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Subsonic Airflow |
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Airflow Subsonic 4
Summary
Airflow pattern, and ultimately lift and drag, will depend upon:
•Angle of attack - airflow cross-sectional area change
•Aerofoil shape (thickness & camber) - airflow cross-sectional area change
•Air density - mass flow of air (decreases with increased altitude)
•Velocity - mass flow of air (changes with aircraft TAS)
The lift force is the result of the pressure differential between the top and bottom surfaces of an aerofoil; the greatest contribution to overall lift comes from the top surface.
Anything (ice in particular, but also frost, snow, dirt, dents and even water droplets) which changes the accurately manufactured profile of the leading portion of the upper surface can seriously disrupt airflow acceleration in that area, and hence the magnitude of the lift force will be affected.
An increase in dynamic pressure (IAS) will increase the lift force, and vice versa.
An increase in angle of attack will increase the lift force, and vice versa, (0° to 16°)
The centre of pressure (CP) of a cambered aerofoil moves forward as the angle of attack increases. The CP of a symmetrical aerofoil does not move under the influence of angle of attack (within the confines of ‘normal range’).
Throughout the normal range of angles of attack, the aerofoil nose-down pitching moment about the aerodynamic centre (AC) will remain constant. The AC is located at the quarter chord position for subsonic flow of less than M 0.4.
The coefficient of lift (CL ) is the ratio between lift per unit wing area and dynamic pressure.
As the angle of attack increases from -4°, the leading edge stagnation point moves from the upper surface around the leading edge to the lower surface.
The greatest positive pressure occurs at the leading edge stagnation point, where the relative flow velocity is zero.
Form (pressure) drag is the result of the pressure differential between the leading edge and trailing edge of the aerofoil.
An increase in dynamic pressure (IAS) will increase form drag, and vice versa.
The coefficient of drag (CD ) is the ratio between drag per unit wing area and dynamic pressure.
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1. |
With reference to aerofoil section terminology, which of the following statements |
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are true? |
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1. |
The chord line is a line joining the centre of curvature of the leading edge to |
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the centre of the trailing edge, equidistant from the top and bottom surface |
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of the aerofoil. |
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2. |
The angle of incidence is the angle between the chord line and the |
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horizontal datum of the aircraft. |
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3. |
The angle between the chord line and the relative airflow is called the |
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aerodynamic incidence or angle of attack. |
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4. |
The thickness/chord ratio is the maximum thickness of the aerofoil as a |
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percentage of the chord; the location of maximum thickness is measured as |
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a percentage of the chord aft of the leading edge. |
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a.1, 2, 3 and 4.
b.1, 2 and 4.
c.2, 3 and 4.
d.2 and 4.
2.The definition of lift is:
a.the aerodynamic force which acts perpendicular to the chord line of the aerofoil.
b.the aerodynamic force that results from the pressure differentials about an aerofoil.
c.the aerodynamic force which acts perpendicular to the upper surface of the aerofoil.
d.the aerodynamic force which acts at 90° to the relative airflow.
3.An aerofoil section is designed to produce lift resulting from a difference in the:
a.negative air pressure below and a vacuum above the surface.
b.vacuum below the surface and greater air pressure above the surface.
c.higher air pressure below the surface and lower air pressure above the surface.
d.higher air pressure at the leading edge than at the trailing edge.
4.On an aerofoil section, the force of lift acts perpendicular to, and the force of drag acts parallel to the:
a.flight path.
b.longitudinal axis.
c.chord line.
d.aerofoil section upper surface.
5.When the angle of attack of a symmetrical aerofoil is increased, the centre of pressure will:
a.have very limited movement.
b.move aft along the aerofoil surface.
c.remain unaffected.
d.move forward to the leading edge.
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6. |
Why does increasing speed also increase lift? |
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a. |
The increased impact of the relative wind on an aerofoil’s lower surface |
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creates a greater amount of air being deflected downward. |
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b. |
The increased speed of the air passing over an aerofoil’s upper surface |
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decreases the static pressure, thus creating a greater pressure differential |
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c. |
between the upper and lower surface. |
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The increased velocity of the relative wind overcomes the increased drag. |
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d. |
Increasing speed decreases drag. |
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7. |
The point on an aerofoil section through which lift acts is the: |
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a. |
midpoint of the chord. |
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b. |
centre of gravity. |
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c. |
centre of pressure. |
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d. |
aerodynamic centre. |
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8. |
The angle between the chord line of the aerofoil section and the longitudinal axis |
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of the aircraft is known as: |
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a. |
the angle of attack. |
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b. |
the angle of incidence. |
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c. |
dihedral. |
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d. |
sweepback. |
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9. |
The angle between the chord line of an aerofoil section and the relative wind is |
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known as the angle of: |
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a. |
incidence. |
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b. |
lift. |
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c. |
attack. |
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d. |
sweepback |
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10. |
A line drawn from the leading edge to the trailing edge of an aerofoil section and |
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equidistant at all points from the upper and lower contours is called the: |
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a. |
chord line. |
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b. |
camber. |
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c. |
mean camber line. |
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d. |
longitudinal axis. |
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11. |
At zero angle of attack, the pressure along the upper surface of a symmetrical |
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aerofoil section would be: |
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a. |
greater than atmospheric pressure. |
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b. |
equal to atmospheric pressure. |
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c. |
less than atmospheric pressure. |
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d. |
non existent. |
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12. |
The angle of attack of an aerofoil section directly controls: |
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a. |
the amount of airflow above and below the section. |
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b. |
the angle of incidence of the section. |
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c. |
the distribution of positive and negative pressure acting on the section. |
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d. |
the angle relative to the horizontal datum |
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Questions 4
13.When the angle of attack of a positively cambered aerofoil is increased, the centre of pressure will:
a.have very little movement.
