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M8.2 Flashcards

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1
Q

What is free stream flow?

A

When the particles of a fluid move in an orderly manner and maintain their relative positions in successive cross sections.

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2
Q

What is ‘laminar flow’?

A

Smooth regular airflow patterns around an object.

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3
Q

What is ‘turbulent flow’?

A

Turbulent flow occurs when air is disturbed and separates from the surface of a moving body, with the formation of eddies in its wake.

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4
Q

What does the continuity equation state?

A

The continuity equation states that the speed of the airflow is inversely proportional to the area of the cross section of the tube if density remains constant.

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5
Q

What is a “diffuser outlet”?

A

When the diameter of a tube increases and the speed decreases.

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6
Q

What is a “jet outlet”?

A

When the diameter decreases and the speed increases.

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7
Q

What does Bernoulli’s Principle state?

A

An increase in dynamic pressure will result in a decrease in static pressure, and vice versa, but total pressure will remain constant.

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8
Q

How is dynamic pressure calculated?

A

Total pressure - Static pressure

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9
Q

What is the point of stagnation?

A

The point at which the speed of the airflow falls to zero and the static pressure equals the total pressure.

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10
Q

What is the boundary layer?

A

It is the layer of fluid in the immediate area of a surface

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11
Q

What is no-slip condition?

A

When the fluid at the skin is brought to rest by friction and be considered stationary. The next molecular layer shears slightly, creating movement with respect to the first layer. Successive layers shears slightly progressively more until it reaches the velocity of the free stream.

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12
Q

What is the fixed boundary?

A

The part of the airflow where the closest layer has zero velocity.

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13
Q

What is the moving boundary?

A

The point at which the air has met free stream velocity.

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14
Q

What factors can make the boundary layer become thinner?

A

An increase in aircraft velocity, decrease in fluid viscosity or the fluid density is increased.

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15
Q

How does air pressure change along the top surface of an airfoil?

A

Air move from high pressure at the leading edge, to its lowest pressure at the thickest point on the airfoil, before returning to a higher pressure again at the trailing edge.

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16
Q

What could disrupt laminar flow over a wing?

A

Distortions in the surface, such as rivets or object protruding into the airflow.

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17
Q

Where on the wing is laminar flow more easily maintained, and why?

A

Laminar flow is more easily achieved near the leading edge of the wing due to a positive pressure gradient, which helps keep the boundary layer attached.

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18
Q

What kind of pressure gradient exists from the thickest point of the wing to the trailing edge, and how does it affect airflow?

A

A negative pressure gradient exists, making it harder for air to stay attached to the surface and increasing the risk of flow separation due to the air running out of energy to overcome this gradient.

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19
Q

What happens if the airflow lacks sufficient energy to stay attached to the wing’s surface?

A

The flow can separate, potentially reversing direction, leading to reduced lift and increased pressure drag.

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20
Q

What is the aerodynamic consequence of sustained flow separation on a wing?

A

The condition worsens, leading to a stall.

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21
Q

Why is turbulent flow more commonly observed than laminar flow in nature?

A

Turbulent flow is more natural and widespread, being seen in clouds,oceans and the surface of the sun.

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22
Q

Does turbulent flow still have zero velocity at the surface of a wing?

A

Yes

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23
Q

How does AOA affect laminar flow?

A

As the AOA increases, it becomes more difficult to maintain laminar flow, and the airflow tends to transition to turbulence.

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24
Q

What is relative airflow?

A

The direction of the airflow relative to the object moving through it.

