Subsonic Airflow

Question 1

Angle of Incidence

Answer 1

Angle of Incidence: The angle between the wing root chord line and the longitudinal axis of the aircraft. (This angle is fixed for the wing, but may be variable for the tailplane).

Question 2

Mean Line or Camber Line

Answer 2

Mean Line or Camber Line: A line joining the leading and trailing edges of an aerofoil, equidistant from the upper and lower surfaces.

Question 3

Maximum Camber

Answer 3

Maximum Camber: The maximum distance of the mean line from the chord line. Maximum camber is expressed as a percentage of the chord, with its location as a percentages of the chord
aft of the leading edge.

Question 4

A symmetrical aerofoil has ____________ because the _______________________.

Answer 4

A symmetrical aerofoil has no camber because the chord line and camber line are coincidental.

Question 5

Relative Air Flow (Relative Wind or Free Stream Flow): Relative Air Flow has three qualities.
If air flow does not possess all three of these qualities, _______________________.

Answer 5

Relative Air Flow (Relative Wind or Free Stream Flow): Relative Air Flow has three qualities.
- DIRECTION - air parallel to, and in the opposite direction to the flight path of the aircraft, in fact the path of the CG; the direction in which the aircraft is pointing is irrelevant.
- CONDITION - air close to, but unaffected by the presence of the aircraft; its pressure, temperature and velocity are not affected by the passage of the aircraft through it.
- MAGNITUDE - The magnitude of the Relative Air Flow is the TAS.
If air flow does not possess all three of these qualities, it is referred to as EFFECTIVE AIRFLOW.

Question 6

Total Reaction, Centre of Pressure (CP) and Lift

Answer 6

Total Reaction: The resultant of all the aerodynamic forces acting on the aerofoil section.
Centre of Pressure (CP): The point on the chord line, through which Lift is considered to act.
Lift: The aerodynamic force which acts at 90° to the Relative Air Flow.

Question 7

Drag, Angle of Attack , EFFECTIVE ANGLE OF ATTACK.

Answer 7

->Drag: The aerodynamic force which acts parallel to and in the same direction as the Relative Air Flow (or opposite to the aircraft flight path).
- Angle of Attack (α or alpha) (can also be referred to as Aerodynamic Incidence) The angle between the chord line and the Relative Air Flow.
- The angle between the chord line and the effective airflow is referred to as the EFFECTIVE ANGLE OF ATTACK.

Question 8

Three dimensional airflow, Two dimensional airflow

Answer 8

Three dimensional airflow: Three dimensional flow is the true airflow over an aircraft and
consists of a hypothetical two dimensional flow modified by various pressure differentials.
Two dimensional airflow: Assumes a wing with the same aerofoil section along the entire span
with no spanwise pressure differential or flow.

Question 9

The greatest positive pressure occurs at the _______________. This stagnation point is located somewhere near the leading edge. As the angle of attack increases from -4° the leading edge stagnation point moves from ______________ It is at the front stagnation point where the flow divides to pass over and under the section. The pressure at the stagnation point is ___________.

Answer 9

The greatest positive pressure occurs at the stagnation point where the relative flow velocity is
zero. This stagnation point is located somewhere near the leading edge. 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. It is at the front stagnation point where the flow divides to
pass over and under the section. The pressure at the stagnation point is Static + Dynamic.

Question 10

Form (pressure) drag

Answer 10

The pressure differential between the leading edge stagnation point and the lower pressure at
the trailing edge creates a force acting backward which is called ‘form (pressure) drag

Question 11

The reduced pressure ‘peak’ moves _______ as the angle of attack increases.
The greatest contribution to overall lift comes from the ________ surface.

Answer 11

The reduced pressure ‘peak’ moves forward as the angle of attack increases.
The greatest contribution to overall lift comes from the upper surface.

Question 12

Favourable gradient
Adverse pressure gradient

Answer 12

A favourable gradient is when air pressure is falling in the direction of airflow.
An adverse pressure gradient is when air pressure is rising in the direction of airflow, such as between the point of minimum pressure on the top surface and the trailing edge.

