Aeroplanlære - Kap 4 (Drag)

Question 1

When moving through a fluid (Air) we encounter a resistance. What is this called?

Answer 1

Drag

Question 2

In two dimensional flow which 2 forms of drag do we have?

Answer 2

Skin friction drag
Form drag

Question 3

What causes form drag?

Answer 3

Caused by the difference in pressure in front and behind the aerofoil

Question 4

What causes skin friction drag?

Answer 4

Caused by shear forces in the boundary layer due to the air’s viscosity

Question 5

What are form drag and skin friction drag together known as?

Answer 5

Profile drag

Question 6

Does profile drag produce lift?

Answer 6

No

Question 7

What is the equation for form drag?

Answer 7

Question 8

What happens to the air molecules when they hit the aerofoil?

Answer 8

Question 9

What is the boundary layer?

Answer 9

The layer from the aerofoil extending out to 99% of the free stream flow

Question 10

Where does skin friction drag take place?

Answer 10

Within the boundary layer

Question 11

When is the most energy produced in this airflow?

Answer 11

When the airflow becomes turbulent

Question 12

Is the laminar boundary layer higher or lower than the turbulent boundary layer?

Answer 12

The laminar is lower than the turbulent boundary layer

Question 13

Does laminar flow produce more or less skin fraction than turbulent air?

Answer 13

The laminar flow produce less and possesses less kinetic energy than turbulent

Question 14

Does airflow speed increase or decrease when getting further from the surface in a laminar boundary layer?

Answer 14

It increases the further from the surface

Question 15

Does airflow speed increase or decrease when getting further from the surface in a turbulent boundary layer?

Answer 15

It increases the further from the surface but less than a laminar boundary layer

Question 16

In which way does turbulent airflow move?

Answer 16

It has flows in many directions

Question 17

Why do turbulent airflow in the turbulent boundary layer create more skin friction drag?

Answer 17

Question 18

What is the point where the laminar boundary layer switches too a turbulent boundary layer?

Answer 18

Transition point

Question 19

What is the equation for reynolds number?

Answer 19

Question 20

What does Reynolds number tell us?

Answer 20

Question 21

Where is the lowest surface pressure located on an aerofoil?

Answer 21

Approximately at the point of maximum thickness

Question 22

From the leading edge too the lowest surface pressure, which way is the pressure gradient?

Answer 22

The same way as the direction of the flow

Question 23

What is the adverse pressure gradient?

Answer 23

When the high pressure from under the wing seeks towards the top of the wing from trailing edge towards leading edge

Question 24

What is the point where turbulent airflow separates from the aerofoil?

Answer 24

Separation point
* Caused by low speed and the adverse pressure gradient

Question 25

Will profile drag occur if there is no lift being generated

Answer 25

Yes

Question 26

What causes induced drag to be produced?

Answer 26

It is caused by the vortices produced whenever lift is produced

Question 27

When is interference drag produced?

Answer 27

Question 28

Which categories of aircraft drag is there and how are they connected?

Answer 28

Question 29

Which factors affect form drag?

Answer 29

Question 30

Which factors affect skin friction drag?

Answer 30

Question 31

Which factors affect interference drag?

Answer 31

When the airflow across one component of an aircraft is forced to mix with the airflow across an adjacent or proximal component

Question 32

Which form of drag increases when airspeed increase?

Answer 32

Parasite drag

Question 33

When using flaps and gears which form of drag increase?

Answer 33

Parasite drag (Greater frontal area)

Question 34

What is the equation for parasite drag?

Answer 34

Question 35

What causes changes in the coefficient of parasite drag?

Answer 35

Question 36

Which form of drag decreases when airspeed increase?

Answer 36

Induced drag

Question 37

Does induced drag increase or decrease with higher weight?

Answer 37

Increase

Question 38

Does a glider have a higher induced drag than short winged aircraft?

Answer 38

No since induced drag is inversely proportional with aspect ratio and a glider has a high wing aspect ratio, therefore less vortices

Question 39

What is the equation for induced drag?

Answer 39

Question 40

What is downwash and the coefficient for induced drag proportional and inversely proportional too?

Answer 40

Question 41

An aircraft is flying straight and level and doubles it's airspeed. What happens too lift and induced drag too keep flying straight and level?

Answer 41

Lift must be reduced to 1/4 Induced drag is equal to lift^2 so that must be 1/4 x 1/4 = 1/16

Question 42

What is total drag?

Answer 42

The sum of parasite drag and induced drag

Question 43

What is Vmd?

Answer 43

Velocity minimum drag

Question 44

When is speed stable and unstable?

Answer 44

Unstable behind velocity minimum drag and stable above

Question 45

What happens to the drag curve when increasing altitude?

Answer 45

It moves right and thereby Vmd is higher

Question 46

What happens to the drag curve when increasing weight?

Answer 46

It moves right and up and thereby Vmd is higher

Question 47

What happens to the drag curve when changing the configuration to a dirty configuration?

Answer 47

It moves left and up and thereby Vmd is lower

Question 48

What is the equation for total drag?

Answer 48

Question 49

How does the coefficient for parasite drag change with the coefficient for lift?

Answer 49

It has a constant stable drag until Clmax but with a slight increase at the end

Question 50

How does the coefficient for induced drag change with the coefficient for lift?

Answer 50

Lift and drag increase slightly until Clmax and then lift decrease after

Question 51

Explain where the different drag coefficients are seen on the drag polar

Answer 51

The coefficient for parasite drag is seen first thereafter it's a combination

Question 52

How do you find the greatest ratio between the coefficient for drag and the coefficient for lift on the total drag curve?

Answer 52

By drawing a tangent line from zero to the graph

Question 53

What is not shown on the lift drag ratio graph?

Answer 53

Blue line = The coefficient for drag Light blue line = The coefficient for lift Orange line = The coefficient for lift / The coefficient for drag X axis = Angle of attack