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25
What is the transition point?
The point at which the airflows changes from laminar to turbulent flow.
26
How does an increase in AOA affect the transition point?
It will move the transition point forwards.
27
How does an increase in AOA affect the separation point?
It will move the separation point forwards.
28
What is flow separation?
When the airflow has too little kinetic energy to overcome the negative pressure gradient, resulting in a reversal of airflow, creating a recirculation region of turbulence and vortices.
29
What are the advantages of laminar flow compared to turbulent flow?
- Less surface friction, enabling lower fuel usage, giving greater range, higher top speed and greater glide ratio.
30
What are the disadvantages of laminar flow compared to turbulent flow?
- Difficult to achieve at high speeds and at high angles of attack. - Particularly difficult to achieve in the area from the thickest part of the wing to the trailing edge. - Creates sudden and unpredictable stalls with high amounts of pressure drag.
31
How can the risk of stalls caused by high AOA be mitigated on commercial aircraft?
- Stalling at wing roots first. - Deliberately causing turbulent flow over the rear part of the wing, trading an increase in skin friction for a more predictable and later separation.
32
What is upwash?
Upwash is the upward deflection of airflow just before it reaches the leading edge of an airfoil. - It is caused by the wing generating lift and altering the surrounding airflow - Upwash contribute to increasing the AOA of the relative airflow
33
What is downwash?
Downwash is the downward deflection of airflow behind an airfoil due to lift generation - It results from the airfoil accelerating air downward, per Newton’s 3rd Law. Downwash affects trailing edge vortices and contributes to induced drag.
34
What are vortices?
Vortices are circular patterns of rotating air formed behind a wing or control surface.They form when the high and low pressure from the bottom and top surfaces of the wing interact at the trailing edge or wing tip.
35
What is a chord line?
A straight line connecting the leading edge and trailing edge.
36
What is the chord?
The distance measured along the chord line.
37
What is the mean camber line?
A line drawn equidistant between the upper and lower surfaces of the profile.
38
What is camber?
The displacement of the mean camber line from the chord line.
39
What is maximum camber?
The greatest distance of the mean camber line from the chord line.
40
How is the location of maximum camber expressed?
Expressed as a percentage along the chord from the leading edge.
41
What is fineness ratio?
The maximum thickness or depth of a section.
42
How is fineness ratio expressed?
Its location is expressed as a percentage of the distance of the chord from the leading edge.
43
What is aspect ratio?
The ratio between length, average width and the average chord of a wing.
44
What is wash in?
Wash in refers to an angle of incidence that is greater towards the wing tip than the wing root.
45
What is wash out?
Wash out refers to an angle of incidence that is greater at the wing root than the wing tip.
46
Describe a geometrically twisted wing.
On a geometrically twisted wing, the camber of the profile is constant across the span of the wing, but the angle of incidence is greater at the root than at the tip. (Chord lines not parallel)
47
Describe an aerodynamically twisted wing.
On an aerodynamically twisted wing, the camber of the profile at the root is greater than the camber at the tip and the angle of incidence is constant across the wingspan. (Chord lines parallel).
48
What is mean aerodynamic chord(MAC)?
The MAC is the average chord length of a tapered or swept wing.
49
What is angle of attack?
The AOA is the difference between the chord line and the relative airflow.
50
What is the angle of incidence?
The angles between the chord line of the profile and the longitudinal axis of the aircraft.
51
What is the centre of pressure?
The point on the chord line where the total sum of a pressure field acts on a body. The total reaction force of a wing acts from the centre of pressure.
52
What are the 4 main types of wing shape?
- Elliptical - Rectangular - Tapered - Swept
53
Does positive sweep mean the wing is swept forwards or backwards?
Backwards
54
How does the sweep of a wing affect is flying characteristic?
The more swept-back a wing is, the more stable the roll of an aircraft is.
55
What is the critical Mach number?
The speed at which shock waves begin to appear. (Can appear only on certain parts of the wing)
56
What is as positive dihedral and what benefits does it have?
When the wingtip is higher than the wing root. This increases roll stability(lateral stability).
57
How does a dihedral wing increase roll stability?
More lift is produced by the down-going wing as it sideslips into the turn, as airflow along it will increase.
58
What are anhedral wings and what characteristic does it have?
When the wingtip is lower than the wing root. It is used on aircraft requiring high roll performance. The downward slope of the wing helps prevent stalls at high AOA.
59
What are the 3 types of drag?
- Induced drag - Parasitic drag - Compressible drag
60
What is induced drag?
The drag on the wing caused by lift
61
What is parasitic drag?
Not lift related (form drag, skin drag(friction), or interference drag.
62
What is compressible drag?
Caused when an aircraft is approaching the speed of sound.
63
What happens to induced drag at low speed?
In low speed flight there is a high AOA and a high lift coefficient, therefore there is a large pressure difference between lower and upper surfaces creating large wing tips vortices. Consequently there is high induced drag.
64
What happens to induced drag at high speeds?
In high speed flight there is low AOA and a low lift coefficient, therefore meaning that there is a lower pressure difference and smaller wing tip vortices.
65
What is form drag?
Form drag is a parasitic drag caused by the pressure distribution on a body. A profile with low form drag will have high friction drag, and vice versa.
66
What is interference drag?
Interference Drag is the turbulence in the airflow caused by sharp corner where components are joined or close to each other. It can be reduced by fairing, such as those found on pylons.
67
What is compressible drag?
Compressible Drag is the result of the adverse pressure gradient which becomes particularly pronounced when approaching and in supersonic flight, due to the thickening of the boundary layer.
68
What is the lift-to-drag ratio?
The ratio of forces that make up the resultant force of a wing. Changes as AOA changes and speed.
69
What is the generation of lift and drag affected by?
- The AOA; as it increases lift will increase up until the critical AOA (usually around 15 degrees) - Airfoil shape; eg more heavily cambered airfoils will generate more lift at the cost of more drag. - Airspeed; drag increases to the square of airspeed.
70
What does the coefficient of lift assume?
That the surface is clean and free from contamination.
71
What are the 4 most common examples of aerofoil contamination?
Snow, ice, dirt, and bird dropping.
72
How does the build-up of ice affect an aerofoil?
It can add extra weight and drag, causing a loss of lift and possibly freezing or unbalancing control surfaces.
73
What are the 3 main types of ice accretion?
- Frost (sometimes knows as Hoarfrost) - Rime Ice - Clear Ice
74
When and where will frost form?
Frost will form in clear air when and aircraft has been parked for a prolonged period (ie overnight) in below zero temperatures. Forms on the cold surfaces of the aircraft.
75
When and where will rime ice form?
Rime Ice is formed when small supercooled water droplets freeze on contact with a surface that is below freezing temperature. It forms on aerofoil and engine inlet leading edges, and thereby affect the aerodynamic characteristics of the wing and airflow into the engines.
76
When and where will clear ice form?
Clear Ice is a heavy coating of glassy ice which forms when flying in areas with a high concentration of large, supercooled water droplets. It is considered the most dangerous as it is heavy, dense tough, and difficult to see. It breaks away from surfaces in large lumps. The main danger of it include aerodynamic instability, unequal wing loading.

M8 (5 decks)

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