yet another cause of annoying noise and vibration. It only occurs on multi-engine aircraft where two propellers are operating on the same wing in close proximity to one another

the tips of adjoining propeller blades passing near each other

This creates an aerodynamic interaction or ‘pulse’ in the intervening air gap. A loud noise may then be generated with its frequency depending on the number of tip passes occurring per second. This induces vibration into the aircraft structure.

Question 174

Blade Position Phasing

the provision of an electronic synchrophasing system ensures that the propellers rotate so that no tip passes occur on adjoining engines.

Answer 174

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Question 175

the blade tips experience sonic flow the resulting turbulence can cause

Answer 175

buffeting and vibration

Question 176

Each power stroke induces a vibration pulse so an eight-cylinder engine would experience four pulses per engine revolution

these power stroke pulsations from a piston engine can transmit into the propeller causing ‘standing wave’ vibration in the blades that can be particularly detrimental within a given RPM range. This is known as the ‘critical range’, and it is normally indicated in red on the engine tachometer.

Answer 176

Prolonged operation in this range would be prohibited. This source of vibration is more damaging than aerodynamically induced vibration. Also, misfiring cylinders will change the frequency and amplitude of vibration. It is the outer part of a blade near the tip that is put at most risk of failure from standing wave vibration.

Turbine engines rotate at much higher speeds than piston engines. They have reduction gears in the region of 16:1. This means the power turbine shaft rotates sixteen times to one revolution of the propeller. It follows that any vibration emanating from the power turbine is going to be at a much higher frequency than propeller vibration frequency