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Aerodynamics > Aerofoil Design Concepts > Flashcards

Aerofoil Design Concepts Flashcards

1
Q

What is the chord line?

A

A straight line connecting the leading edge and the trailing edge of the aerofoil

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2
Q

What is the chord?

A

The length of the chord line.

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3
Q

What is the Mean chord/Mean Aerodynamic Chord (MAC)?

A
  • The chord averaged from the root to the wingtip.

- For a swept wing, this is measured along a line parallel to the fuselage reference line.

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4
Q

What is the thickness/chord ratio?

A
  • The thickness section of the wing, expressed as a percentage of the chord length.
  • A typical training aircraft wing has a thickness/chord ratio of around 12%.
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5
Q

What is the leading edge radius?

A

A measure of the sharpness of the leading edge as a percentage ratio of the chord.

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6
Q

What are some leading edge devices?

A

Slats and Kruger flaps which change the leading edge radius to convert a high speed aerofoil into a high lift aerofoil.

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7
Q

What is the mean camber line?

A

The line formed by a point halfway between the upper surface and lower aerofoil surface, joining the two ends of the chord line.

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8
Q

What is the aerofoil camber?

A

The distance of the mean camber line from the chord line.

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9
Q

What is positive camber? What about negative?

A
  • If the mean camber line is above the chord line

- If the mean camber line is below the chord line, it has negative camber.

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10
Q

What is maximum camber?

A

The maximum distance between the chord line and the mean camber line.

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11
Q

What is the free stream flow?

A

The air is the region where pressure, temperature and relative velocity are unaffected by the passage of the aerofoil or aircraft through it.

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12
Q

What is the relative airflow (RAF)?

A

The airflow relative to the A/C that is not affected or influenced by the passage of an aerofoil or aircraft.

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13
Q

What is the local RAF?

A

Changed/unchanged relative airflow caused by the downwash behind the wing.

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14
Q

What is upwash?

A

The air approaching the leading edge that starts to flow upwards ahead of the wing due to the area of low pressure that exists above an aerofoil.

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15
Q

What are tip vortices/spanwise flow?

A
  • Spanwise flow is air moving towards the wingtip on the bottom surface and moving towards the root on the top surface.
  • This creates vortices at the trailing edge of the wing, which are greatest at the wingtip.
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16
Q

What determines the strength of the vortices?

A

The pressure differential above and below the aerofoil.

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17
Q

What are sharp leading edges/Rams horn vortices?

A

Purposefully induced vortices that re-energise the flow and keep it attached to the wing for longer.

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18
Q

What is lift and where does it act?

A
  • The net force produced by an aerofoil
  • Acts through the CoP
  • Always perpendicular to the RAF.
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19
Q

What is drag?

A
  • The net force incurred by an aerofoil

- Parallel to and in the same direction as the RAF

20
Q

What is the Total Reaction (TR)?

A
  • The resultant of the Lift and Drag forces on the aerofoil.
  • This is the direction the wing would actually move when subjected to both lift and drag.
21
Q

What is the Lift/Drag Ratio?

A
  • When producing lift, an aerofoil will also produce drag.

- This is due to wasted energy caused by the creation of vortices.

22
Q

What is the angle of attack (AoA)?

A
  • The angle between the aerofoil chord line and the RAF
23
Q

What is the critical angle?

A

The AoA beyond which the airflow separates from the top of the wing surface, causing a sudden large decrease in lift.

24
Q

Is the best lift/drag ratio dependent on speed or angle of attack?

A

Angle of attack.

25
Q

For most general purpose aerofoils, what is the best lift to drag ratio achieved at?

A

4 deg AoA

26
Q

What is the Lift formula?

A

Lift = Cl x 1/2 x rho x V^2 x S

27
Q

What is re-circulation theory?

A
  • Proposes that there is a large displacement of air ahead and especially behind the wing, resulting in a re-circulation pattern around the wing.
  • Due to Newton’s 3rd law, the equal and opposite reaction to this results in the lifting force upon the wing.
28
Q

How many stagnation points are there?

A
  • 2
  • 1 at leading edge
  • 1 at trailing edge
29
Q

Is the static pressure at stagnation points higher than free stream static pressure?

A

Yes

30
Q

Where does the lift force act through?

A

Centre of Pressure

31
Q

As the AoA is increased, where does the CoP go?

A

Towards the leading edge of the airfoil.

32
Q

What occurs as the airflow passes the point of maximum thickness ?

A

It starts to flow down, pressure increases as the velocity decreases.

33
Q

What is the cause of the trailing edge stagnation point?

A
  • Colliding airflow from bottom and upper surfaces causes an area of high pressure called the trailing edge stagnation point.
  • Same pressure as the leading edge stagnation point.
34
Q

What is the adverse pressure gradient?

A

Pressure increase once past the point of maximum camber will result in an adverse pressure gradient.

35
Q

Where does the adverse pressure gradient extend from and to?

A

Extends from the point of maximum camber up to the trailing edge stagnation point.

36
Q

Where is point of maximum thickness for laminar flow wings?

A

At 50% of the chord.

37
Q

What is the aspect ratio?

A

Ratio of the wingspan to chord (wingspan squared/S).

38
Q

What are low aspect ratio wings typically used on?

A

High speed aircraft such as fighter and aerobatic aircraft

39
Q

What are medium aspect ratio aircraft typically used on?

A

Advanced training aircraft

40
Q

What is true with high aspect ratio wings regarding spanwise flow?

A

Reduced spanwise flow.

41
Q

When does lift dependent drag become an issue?

A

At low speed flight.

42
Q

What does the downwash do behind the wing?

A
  • Reduces the local angle of attack on the wig and tilts the lift vector backwards.
  • Increased downwash means more lift dependent drag
43
Q

Will low aspect ratio wings or high aspect ratio wings stall at a greater angle of attack?

A

Low aspect ratio wings will stall at a higher angle of attack

44
Q

Which aspect ratio wing , low or high, has greater structural strength?

A

Low

45
Q

Why do low aspect ratio wings stall at a higher angle of attack?

A

Because of the greater vortices which will reduce the effective relative airflow and angle of attack, requiring a further pitch back to get stall.

Aerodynamics (13 decks)

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