Chapter 4 Drag

Question 1

What is the drag formula?

Answer 1

Cd 1/2 p V^2 S

Question 2

From looking at the drag formula what can it tell us about what can increase an object drag?

Answer 2

• angle of attack
• shape of airfoil
• amount of kinetic energy

Question 3

Total drag is made out of:
1)
2)
3)

Answer 3

1) profile drag, 2) parasitic drag, 3) induced drag

Question 4

Where can you find parasitic drag?

Answer 4

On bits of a helicopter that doesn’t produce lift

Question 5

Why is it important to keep parasitic drag to a minimum?

Answer 5

Because it increases with velocity squared

Question 6

What is profile drag a combination of?

Answer 6

1- form drag. Formed in the frontal and rear areas of rotor blade

2- skin friction. The slow down of air close to the skin of the blade, called “the boundary layer”

Question 7

What is the boundary layer?

Answer 7

The very thin layer of air closest to the skin which is virtually motionless

Question 8

How many types of boundary layer are there and what are they called?

Answer 8

There are two types of boundary layer, the laminar type and turbulent type.

Question 9

Where can laminar type be found? Name a property of it.

Answer 9

The laminar type can only be found in a region of decreasing air pressure, meaning it will only be found at the leading edge of the airfoil.

This layer of air is very thin and extremely brittle which means that even small specks of dust or ice can cause a layer to fracture into turbulence or even separate from the surface

Question 10

What is the turbulent boundary layer made of and name 2 properties of it?

Answer 10

It consists of revolving and disturbed air molecules.

1- it is thicker and it produces more drag than laminar type
2- unlike laminar it sticks to the skin surface more despite specs of air or small obstruction

Question 11

What are the three main factors that determine the amount of skin friction made?

Answer 11

Surface roughness - the rougher of the airfoil, the thicker a boundary layer

Shape of the airfoil - the further back the point of maximum thickness, the less skin friction drag (ignoring airspeed)

airspeed - the higher the speed of Air past the blade, the greater the skin friction drag

Question 12

Explain what the transition point is?

Answer 12

The point where the lemonade boundary layer changes into the turbulent boundary layer. When the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, upper surface transition point moves forward. Hence skin friction drag increases as airspeed increases and vice versa.

Question 13

Explain what the separation point is?

Answer 13

Where turbulent boundary layer thickens and separates from the air foil and where wake commences.

Question 14

What is induced drag?

Answer 14

Induced drag occurs whenever a moving object such as an airfoil redirects the airflow coming at it

Question 15

Why do tip vortices occur?

Answer 15

Whenever an airfoilproduces lift, there will be air pressure differences above and below the airfoil.

Naturally the greater pressure gradient will try to force air from the high pressure region into the low pressure region, this happens around the blade tips creating spiral vortices

Question 16

Explain how due to vortex’s induced drag is increased?

Answer 16

When it starts and the higher pressured air moves into the lower pressure area (the upwards component of the vortex), it initially had no effect on the behaviour of the relative air flow onto the blade. But the downwards component which occurs behind the blade, increases downwash, which then increases the induced drag.

Question 17

What is a vortex?

Answer 17

It’s a region in a fluid in which the flow revolves around an axis line, which may be straight or curved.

Question 18

What is downwash?

Answer 18

change in direction of air deflected by the airfoil

Question 19

Tip vortices

When we create more lift created, what can we say happens to the vortex?

Answer 19

It too will increase.

the greater the angle of attack, the greater the lift coefficient, the stronger the effects of tip vortices and the greater the amount of induced drag

Question 20

What is aspect ratio?

Answer 20

It is the ratio of a blade span to it’s chord

Question 21

Is the size of a tip vortices the same in helicopters with different span?

Answer 21

Yes.

Question 22

Finish this sentence

The height of the aspect ratio of a blade…

Answer 22

The smaller the amount of induced drag it produces

Question 23

What are some ways to reduce induced drag?

Answer 23

Changing tip design (eg. Pointing up tip)

Adding a washout design

Question 24

What the washout design?

Answer 24

characteristic of wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing.

The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip.

Question 25

Define angle of incidence

Answer 25

angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis

Question 26

When the speed of a helicopter increases from 20 knots to 60 knots, parasitic drag increases by a factor of… (3/4/6/9)

Answer 26

Bottom of Page 28

A tripling of airspeed increases parasitic drag 9 times

Question 27

With an increase in air speed, skin friction drag (increases/decreases) because the transition point moves (forward/aft) and the boundary layer (thickens/thins)

Answer 27

Check to be sure

Increases

Forward

Thins

Question 28

With an increasing airspeed, induced drag (increases or decreases) and parasitic drag (increases or decreases)

Answer 28

You can check the graph

Induced drag - Decreases

Parasitic drag - Increases

Question 29

Draw an airfoil with all the types of boundary layers and transition points

Answer 29

Should have on the drawing and in correct place

• Relative airflow arrow
• laminar boundary layer
• transition point
• turbulent boundary area
• separation point
• wake