b.move forward along the chord line.
c.remain unaffected.
d.move back along the chord.
14.The term “angle of attack’’ is defined as the angle:
a.formed by the longitudinal axis of the aeroplane and the chord line of the section.
b.between the section chord line and the relative wind.
c.between the aeroplane’s climb angle and the horizon.
d.formed by the leading edge of the section and the relative airflow.
15.Which of the following statements is true?
1.Relative airflow, free stream flow, relative wind and aircraft flight path are parallel.
2.Aircraft flight path, relative airflow, relative wind and free stream flow are parallel, but the aircraft flight path is opposite in direction.
3.The pressure, temperature and relative velocity of the free stream flow are unaffected by the presence of the aircraft.
4.The relative wind is produced by the aircraft moving through the air.
5.The direction of flight is parallel with and opposite to the relative airflow.
a.5 only.
b.3, 4 and 5.
c.1 and 2.
d.1, 2, 3, 4 and 5.
16.Which of the following statements are correct?
1.Maximum camber is the maximum distance between the top and bottom surface of an aerofoil section.
2.The thickness/chord ratio is expressed as a percentage of the chord.
3.It is easier for air to flow over a well-rounded leading edge radius than a sharp leading edge.
4.Two dimensional airflow assumes a wing with the same aerofoil section along its entire span, with no spanwise pressure differential.
5.Air flowing towards the lower pressure of the upper surface is called upwash.
a.1, 2, 3, 4 and 5.
b.2, 3 and 4.
c.2, 3, 4 and 5.
d.1 and 5.
17.When considering an aerofoil section at a constant angle of attack, which of the following statements is true?
a.If the static pressure on one side is reduced more than on the other side, a pressure differential will exist.
b.If dynamic pressure is increased, the pressure differential will decrease.
c.The pressure differential will increase if the dynamic pressure is decreased
d.Dynamic pressure and pressure differential are not related.
Questions 4
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18. |
Considering an aerofoil section subject to a constant dynamic pressure, which of |
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the following statements is correct? |
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a. |
If the angle of attack is increased from 4° to 14°, the pressure differential will |
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not change but lift will be greater due to increased dynamic pressure acting |
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on the lower surface. |
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b. |
Up to about 16°, increasing the angle of attack will increase the pressure |
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differential between the top and bottom surface of the aerofoil. |
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c. |
Changing the angle of attack does not affect the pressure differential, only |
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d. |
Up to about 16°, increasing the angle of attack decreases the pressure |
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changes in dynamic pressure affect the pressure differential. |
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differential between the top and bottom surface of the aerofoil section. |
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19. |
When considering the effect of changing angle of attack on the pitching moment of |
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an aerofoil, which of the following statements is correct? |
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1. |
At ‘normal’ angles of attack the pitching moment is nose-up. |
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2. |
The pitching moment about the aerodynamic centre (AC) is constant at |
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normal angles of attack. |
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3. |
The aerodynamic centre (AC) is located approximately at the 25% chord |
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point. |
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4. |
The moment about the aerodynamic centre (AC) is a product of the distance |
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between the aerodynamic centre (AC) and the centre of pressure (CP) and |
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the magnitude of the lift force. |
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a. |
1, 2, 3 and 4. |
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b. |
4 only. |
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c. |
3 and 4. |
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d. |
2, 3 and 4. |
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20. |
Ice contamination of the leading portion of the aerofoil has which of the following |
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consequences? |
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1. |
The profile of the leading portion of the surface can be changed, preventing |
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normal acceleration of the airflow and substantially reducing the |
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magnitude of the lift force. |
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2. |
Form (pressure) drag will be increased because of the increased frontal area |
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of the aerofoil section. |
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3. |
Loss of lift will have a greater effect than an increase in form (pressure) |
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drag. |
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4. |
At ‘normal’ angles of attack lift can be lost entirely if enough ice |
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accumulates. |
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a. |
1, 2, 3 and 4 |
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b. |
1, 3 and 4 |
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c. |
1, 2 and 3 |
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d. |
3 and 4 |
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Questions 4
Questions 4
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