17.1 Flashcards
(91 cards)
1
Q
What principle does a propeller work under?
A
The reaction principle
2
Q
A propeller converts torque into?
A
Thrust
3
Q
How does a propeller move the air?
A
A large mass of air, slowly rearwards
4
Q
Coarse pitch moves the blade to?
A
In a more horizontal position
5
Q
Fine pitch makes the blade move?
A
In a more vertical position
6
Q
What are the effects of low pitch?
A
Good low speed acceleration, and good rate of climb
7
Q
What are the effects of high pitch?
A
High speed performance and economy
8
Q
What must be done to a propeller to ensure its thrust remains the same along the entire length of the propeller?
A
Must incorporate washout, the blade twist ensure equal thrust from hub to tip
9
Q
What is blade angle?
A
The angle between the chord of the blade and the rotational plane
10
Q
On a fixed pitch propeller what is the angle of incidence?
A
The angle between the chord of the blade and the rotational plane
11
Q
Is the propeller blade angle the same along its entire length?
A
No, as the washout imparts twist on the blade
12
Q
Where is the reference blade station typically located?
A
0.5 to 0.75 of the radius of the propeller
13
Q
What is blade angle of attack?
A
The angle between the profile chord and the relative airflow
14
Q
With forward movement, what two forces are created?
A
The air flow velocity, resulting from aircraft airspeed v
Propeller rotational velocity u
15
Q
What is the helix angle (angle of advance)?
A
The angle of advance φ is the angle between the rotational plane of the propeller and the relative airflow (RAF) w. The angle of advance increases with increasing airspeed v.
16
Q
How do you calculate blade angle?
A
Blade angle = helix angle + angle of attack
17
Q
A change in airspeed or rotational velocity creates a change in what?
A
The relative airflow direction and velocity
18
Q
A change in airspeed or rotational velocity can lead to?
A
A negative angle of attack
19
Q
If an aircraft is at idle power in a decent, what will happen?
A
The propeller angle of attack will decrease, and the airflow will now drive the prop creating a negative torque
20
Q
What is said about a propellers angle of attack for different purposes?
A
As a certain angle of attack is optimal for any given propeller, fixed pitch propellers can only operate efficiently within a given aircraft speed range. Therefore, fixed pitch propellers are designed for a particular type of flight
regime. For example: Good climb performance
High cruising speeds
Towing
21
Q
Explain propeller brake moment?
A
At a constant rotational speed the sum of propeller brake moment and engine torque is zero. In simple terms Propeller Brake moment can be considered to
be the effort with which is required to be able to spin the propeller
22
Q
What is the effect of AoA with changing RPM?
A
We can see that with increasing RPM for a propeller with a given pitch at a constant airspeed AoA increases and similarly when RPM is reduced at a constant airspeed AoA is reduced
23
Q
What is the effect of Brake Moment when Changing the Blade Angle?
A
A reduction in blade angle leads to a reduction the brake moment. With constant motive power the rotational speed will increase
24
Q
How can brake moment be maintained?
A
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
25
Explain brake moment whilst wind-milling
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
26
What must be available when in reverse pitch?
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 propellers rotational speed
27
Can brake thrust exceed take off thrust?
Yes, 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
28
What effects the amount of lift a prop can make?
Shape, RPM and angle of attack
29
What is done to the aerofoil shape of a prop to ensure near constant pitch?
Must be twisted from root to the tip
30
What is pitch distribution?
The blade angle becomes smaller the further it is from the centre axis in order to keep a nearly constant angle of attack
31
Explain root losses
A thickened root area can withstand high stresses but loses aerodynamic efficiency. Airflow at the root is affected by the engine
32
Explain tip losses
Tip vortices and induced drag cause tip losses at high rotational speeds. Further losses are caused by compressibility effects
33
Explain 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
34
Explain prop slip
Slip is the difference between geometric pitch and effective pitch which is, in effect, the efficiency losses of the propeller. Efficiencies can vary from between 50% to 90% depending on the amount of slip
35
Explain prop pitch
Propeller pitch is defined as the distance that a propeller will move forward in one revolution. This is based on the blade angle at the 75% blade station.
Master Reference Station
36
Explain prop geometric pitch
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
37
Explain prop 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
38
Explain prop efficency
Propeller efficiency is, in basic terms, the performance produced by the propeller in relationship to its motive performance
39
What is motive performance?
Motive performance is the same as the output power of the engine, which for a piston engine is brake power and for a turboprop is shaft power
40
What is the thrust performance and its calculation?
The performance produced is the thrust performance of the propeller. Thrust performance can be calculated from thrust and airspeed. Propeller efficiency can also be calculated by dividing effective pitch by geometric pitch
41
What are the normal propeller efficiencies?
Normal propeller efficiency ranges are in the region of 0.8 to 0.9 (80% -90%)
42
What forces must a prop be able to counter?
As a propeller rotates, various forces interact to cause torsion, tension, bending and compression loads which the propeller must be designed to withstand
43
Static Loads- Explain centrifugal force
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 7 500 times the weight of the propeller blade
44
Static Loads- explain thrust bending force
Thrust Bending Force attempts to bend the propeller blade tips forwards. This is due to the lift (thrust) flexing the thin blade section
45
Static Loads- explain torque bending force (brake moment)
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
46
Static Loads- explain aerodynamic twisting force (ATM)
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
47
Static Loads- explain centrifugal twisting moment (CTM)
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
48
Where are all forces felt on a prop?
All of these loads are all felt at the blade root. Therefore, the greatest stresses will occur in this area as well as on the hub
49
With dynamic loads where are the vibrations created?
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
50
Where are aerodynamic forces stongest?