Question 13

The location of the CP is a function of _______________.

What is the Centre of Pressure.

Answer 13

The location of the CP is a function of camber and section lift coefficient.i.e. angle of attack.
The whole surface of the aerofoil contributes to lift, but the point along the chord where the distributed lift is effectively concentrated is termed the Centre of Pressure.

Question 14

What happens to the CP movement with increase in AOA ? When is the CP location at its most forward position ?

Answer 14

The CP moves forward and the magnitude of the lift force increases with increase in angle of attack until the stall is reached when the lift force decreases abruptly and the CP generally moves back along the chord.
Note that the CP is at its most forward location just before the stall (C L MAX)

Question 15

An aerodynamic force coefficient is a _________ ratio between the _________________

Answer 15

An aerodynamic force coefficient is a dimensionless ratio between the average aerodynamic pressure and the airstream dynamic pressure.

Question 16

Lift coefficient (C L ) is the ratio between _______________ and a drag coefficient (C D) is the ratio between _______________________________.

Answer 16

Lift coefficient (C L ) is the ratio between lift divided by the wing planform area and dynamic pressure and a drag coefficient (C D) is the ratio between drag divided by the wing planform area and dynamic pressure.

Question 17

The use of the coefficient of an aerodynamic force is necessary since the force coefficient is:

Answer 17

The use of the coefficient of an aerodynamic force is necessary since the force coefficient is:
- An index of the aerodynamic force independent of area, density and velocity. It is derived from the relative pressure and velocity distribution.
- Influenced only by the shape of the surface and angle of attack since these factors determine the pressure distribution.

Question 18

Aerodynamic Centre (AC), How does the pitching moment remain constant about the AD centre ?

Answer 18

‘The point where all changes in the magnitude of the lift force effectively take place’, AND: ‘The point about which the pitching moment will remain constant at ‘normal’ angles of attack’. Since an increase in angle of attack will increase the lift force, but also move the CP towards the AC (shortening the lever arm), the moment about the AC remains the same at any angle of attack within the “normal” range.
- The aerodynamic centre (AC) is an aerodynamic reference point. The most direct application being to the longitudinal stability of an aircraft

Question 20

The Lift force is the result of the pressure differential between _____________; the greatest contribution to overall lift comes from ______________.

Answer 20

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.

Question 21

The centre of pressure (CP) of a cambered aerofoil moves _______________.
The (CP) of a symmetrical aerofoil ____________ under the influence of angle of attack

Answer 21

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

Question 22

Throughout the normal range of angles of attack the aerofoil nose down pitching moment ___________. The AC is located at the quarter chord position for subsonic flow _____________.

Answer 22

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 M0.4.

Question 23

The coefficient of lift (C L ) is the ratio between ____________ and ______________.

Answer 23

The coefficient of lift (C L ) is the ratio between lift per unit wing area and dynamic pressure.

Question 24

–The greatest positive pressure occurs at the ___________, where the relative flow velocity is _______.
–Form (pressure) drag is the result of _____________________
–An increase in dynamic pressure (IAS) will __________________.

Answer 24

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.

Question 25

With reference to aerofoil section terminology, which of the following statements is true: 1 The chord line is a line joining the centre of curvature of the leading edge to the centre of the trailing edge, equidistant from the top and bottom surface of the aerofoil. 2 The angle of incidence is the angle between the chord line and the horizontal datum of the aircraft. 3 The angle between the chord line and the relative airflow is called the aerodynamic incidence or angle of attack. 4 The thickness/chord ratio is the maximum thickness of the aerofoil as a percentage of the chord; the location of maximum thickness is measured as a percentage of the chord aft of the leading edge. a. 1, 2, 3 and 4 b. 1, 2 and 4 c. 2, 3 and 4 d. 2 and 4

Answer 25

C

Question 26

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

Answer 26

D

Question 27

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.

Answer 27

C

Question 28

On an aerofoil section, the force of lift acts perpendicular to, and the force of drag acts parallel to the: a. flightpath. b. longitudinal axis. c. chord line. d. aerofoil section upper surface.

