Questions on Lift and Drag

Question 1

What happens to total drag when accelerating from CL MAX to maximum speed?

Answer 1

Decrease then Increase.

CLmax in level flight is highest at low speed. At low speed total drag is high. Thus accelerating to VMD the drag first decreases then accelerating to max speed it increases again.

Question 2

The maximum angle of attack for the flaps down configuration, compared to flaps up is “Smaller”

Answer 2

Flaps down (is like having a cambered wing section) gives higher CLmax values at a smaller angle of attack.

Question 3

Decreasing sweep angle decreases stall speed….

Answer 3

Since straight wings have higher CLMax.

Question 4

Drag acts in the same direction as the relative airflow and lift….

Answer 4

perpendicular to it.

Question 5

The Cl-alpha curve of a positive cambered aerofoil intersects with…

Answer 5

the vertical axis of the Cl-alpha graph “Above the origin”.

Question 6

If the weight of an aircraft is increased the maximum lift/drag ratio will….

Answer 6

“Not be affected”.

Question 7

Suction above the wing is the greatest factor causing…

Answer 7

lift.

Question 8

Lift is generated when a certain mass of air is accelerated….

Answer 8

downwards.

Question 9

Lift is generated when the flow direction of a certain mass of air is

Answer 9

changed.

Question 10

An aeroplane maintains straight and level flight while the IAS is doubled. The change in lift coefficient will be…

Answer 10

0.25

Question 11

The aerodynamic drag of a body, placed in a certain airstream depends amongst others on the…

Answer 11

airstream velocity.

Question 12

For a given angle of attack the lift/drag ratio is unaffected by…

Answer 12

density change.

Question 13

If the weight of the aircraft is increased, the maximum lift/drag ratio will remain the same but occur at….

Answer 13

a higher speed.

Question 14

What is the purpose of a slat on the leading edge?

Answer 14

It allows a greater angle of attack.

Question 15

What is true regarding deployment of Slats/Krueger flaps?

Answer 15

Slats form a slot, Krueger flaps do not.

Question 16

The aerodynamic centre is the point where all

Answer 16

changes in the magnitude of the lift force effectively take place.

Question 17

A body is placed in a certain airstream. The density of the airstream decreases to half of the original value

Answer 17

The aerodynamic drag will decrease with a factor of 2. Drag and density have a direct relation (Drag equation).

Question 18

When considering the aerodynamic forces acting on an aerofoil section, lift increases….

Answer 18

linearly and drag increases exponentially with an increase in angle of attack.

Question 19

For an aircraft flying at a constant IAS, the drag will be the same at altitude as

Answer 19

at sea level.

Question 20

For aircraft of the same weight, flying at the same IAS the angle of attack will be the same at altitude as at

Answer 20

sea level.

Question 21

To fly at a given IAS, the thrust required at altitude will be the same as at sea level, but will require

Answer 21

the throttle to be advanced.

Thrust required = Drag.

Drag = 1/2 rho V^2 S Cd

Fixed IAS = Fixed 1/2 rho V^2

Thus thrust remains unchanged.

Power = Rate of doing work (FxD/T)

F = Drag and D/T is Speed (TAS)

So power required = Drag x TAS

Less density, same IAS means increased TAS.

Therefore power required is increased.

Question 22

An aeroplane in straight and level flight is subjected to a strong vertical gust. The point on the wing, where the instantaneous variation in wing lift effectively acts is known as

Answer 22

the “Aerodynamic Center”.

Question 23

At an aeroplane minimum drag speed, the ratio between induced drag and profile drag is…

Answer 23

1/1

Question 24

Extending airbrakes during an approach will reduce

Answer 24

the minimum drag speed (VDmin)

Question 25

The effects of very heavy rain (tropical rain) on the aerodynamic characteristics of an aeroplane are....

Answer 25

decrease of CLmax and increase of drag.

Question 26

Total Drag at high Mach numbers is a combination of...

Answer 26

induced drag, wave drag, form drag, skin friction drag and interference drag.

Question 27

The value of the parasite drag in straight and level flight at constant weight varies...

Answer 27

linearly with the square of the speed.

Question 28

An aircraft is required to cruise, maintaining Vmd. As the weight decreases the IAS must be _______, and the angle of attack _______

Answer 28

decreased, remain constant.

Question 29

When an aircraft selects its undercarriage and flaps down in flight, its VIMD will _______ and if it maintained the clean configuration VIMD its speed stability would ________

Answer 29

(i) REDUCE, (ii) INCREASE.

Question 30

What must happen to the CL when flaps are deployed while maintaining a constant IAS in straight and level flight? It remains constant.

Answer 30

The wingtip vortices reduce. Wingtip vortices are proportianal to angle of attack. Since the angle of attack is reduced to keep CL constant after lowering flaps to maintain straight and level flight at the same speed.

Question 31

Flap selection at constant IAS in straight and level flight will ________ the maximum lift coefficient (CLmax) and the drag.

Answer 31

increase

Question 32

Reduction in CLMax is the most important result/problem caused by ...

Answer 32

ice formation.

Question 33

In the last part of the rotation (during take off) the aerodynamic effect of ice located on the wing leading edge is

Answer 33

most critical.

Question 34

At a ___ weight the maximum level flight speed will be ___ because of a change in ___ drag: a) lower/less/parasite b) lower/less/induced c) higher/less/induced d) higher/less/parasite

Answer 34

c) higher/less/induced

Question 35

For an aircraft flying at a speed above Vmd: a) a speed increase causes a drag increase which will cause a deceleration b) a speed increase causes a drag decrease causing further acceleration c) a speed increase causes a drag increase causing an acceleration d) a speed decrease causes a drag increase causing a deceleration

Answer 35

a) a speed increase causes a drag increase which will cause a deceleration

Question 36

In which situation would the wing lift of an aeroplane in straight and level flight have the highest value? (the engines are mounted below the wing).

Answer 36

Forward CG and Idle Thrust

Question 37

If the CG is ahead of the wing CP and there is no thrust/drag couple, for level flight, the wing lift must be ________

Answer 37

greater than the weight. With a forward CG there will be a down force at the horizontal stabilizer. Total lift will be weight + tail down force. In that case lift will be more than the weight.