Aerofoils

Question 1

The term ‘coefficient’, when applied to lift and drag for a particular aerofoil is most closely related to
[a] angle of attack [b] camber
[c] speed [d] wing area

Answer 1

The coefficient indicates what proportion of the free airstream’s energy is going into
the production of lift or drag. A bigger coefficient means a bigger ‘slice’ of the available
energy is being tapped. It is the angle of attack that decides how big that ‘slice’ is.

Question 2

The angle of attack of an aerofoil is the angle at which the relative airflow approaches the
[a] top surface [b] bottom surface
[c] chord line [d] mean camber line

Answer 2

C
By definition

Question 3

The net effect of all forces acting on an aerofoil can be attributed to a single imaginary force called
[a] lift [b] the total reaction
[c] induced drag [d] total drag

Answer 3

B
The total reaction accounts for all aerodynamic forces acting on the aerofoil [or
aeroplane] by replacing them with a single force acting at a specified point, just as the
centre of gravity replaces all items of weight with a single force at a specified point.

Question 4

As the angle of attack of an aerofoil is increased from zero to the stalling angle at constant indicated air speed,
the magnitude of the force of lift
[a] increases then decreases [b] decreases continuously
[c] decreases then increases [d] increases continuously

Answer 4

D
The question specifies up to the stalling angle not beyond. Lift continues to increase
up to the stalling angle then begins to decrease after the stalling angle is exceeded.

Question 5

Any increase in the angle of attack beyond the stalling angle at constant indicated air speed, will cause
[a] less lift and more drag [b] more lift and less drag
[c] more lift and more drag [d] less lift and less drag

Answer 5

A
Lift decreases after the stalling angle is exceeded, but drag continues to increase
beyond the stalling angle.

Question 6

As the angle of attack of an aerofoil is increased beyond the stalling angle at constant indicated air speed, the
magnitude of the force of lift
[a] increases then decreases [b] decreases continuously
[c] decreases then increases [d] increases continuously

Answer 6

B
The stalling angle produces the maximum lift coefficient, so at constant IAS it produces
the maximum possible amount of lift. Further increases in angle of attack
produce a reduced lift coefficient and therefore less lift. Comment

Question 7

Laminar flow in the boundary layer tends to persist
[a] across most of the upper surface [b] across most of the lower surface
[c] while ever the aerofoil section is thickening [d] to the separation point

Answer 7

C
While the aerofoil section is thickening, the airflow is travelling ‘up hill’. The boundary
layer tends to remain laminar

Question 8

A laminar flow aerofoil has the point of maximum thickness
[a] well forward and produces less parasite drag
[b] near 50% of the chord and produces less induced drag
[c] well forward and produces less induced drag
[d] near 50% of the chord and produces less parasite drag

Answer 8

D
Turbulent flow in the boundary layer gives rise to more parasite drag than laminar
flow. Since laminar flow tends to persist while ever the aerofoil section is thickening,
moving the maximum thickness further back encourages a more extensive area of
laminar flow and therefore reduced parasite drag.

Question 9

An aerofoil is said to be at its stalling angle if any increase or decrease in angle of attack
[a] produces less lift and less drag [b] produces a lower lift/drag ratio
[c] produces less lift [d] produces more drag

Answer 9

C
The stalling angle produces the maximum lift coefficient. Any other angle of attackgreater
or smaller- produces a lower lift coefficient.

Question 10

As the angle of attack is increased from zero to the stalling angle, the centre of pressure of most aerofoils
[a] moves further forward
[b] moves further aft
[c] remains stationary
[d] moves forward, then aft

Answer 10

A
At higher angles of attack the front portion of the wing contributes more of the total
lift. The centre of pressure moves towards added lift just as the centre of gravity moves
towards added weight.

Question 11

Subsonic air flowing through the throat of a venturi suffers
[a] an increase in both pressure energy and kinetic energy
[b] an increase in pressure energy and a decrease in kinetic energy
[c] a decrease in pressure energy and an increase in kinetic energy
[d] a decrease in both pressure energy and kinetic energy

Answer 11

C
The total energy remains constant throughout. As the kinetic energy [speed] increases,
the pressure energy must decrease [see page 1.2.3].

Question 12

If angle of attack is increased from zero to beyond the stalling angle at constant indicated air speed, the magnitude
of the force of drag
[a] increases then decreases
[b] decreases continuously
[c] decreases then increases
[d] increases continuously

Answer 12

D
The force of drag increases as angle of attack is increased. The stalling angle has no
particular significance to the behaviour of drag-it simply continues to increase after the
stalling angle is exceeded.

Question 13

As the angle of attack of a cambered aerofoil is increased in flight towards the stalling angle, the effect on the
total reaction is that
[a] the point on the chord through which the total reaction acts moves aft
[b] the point on the chord through which the total reaction acts remains stationary
[c] the total reaction increases up to the best lift/drag ratio angle, then moves aft
[d] the magnitude of the total reaction continues to increase up to the stalling angle

Answer 13

D
As the angle of attack increases a greater and greater proportion of the energy of the
free airstream is absorbed. The total reaction continues to increase up to the stalling
angle.

Question 14

The value of the minimum drag coefficient of a particular aerofoil is a factor in determining
[a] the maximum range of the aeroplane [b] the minimum speed in level flight
[c] the stalling speed of the aeroplane [d] the maximum speed in level flight

Answer 14

D
Minimum drag coefficient is not minimum drag-it is minimum angle of attack [see
page 1.3.9]. To fly at the minimum angle of attack in level flight, you must fly at the
highest possible speed.

Question 15

As the angle of attack of an aerofoil is increased from about 4° to the stalling angle, at constant indicated air
speed,
[a] lift increases and drag increases [b] lift increases and drag decreases
[c] lift decreases and drag increases [d] lift decreases and drag decreases

Answer 15

A

Question 16

The angle of attack of an aerofoil is defined as the angle between
[a] the chord line and the longitudinal axis
[b] the aircraft’s nose and the natural horizon
[c] the chord line and the relative airflow
[d] the aircraft indicator and the horizon bar on the artificial horizon

Answer 16

C
By definition

Question 17

An angle of attack increase in straight and level flight is accompanied by
[a] an increase in the total reaction and a forward movement in the centre of pressure
[b] an increase in the total reaction and an aft movement in the centre of pressure
[c] a decrease in the total reaction and a forward movement in the centre of pressure
[d] a forward movement in the total reaction and no change in the centre of pressure

Answer 17

A

Question 18

As the angle of attack of an aerofoil is increased towards the stalling angle at constant IAS, the effect on the
coefficients of lift and drag is -
[a] the coefficient of lift increases and the coefficient of drag increases.
[b] the coefficient of lift decreases and the coefficient of drag decreases.
[c] the coefficient of lift increases and the coefficient of drag decreases.
[d] the coefficient of lift decreases and the coefficient of drag increases.

Answer 18

A
The coefficient of lift and drag is simply a measure of the ‘degree of success’ the
aerofoil is having in generating lift and drag from the total energy available in the free
airstream. As angle of attack increases, more lift and more drag are generated for a
given airspeed.