MODULE 6: DRAG MEASUREMENTS

Question 1

4 forces of flight:

Answer 1

• Lift
• Weight
• Drag
• Thrust

Question 2

As an aircraft moves/travels through air, every part of it that is exposed to the airflow produces a different type of resistance to the forward motion which constitutes the Total Drag.

Answer 2

Drag

Question 3

Forms of drag:

Answer 3

• Parasite drag
• Induced drag

Question 4

This is comprised of all the forces that work to slow an aircraft’s movement which includes resistance of air moving over the aircraft’s surface.

Answer 4

Parasite drag

Question 5

This is an inevitable consequence of lift and is produced by the passage of an airfoil through the air.

Answer 5

Induced drag

Question 6

Factors that affect the type of flow in the boundary layer:

Answer 6

• Shape
• Texture
• Flow quality
• Pressure gradient
• Reynolds number
• Heating of the fluid

Question 7

This type of drag is the aerodynamic resistance due to the contact of moving air with the surface of an object, and in this particular case that object would be an aircraft. Every surface, no matter how apparently smooth, has a rough, ragged surface when observed under a microscope.

Answer 7

Skin friction drag

Question 8

A _________ cannot exist when pressure is rising in the direction of the flow.

Answer 8

Laminar flow

Question 9

On a curved surface, such as an airfoil, the transition point is usually __________.

Answer 9

At or near to the point of the maximum thickness

Question 10

Because of the adverse pressure gradient existing on a curved surface the transition point will be ________ than if the surface was flat.

Answer 10

Further forward

Question 11

This is generated due to the aircraft’s shape and the airflow around it. When the air has to separate to move around a moving aircraft and its components, it eventually rejoins after passing the body. How quickly and smoothly it rejoins is representative of the resistance it creates, which creates additional force to overcome.

Answer 11

Form drag

Question 12

To reduce form drag, it is necessary to ________.

Answer 12

Delay the separation as close to the trailing edge as possible (Streamlining)

Question 12

________ increases the ratio between the length and depth of the body, reducing the curvature of the surfaces and thus the adverse pressure gradient.

Answer 12

Streamlining

Question 13

________ is the measure of streamlining.

Answer 13

Fitness ratio

Question 14

An example of this would be the intersection created by the connection of the wing and the fuselage at the wing root.

Answer 14

Interference drag

Question 14

The most interference drag is observed when_______. _______ are used to reduce this tendency.

Answer 14

Two surfaces meet at perpendicular angles

Question 15

Factors affecting parasite drag:

Answer 15

• Indicated air speed
• Configuration
• Airframe contamination

Question 15

Formula of parasite drag:

Answer 15

Dp = qSCdp

Question 16

Parasite drag varies with the ________.

Answer 16

Square of the IAS

Question 17

What is the relationship of the variation of parasite drag to the parasite area?

Answer 17

Directly proportional

Question 18

Contamination by these will increase the parasite drag coefficient.

Answer 18

By ice, frost, snow, mud, or slush

Question 19

________ is an inherent component, a rather undesirable by-product of lift.

Answer 19

Induced drag

Question 20

_______ modify upwash and downwash in the vicinity of the wing which produces a rearward component to the lift vector known as the “________”.

Answer 20

Wing tip vortices; Induced drag

Question 21

Wingtip vortices induce downwash, which tilts the lift vector ________, creating induced drag.

Answer 21

Backward

Question 22

Wing tip vortices create certain ________ in the airflow in the vicinity of the wing, both in front of and behind it. These strengthen upwash and downwash which reduces the _______.

Answer 22

Vertical velocity components; Effective angle of attack

Question 23

What is the relationship between the strength of the vortices and the effective angle of attack?

Answer 23

Inverse relationship

Question 24

_______ must be equal to weight in flight so induced drag will depend on the weight of the aircraft.

Answer 24

Lift

Question 25

Factors affecting induced drag:

Answer 25

- Size of the lift force - Speed of the aircraft - AR of the wing

Question 26

Induced drag will be _______ at higher aircraft weights.

Answer 26

Greater

Question 27

Certain maneuvers require the lift force to be greater than the aircraft weight. This relationship of lift to weight ratio is also known as the _______.

Answer 27

Load factor

Question 28

Formula of coefficient of induced drag:

Answer 28

Cdi = CL^2/piARe

Question 29

What is the relationship of the load factor to the induced drag?

Answer 29

Directly proportional

Question 30

What is the relationship between the induced drag and the speed of the aircraft? How does increased speed mean less induced drag?

Answer 30

Inverse relationship; less downwash = less lift inclination = less induced drag

Question 31

What is the relationship between the AR of the wing and the induced drag? How does a higher AR decrease the induced drag?

Answer 31

Inverse relationship; higher AR = longer span = less wing vortices effect; less overall change in downwash = smaller rearward lift tilt = less induced drag

Question 32

Factors to consider if one decides to continue the increase of AR:

Answer 32

- Excessive wing bending moments - Reduced rate of roll - Reduced ground clearance in roll during takeoff and landing.

Question 32

How to reduce excessive wing bending moments?

Answer 32

By carrying fuel in the wings and mounting the engines in pods beneath the wing

Question 33

Reduced rate of roll is caused by the down-going wing experiencing an _______.

Answer 33

Increased angle of attack

Question 34

These are small vertical aerofoils which form part of the wing tip, shaped and angled to the induced flow, and generate a small forward force.

Answer 34

Winglets

Question 35

This is placed at the wing tip to restrict the tip vortices and have a similar effect to an increased aspect ratio, but without the extra bending loads.

Answer 35

Wing-end plates

Question 36

This will have a similar beneficial effect to an end plate, which is to reduce induced drag.

Answer 36

Tip tanks

Question 37

These partly block the air flowing from the bottom to the top surface of the wing, reducing the strength of the tip vortices.

Answer 37

Winglets

Question 38

This can affect the strength of the tip vortices, and designs such as turned down or turned up wing tips have been used to reduce induced drag.

Answer 38

Wing-tip shape

Question 39

The _______ of an aircraft in flight is the sum of its induced drag and parasite drag.

Answer 39

Total drag

Question 39

Formula of total drag:

Answer 39

D = qsCd

Question 40

The _______ is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and flow conditions on aircraft drag.

Answer 40

Drag coefficient

Question 41

Formula of total drag coefficient:

Answer 41

Cd = Cdo + Cdi

Question 42

What is the effect of the surface roughness to the turbulent flow and the laminar flow?

Answer 42

Surface roughness increases the skin friction drag. The surface from the turbulent flow is rougher than that of the laminar, hence, it gives a greater skin friction drag than that of the laminar.