Aerodynamic forces have a greater vibration effect at the tip of the blade where the effects of transonic speeds cause buffeting and vibration. Any vibrations may be decreased by the use of the correct aerofoil shape and tip design
51
What determines the natural frequency range of a prop?
The natural frequency of a propeller blade will depend on its length, shape, blade root and material
52
What are the basic frequency ranges of different materials used in props?
```
The basic frequency ranges from 20 Hz (metal) to
60 Hz (wood). The blade’s natural frequencies will also change over the RPM range due to differing centrifugal loads
```
53
Where are the highest vibrational loads felt on a prop?
The highest vibrational loads are felt in the area of about 80% of the blade length, making this region particularly susceptible to failure
54
As engine power increases what can be done to improve prop performance?
Increase the blade angle
Increase the diameter of the propeller disk
Increase the RPM of the propeller
Increase the camber of the blade
Increase the chord of the propeller blades
Increase the number of blades
55
Explain how increasing the blade angle improves the prop
Increase the blade angle (or the pitch) of the propeller blades. This will increase the angle of attack of the blades allowing them to impart greater
energy to the air flow
56
Explain how increasing the diameter of the propeller disc improves the prop
Increase the diameter of the propeller disc. This makes the blades longer. They will therefore transfer more energy by affecting a larger volume of air
57
Explain how increasing the RPM of the propeller improves the prop
Increase the RPM of the propeller. The same amount of energy is transferred to the air but in a shorter time
58
Explain how increasing the RPM of the propeller improves the prop
Increase the camber of the blade. Increasing the camber of the blade creates a greater thrust force just like increasing the camber of a wing creates a greater lift force
59
Why is increasing bland angle impractical?
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
60
Why is increasing blade length impractical?
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
61
Why is increasing rpm impractical?
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
62
Why is changing the aerofoil camber impractical?
The blade aerofoil are 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
63
What is the prop tip to fuselage clearence?
1 inch (25.4mm)
64
What is the prop to nose wheel clearence?
1/2 inch (13.7mm)
65
What is the prop tip to ground clearance? (nose wheel)
7 inches (17.78cm)
66
What is the prop tip to ground (tail wheel in flight attitude)
9 inches (22.86cm)
67
What is the prop tip to water clearance? (floatplane)
18 inches (45.72cm)
68
What is solidity?
Solidity simply means the area of the propeller disk 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
69
What modifies solidity?
Both the blade chord or the number of blades will have the effect of increasing the solidity of the propeller disk
70
What can be the effect of increasing blade chord?
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
71
What is the effect of a running engine and propeller spinning?
Torque with a reaction torque in the opposite direction
72
How can torque be combatted?
To compensate for this, 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. Generally, the compensating factors are permanently set so as to compensate
for this force at cruise, since most of the aircraft’s operating lift is at that speed. However, aileron trim tabs permit further adjustment for other speeds
73
At slow speeds what happens to the slipstream off the prop?
The high speed rotation of an aircraft propeller gives a spiralling rotation to the slipstream. At high propeller speeds and low forward speed (as in take off,
approaches and power on stalls), this spiralling rotation is very compact and exerts a strong side wards force on the aircraft’s vertical tail surface
74
At high speeds what happens to the slipstream off the prop?
As the forward speed increases, however, the
| spiral elongates and becomes less effective
75
How is the corkscrew slip stream counter acted?
To compensate, 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)
76
If a propeller is vibrating, what can cause this?
If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates throughout the entire rpm range, although the intensity
of the vibration may vary with the rpm
77
What indicates a prop to engine mismatch?
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
78
How can prop vibration be spotted?
the cause of the vibration can be determined by observing the propeller hub, dome, or spinner while the engine is running within a 1,200- to 1,500-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
79
What should be spotted and be the cause of prop vibrations?
When propeller vibration is the reason for excessive vibration, the difficulty is usually caused by propeller blade imbalance, propeller blades not tracking, or
variation in propeller blade angle settings. Check the propeller blade tracking and then the low-pitch blade angle setting to determine if either is the cause
of the vibration. If both propeller tracking and low blade angle setting are correct, the propeller is statically or dynamically unbalanced and should be replaced, or re-balanced if permitted by the manufacturer
80
What are the 5 components of prop noise?
```
Rotation noise
Vortex noise
Displacement noise
Blade vibration noise
Noise caused by inconsistent airflow
```
81
What is rotation noise?
The rotating pressure field of the propeller produces rotation noise. At Mach (M) numbers of the blade tips between M = 0.5 and M = 0.85 and an undisturbed flow of air to the blade this noise exceeds all other noise components
82
What is 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
83
What is 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 Mach numbers above 0.9 this noise source equals that of rotation noise
84
What is 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
85
What is 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. This results in noise. A similar occurrence is possible if the airflow on the
preceding blade stalls as a result of excessive load
86
By how much is noise increased when engine power is doubled?
5dB
87
if prop diameter is increased with peripheral speed constant, what happens to the noise level?
noise is reduced by 6dB
88
If prop diameter is increased and rotational speed increases then what happens to the noise?
But if the propeller diameter is increased with
| rotational speed remaining constant, the noise level increases
89
What is the effect of adding prop blades on noise?
If with constant RPM, the same power and the same propeller diameter the 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
90
What does increased tip speed do to noise?
An increase in the airflow Mach number of the propeller blade tips from M=0.63 to M=0.87 with propeller data and power remaining constant leads to
an increase in noise level of about 6 dB
91
What are the blade shape factors and the noise levels?
```
Blade tip shape: 3 - 6 dB
Profile type: 2 - 3 dB
Blade contour: 1 - 2 dB
Blade twist: 1 - 2 dB
Profile camber: 1 - 2 dB
Profile section ratio: 1 - 2 dB
```
