17.1 Fundamentals Flashcards
1
Q
propellers made their debut when
A
late 19th century
2
Q
first propeller designs included
A
simple wood and fabric paddles to complex multi-bladed wire-braced designs, some of which were used successfully to propel dirigibles.
3
Q
the increase in aircraft size, speed, and engine power required further improvements in propeller design.
at what time period
A
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.
4
Q
The advantage of being able to alter the propeller blade angle in flight led to the
A
acceptance of the two-position propeller and the development of the constant-speed propeller system
5
Q
Propeller -
A
A device, consisting of a rotating hub with two or more radiating blades; used to propel an aircraft.
5
Q
Further developments during the period leading up to, and during World War II included the
A
feathering propeller and the reversing propeller.
6
Q
Hub -
A
The central portion of a propeller which carries the blades.
7
Q
Blade -
A
Aerofoil section that is attached to the hub
8
Q
Blade root/shank
A
The thickened portion of the blade nearest to the hub.
9
Q
Blade station
A
A distance measured from the centre of rotation, normally measured in inches or centimetres
10
Q
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
11
Q
Blade face -
A
The flat thrust producing side of a propeller blade
12
Q
Blade back
A
The curved side of the propeller blade facing the direction of flight
13
Q
Blade chord line
A
A line through the blade profile at the points between the face and back surfaces
14
Q
Plane of rotation
A
The plane in which the propeller rotates. This is 90° to the engine centreline
15
Q
Blade angle
A
The angle between the blade chord line and the plane of rotation
16
Q
Pitch
A
Distance advanced in one complete revolution
17
Q
Pitch change mechanism
A
Device to alter blade angle
18
Q
Fine pitch
A
Vertical blade angles. Also referred to as “Low pitch”.
19
Q
Coarse pitch
A
Horizontal blade angles. Also referred to as “High pitch”.
20
Q
Reverse pitch
A
Turning the propeller blades to a negative angle to produce braking or reversing thrust.
21
Q
Dome assembly
A
Encases the pitch change mechanism.
22
Q
Spinner
A
An aerodynamic fairing that covers the centre of the propeller
23
Q
Tractor propeller
A
A propeller mounted in front of the leading edge of the wing or on the nose of the aircraft.
24
Pusher propeller
A propeller mounted behind the trailing edge of the wing, or at the rear of the fuselage.
25
a propeller accelerates a large mass of air slowly rearwards.
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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
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27
A propeller works on the reaction principle (Newton’s Third Law of Motion)
for every action there is an equal and opposite reaction.
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cross section of a typical propeller blade is an aerofoil section like that of an aircraft wing
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ne surface of the blade is cambered or curved, like the upper surface of an aircraft wing and is known as the
Blade Back
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the other surface is flat like the bottom surface of a wing and is known as the
Blade Face
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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.
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32
A typical propeller system has an efficiency of about 80% up to a speed of 800 km/h (497 mph).
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33
propeller’s efficiency is the ratio between what
the power developed by the propeller and the power obtained from the aircraft’s power plant
34
The “Momentum Theory” was developed by who
W.J.M. Rankine and R.E. Froude
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
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.
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
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the blade element theory deals primarily with the aerodynamic forces acting on the propeller blades.
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.
38
Blade element theory (2)
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.
39
V₀ = Axial flow at propeller disc
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V₂ = Angular flow velocity vector
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V₁ = Section local flow velocity vector, summation of vectors V₀ and V₂
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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.
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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.
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44
The change of reference line from chord to zero lift line.
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for any single revolution of the propeller, the amount of air displaced depends on what
the blade angle, which determines the quantity or amount of mass of air the propeller moves.
46
blade angle, usually measured in what
degrees
47
blade angle is what θ (Theta)
the angle between the chord of the blade and the plane of rotation. It is measured at the master reference station
48
the chord line is often drawn along the face of the propeller blade.
.because most propellers have a "flat face"
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.
An increase or decrease in one is usually associated with an increase or decrease in the other
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".
