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M17 > 17.1 - Fundamentals > Flashcards

17.1 - Fundamentals Flashcards

(65 cards)

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1
Q

What is a tractor propeller and a pusher propeller

A

Tractor is mounted in front of wing or fuselage.
Pusher is mounted on back of fuselage or wing.

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2
Q

What is the purpose of a propeller

A

Move the a/c through the air.

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3
Q

What principal does a prop work on

A

Newton’s third law
Reaction principle.

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4
Q

What is thrust equal to

A

The pressure jump x propeller disc area.

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5
Q

What is prop efficiency

A

The ratio between the power of the engine and the power developed by the prop.

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6
Q

What is an the most efficient AOA

A

2-4 degrees

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7
Q

What is meant by AOA

A

The angle between the chord of the blade and the relative airflow.

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8
Q

How does rpm and airflow influence AOA

A

Increasing rpm increases AOA
Increasing airspeed decreases AOA

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9
Q

What is meant by angle of advance

A

Angle between rotational plane and relative airflow
Angle increases with airspeed and decreases with rpm.

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10
Q

What are the other names for angle of advance

A

Angle of helix
Phi.

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11
Q

What is the definition of relative airflow.

A

The speed and direction of air movement past an aero foil.
Has both direction and velocity. Governed by rpm and forward airspeed.

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12
Q

Windmilling.

A
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13
Q

What is meant by angle of advance

A

The difference between the plane of rotation and relative airflow.
Angle increases with airspeed.

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14
Q

What is relative airflow defined as

A

The speed and direction of the airflow over an aero foil
Has both direction and magnitude.
Governed by airspeed and rpm.

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15
Q

What are other names for angle of advance.

A

Helix angle
Phi.

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16
Q

Windmilling.

A

A change in airspeed or rotational velocity results in a change of relative airflow.
These changing parameters can lead to a negative AOA.
At a negative AOA the incoming airflow will apply a turning force to the propeller
A windmilling propeller spins the same way as normal
Airflow drives the engine and produces a negative torque.

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17
Q

What 3 types of flight can fixed pitch propellers be designed for.

A

Good climb performance.
High cruising speeds.
Towing.

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18
Q

Propeller brake moment.

A

Considered to be the effort required to turn a propeller
At a constant speed the net engine torque and prop braking moment is said to be 0.

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19
Q

Brake moment when changing the blade angle.

A

A reduction in blade angle leads to a reduction of brake moment.

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20
Q

Reverse angle.

A

Pilot selectable feature.
Negative pitch angle.
Air is pushed forward, creating reverse thrust.
Used to stop in a landing roll, control taxiing, backing away from an obstacle.

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21
Q

Brake moment at reverse pitch.

A

Requires a corresponding increase in engine power bc brake moment increases.

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22
Q

Beta range

A

Used on the ground.
Maximum reverse to flight idle.
Throttle lever needs to be raised to enter beta range.

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23
Q

What does the lift on a prop depend on.

A

Aerofoil shape, rpm , AOA.

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24
Q

What is pitch distribution.

A

The blade angle becomes smaller the further away from the centre axis , to keep a nearly constant AOA.

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25
What is geometric twist.
Prop blade twisted from root to tip to maintain a nearly constant angle of pitch
26
What is blade washout.
Twisted blade from root to tip to maintain AOA.
27
What is geometric pitch.
Theoretical. It is as if a prop were to spiral through the air. How far it would have move forward in one rotation.
28
What is effective pitch.
The actual distance moved forward in one rotation.
29
What is propeller slip
It is the difference between geometric pitch and effective pitch.
30
What is a normal propeller refficeiency.
80 - 90 %
31
What is prop efficiency.
How well rotational force in converted into thrust.
32
What are the 5 static loads.
Centrifugal force , the greatest load felt. Thrust bending force , attempts to bend prop tips. Torque bending force , opposite to direction of rotation. Aerodynamic twisting moment ATM , turns blade to a course angle. Centrifugal twisting moment CTM , turns blade to a finer angle - CTMs are always greater than ATMs
33
Is damage felt at the blade roots.
No damage or repairs are allowed here.
34
Dynamic loads.
Vibration from gearing or piston engine, can be induced by aerodynamic forces. Greater vibration effect towards blade tip. Highest vibration at outer nodal point.
35
What is significant about the outer nodal point
It is 80% of the blade length and is where highest vibrational loads are felt.
36
Power absorption.
Ways of designing blades capable of absorbing higher power: - increase the blade angle - increase diameter of the prop disc ( longer blades ) - increase rpm of prop - increase camber of blade - increase chord. - increase number of blades
37
What is the best way of increasing power absorption capabilities. And why is it best.
Increasing number of blades as it increases solidity means more power is transferable to the air. Other ways have drawbacks.
38
Blade clearances. Fuselage Ground Water Nosewheel
1 inch from fuselage. 7 inch from ground with a nose wheel 9 inch from ground with a tail wheel 1/2 inch from prop to nosewheel 18 inch from prop to water.
39
Effective of engine and prop torque on the a/c
Creates a/c roll to one side due to torque effect in flight. Older a/c were rigged to make one wing produce more lift. Modern a/c offset the engine to counteract the torque effect. On the ground there is a yawing moment which is required to be controlled by the pilot by moving the rudder.
40
Twist effect of propeller wash
To compensate the vertical stab is off set by 1 or 2 degrees.
41
Gyroscopic effect
Prop acts as a gyro Yawing induces a pitching movement. Vice versa.
42
Asymmetric loading. (P factor)
During climb or descent the down going prop has a larger AOA than the upgoing prop, this results in a yawing moment. On twin engine a/c this is avoided by having the props spin in opposite directions. On twin engine a/c where they spin in the same direction the critical engine is the side where the downgoing prop is inboard.
43
Natural vibration
When a body oscillates under its own gravitational or elastic forces.
44
Resonant frequency
A natural frequency that occurs when it is struck.
45
Forced vibration.
When a vibrating body contacts another body
46
Causes of vibration
Aerodynamic or mechanical.
47
What does it mean f the prop only vibrates at one rpm range.
Means it is a poor engine-prop match.
48
Blade shake
When there is a certain amount of movement in the blades. It is by design.
49
Wobble
Spinner out f balance.
50
How to decide if the prop is vibrating or the engine.
1200-1500 rpm Observe prop hub and dome
51
Prop noise
Tonal = caused by displacement of air Broadband = airflow over the prop.
52
Rotation noise
Tips of blades.
53
Vortex noise.
Vortices leaving Trainline edge and tips.
54
Displacement noise
Displacement of the air.
55
Blade vibration noise.
Noise occurs with periodic stalls.
56
Power, on noise level
Double power = + 5dB
57
Prop diameter, on noise level
Double prop diameter at same tip speed = -6dB
58
Effect of Number of prop blades on noise level
Increases by 1.1dB
59
Blade shape with least noise
Scimitar
60
Blade shape with most noise
Straight tips.
61
Blade material with least noise
Wood or composite.
62
Beat frequencies
Twin engines.
63
Constructive beat frequencies.
Waves overlap creating larger wave.
64
Destructive interference beat frequencies
Waves overlap in a ways that cancel each other out.
65
Blade position phasing.
A potential cause for vibration and noise. Multi engine - props close together + + = bad + x = good Blades tips pass close together creating a pulse.

M17 (6 decks)

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