Answer 28

A

Question 29

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.

Answer 29

C

Question 30

Why does increasing speed also increase lift? a. The increased impact of the relative wind on an aerofoil's lower surface creates a greater amount of air being deflected downward. b. The increased speed of the air passing over an aerofoil's upper surface decreases the static pressure, thus creating a greater pressure differential between the upper and lower surface. c. The increased velocity of the relative wind overcomes the increased drag. d. Increasing speed decreases drag.

Answer 30

B

Question 31

The point on an aerofoil section through which lift acts is the: a. midpoint of the chord. b. centre of gravity. c. centre of pressure. d. aerodynamic centre.

Answer 31

C

Question 32

The angle between the chord line of the aerofoil section and the longitudinal axis of the aircraft is known as: a. the angle of attack. b. the angle of incidence. c. dihedral. d. sweep back.

Answer 32

B

Question 33

The angle between the chord line of an aerofoil section and the relative wind is known as the angle of: a. incidence. b. lift. c. attack. d. sweepback

Answer 33

C

Question 34

A line drawn from the leading edge to the trailing edge of an aerofoil section and equidistant at all points from the upper and lower contours is called the: a. chord line. b. camber. c. mean camber line. d. longitudinal axis.

Answer 34

C

Question 35

At zero angle of attack, the pressure along the upper surface of a symmetrical aerofoil section would be: a. greater than atmospheric pressure. b. equal to atmospheric pressure. c. less than atmospheric pressure. d. non existent.

Answer 35

C

Question 36

The angle of attack of an aerofoil section directly controls: a. amount of airflow above and below the section. b. angle of incidence of the section. c. distribution of positive and negative pressure acting on the section. d. the angle relative to the horizontal datum

Answer 36

C

Question 37

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

Answer 37

B

Question 38

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.

Answer 38

B

Question 39

Which of the following statements is true: 1 Relative airflow, free stream flow, relative wind and aircraft flightpath are parallel. 2 Aircraft flightpath, relative airflow, relative wind and free stream flow are parallel, but the aircraft flightpath 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

Answer 39

D

Question 40

Which of the following statements is 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

Answer 40

C

Question 41

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.

Answer 41

A

Question 42

Considering an aerofoil section subject to a constant dynamic pressure, which of the following statements is correct: a. If the angle of attack is increased from 4° to 14° the pressure differential will not change but lift will be greater due to increased dynamic pressure acting on the lower surface. b. Up to about 16°, increasing the angle of attack will increase the pressure differential between the top and bottom surface of the aerofoil. c. Changing the angle of attack does not affect the pressure differential, only changes in dynamic pressure affect the pressure differential. d. Up to about 16°, increasing the angle of attack decreases the pressure differential between the top and bottom surface of the aerofoil section.

Answer 42

B

Question 43

When considering the effect of changing angle of attack on the pitching moment of an aerofoil, which of the following statements is correct: 1 At ‘normal’ angles of attack the pitching moment is nose up. 2 The pitching moment about the aerodynamic centre (AC. is constant at >normal= angles of attack. 3 The aerodynamic centre (AC. is located approximately at the 25% chord point. 4 The moment about the aerodynamic centre (AC. is a product of the distance between the aerodynamic centre (AC. and the centre of pressure (CP) and the magnitude of the lift force. a. 1, 2, 3 and 4 b. 4 only c. 3 and 4 d. 2, 3 and 4

Answer 43

D

Question 44

Ice contamination of the leading portion of the aerofoil has which of the following consequences: 1 The profile of the leading portion of the surface can be changed, preventing normal acceleration of the airflow and substantially reducing the magnitude of the lift force. 2 Form (pressure) drag will be increased because of the increased frontal area of the aerofoil section. 3 Loss of lift will have a greater effect than an increase in form (pressure) drag. 4 At ‘normal’ angles of attack lift can be lost entirely if enough ice accumulates. a. 1, 2, 3 and 4 b. 1, 3 and 4 c. 1, 2 and 3 d. 3 and 4

Answer 44

A