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51
The pitch of a propeller can be designated in inches.
A propeller designated as a “74–48” is 74 inches in length and has an effective pitch of 48 inches
52
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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.
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fine pitch =
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).
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coarse pitch =
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.
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.
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.
57
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”.
58
Angle of Attack α (Alpha)
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.
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
- The airflow velocity, resulting from aircraft airspeed
- Propeller rotational velocity
60
(RAF) relative airflow is produced from what
- airflow velocity, resuting from aircraft speed
- propeller rotational velocity
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.
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62
Blade angle = Helix angle + Angle of Attack
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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.
As the propeller rotates and advances through the air, the actual path that the blades follow describes a helix.
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.
The rotational velocity is also known as the "tangential velocity"
65
the helix angle is related to the advance per rev, or effective pitch (B–C).
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66
any change in propeller RPM or advance per rev induces a change in the helix angle (AB–AC)
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67
The propeller tip’s helical path is approximately 45° from the vertical and increases towards the blade root.
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.
68
Helix angle tan θ = P/2 πR P over 2 Pi R
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69
Effective Pitch (P) = 2 π R tan θ
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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
Negative pitch angles result in the thrust generated by the propeller being directed forward against the directional motion of the aircraft.
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)
In reverse pitch, the air is pushed towards the front of the aircraft creating a reverse thrust.
72
Reverse pitch results in braking action, rather than the forward thrust of the aircraft. It is used for
backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the aircraft to a stop during the landing roll
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.
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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"
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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.
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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
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.
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.
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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
"Flight Fine" (or sometimes referred to as "flat pitch" and produces no thrust) to ‘Reverse’ and is only available on the ground.
79
the beta range of operation consists of power lever positions from flight idle to maximum reverse
.
80
Since a propeller blade is a rotating aerofoil, it produces lift by aerodynamic action. The amount of lift produced depends on what
the aerofoil shape, Revolutions Per Minute (RPM), and Angle of Attack (AOA) of the blade sections.
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.
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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
propellers geometric twist
83
The geometric twist is the angle between the blade chord and its plane of rotation that varies along the blade’s length
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84
the actual blade twist is designed to provide the correct angle of attack at the design cruise speed.
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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.
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
pitch distribution
87
in addition to the angle of incidence, the profile shape also changes for what reasons
static and aerodynamic reasons
88
Root Losses
A thickened root area can withstand high stresses but loses aerodynamic efficiency. Airflow at the root is affected by the engine.
89
Tip Losses
Tip vortices and induced drag cause tip losses at high rotational speeds. Further losses are caused by compressibility effects.
90
Blade Washout
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
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
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92
Geometric pitch is
the theoretical distance a propeller should advance in one revolution
93
effective pitch is
the distance it actually advances; this takes into account that air is compressible and the effects of drag.
94
geometric, or theoretical pitch is based on no slippage, but actual or effective pitch includes propeller slippage in the air
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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.
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.
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
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.
97
Geometric pitch is usually expressed in pitch inches and calculated by using the following formula:
GP = 2 x πR x Tangent of blade angle at 75 percent station
97
to calculate the effective pitch:
effective pitch = Aircraft speed X duration of a Revolution
98
Propeller efficiency is used to define how well a propeller transmits its rotational force or energy into thrust.
the amount of thrust generated by a propeller is controlled by the angle at which its blades attack the air
99
it is the design and shape of a propeller that defines its efficiency more than the speed at which it turns
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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.
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101
Propeller efficiency is measured as
brake power for piston engine aircraft and shaft power for turboprop aircraft, and not thrust.
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
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103
the advance ratio is the ratio of the aircraft's airspeed and the propeller speed.
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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.
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105
static loads = (5 of them)
- Centrifugal Force
- Thrust bending force
- Torque bending force
- Aerodynamic Twist Moment
- Centrifugal Twist Moment
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.
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Thrust Bending Force attempts to bend the propeller blade tips forwards. This is due to the lift (thrust) flexing the thin blade section.
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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
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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.
In reverse pitch, the ATM will turn the blade to a coarser Negative Blade Angle.
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
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111
The magnitude of CTM depends on blade chord, weight and RPM. Any increase of the three mentioned parameters will increase CTM.
The ATM acts with the CTM to fine off the blades only when the propeller is Windmilling.
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.
No damage or repair work is permitted within the blade root area
113
Dynamic loads
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
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
Aerodynamic forces have a greater vibration effect at the tip of the blade where the effects of transonic speeds cause buffeting and vibration
115
Aero-elastic flutter is speculated to be a dominant mechanism causing rapid fatigue failure near a tip when insufficient tip stiffness exists
any vibrations may be decreased using the correct aerofoil shape and tip design.
116
more powerful engines require more propeller blades.
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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.
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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
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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.
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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):
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)
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
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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.
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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.
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124
Solidity is the ratio of total blade area to total prop disc area, which can be found from the following formula:
Solidity = S/ π r²
S = total blade area
π r² = prop disc area
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.
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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
- Torque reaction from engine and propeller
- Twist effect of the propeller wash
- Gyroscopic action of the propeller
- Asymmetric loading of the propeller (P-factor)
127
Torque reaction involves Newton’s Third Law of Motion; for every action, there is an equal and opposite reaction
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
128
to compensate torque reaction inflight what was done
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.
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:
Size and horsepower of engine
Size of propeller and the RPM
Size of the aircraft
Condition of the ground surface
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
just under 2%
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
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.
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.
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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.
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134
two fundamental properties of gyroscopic action: rigidity in space and precession
the one of interest with regards to propellers is precession.
135
Precession is the resultant action, or deflection, of a spinning rotor when a deflecting force is applied to its rim.
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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.
Depending on where the force is applied, the aircraft will yaw left or right, pitch up or down, or in a combination of both
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.
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
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
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.
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)
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.
140
windmilling when the propeller drives the engine
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fixed pitch propellers are designed for a particular type of flight regime. For example:
Good climb performance
High cruising speeds
Towing
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Propeller Brake Moment
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.
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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.
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.
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as airspeed decreases, AoA increases resulting in an increase in Brake Moment which would decrease RPM (u) for a given power setting
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.
145
Effect of Changing RPM
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.
146
A reduction in blade angle leads to a reduction of Brake Moment. With constant motive power the rotational speed will increase.
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.
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Natural Vibration
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
148
Resonant Frequency
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.
149
Forced Vibration
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
150
Causes of Vibration
The sources of vibration in a propeller may be grouped into two main categories. These are mechanical and aerodynamic.
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.
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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.
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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..
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When the blade is not rotating there is a certain amount of movement in the mountings, this is called
Blade Shake
154
Once the engine is running, centrifugal force holds the blades firmly (approximately 30 000 – 40 000 pounds) against blade bearings.
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Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180°, and reinstalled.
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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.
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Resonance
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.
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If propeller noise is analysed, it can be broken down into the following components:
Rotation noise
Vortex Noise
Diseplacement Noise
Blade Vibration Noise
Noise caused by inconsistent airflow
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vortex noise =
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.
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Displacement Noise
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.
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Blade Vibration Noise =
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.
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Noise Caused by Inconsistent Airflow =
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.
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Influences on the Level of Noise
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
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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.
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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%.
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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.
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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).
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The following list shows by about how much propeller noise can be changed by various influencing factors:
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
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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.
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Beat Frequencies
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
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Constructive interference happens when two waves overlap in such a way that they combine to create a larger wave
Destructive interference happens when two waves overlap in such a way that they cancel each other out.
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only way to remove Beat frequencies is to
to guarantee the removal of this problem is to use a synchronising system.
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Blade Position Phasing
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.
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Blade Position Phasing
the provision of an electronic synchrophasing system ensures that the propellers rotate so that no tip passes occur on adjoining engines.
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the blade tips experience sonic flow the resulting turbulence can cause
buffeting and vibration
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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.
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
