Flight

Question 1

What is lift

Answer 1

the force that is perpendicular to the direction of fluid flow

Question 2

What happens when an air flow is superimposed with a rotating movement of air around a cylinder?

Answer 2

air will be induced to move with a no slip condition. you will create a region of high/low pressure.

Question 3

state the direction of the lift force for (air direction, rotation direction)

a. to the right, clockwise
b. to the right, anti-clockwise
c. to the left, clockwise
d. to the left, anti-clockwise

Answer 3

a. upwards
b. downwards
c. upwards
d. downwards

Question 4

Explain the magnus effect using Newton’s 3rd law

Answer 4

-air is directed downwards and since momentum needs to be conserved the cylinder is propelled upwards

Question 5

Explain the magnus effect using Bernoullis

Answer 5

you would impart a greater velocity on one end ( causing greater dynamic and lower static pressure) ; the end which has a slower velocity (have a smaller dynamic and greater static pressure). the pressure gradient (want to move from greater to lower pressure)

Question 6

How do aerofoils assist in flight

Answer 6

1. the air passing through the top surface travels faster than the air passing through the bottom surface
2. by Bernoulli the air on top has greater dynamic pressure than the air at the bottom ( so the air at the bottom has a higher static pressure and there is lower pressure below the wing) this results in lift

Question 7

How is the aerofoil related to a bound vortex

Answer 7

It has the same air flow patterns without actually rotating

Question 8

for a positive angle of attack must the relative wind be above or below the chord line

Answer 8

must be below the chord line for a positive angle of attack

Question 9

Draw the leading edge, chord, trailing edge and a positive angle of attack

Answer 9

L 16 slide 22

Question 10

what direction does the wing tilt the air at a positive angle of attack, what does this lead to?

Answer 10

air is tilted forwards and downwards which causes the wing to be pushed backwards (drag) and upwards ( lift)

Question 11

What are 4 forces acting on a flying animal

Answer 11

weight, lift

thrust, drag

Question 12

What happens when lift> weight
lift < weight
lift =weight

Answer 12

bird accelerates upwards, bird accelerates downwards, bird remains at height

Question 13

What happens when
thrust > drag
thrust < drag
thrust = drag

Answer 13

thrust > drag –>accelerates forward
thrust < drag –>deccelerates ( backward)
thrust = drag –>constant speed

Question 14

how is lift and drag affected by velocity

Answer 14

proportionate to v^2

Question 15

How would you normalize a lift force vs angle of attack graph

Answer 15

Plot coefficient of lift ( Cl = F_L / 1/2pv^2 * A )vs angle of attack

Question 16

How does the lift vs angle of attack graph differ between a symmetric and asymmetric aerofoil (DRAW IT OUT)

Answer 16

slide 11 L 17

Question 17

Why is the lift produced from a asymmetric aerofoil typically greater than that produced by a symmetric aerofoil

Answer 17

the air traveling at the top is moving at a greater speed than that at the bottom so the static pressure is even lower , greater pressure difference = more lift

Question 18

How is the optimal angle of attack chosen

Answer 18

It is the the angle that corresponds to the highest lift to drag ratio.

Question 19

What is a polar plot.

Answer 19

It is a lift vs drag graph that is used to find the optimum angle of attack ( go from y axis to the first pt that touches the curve)

Question 20

What are the factors that can be used to increase the lift

Answer 20

1. increase the velocity
2. increase the planform area of the wings ( but also increases drag)
3. increase the angle of attack

Question 21

What is stalling

Answer 21

It is the angle of attack where the lift force can no longer increase but starts to decrease, I think it’s cause you introduce an early boundary layer separation

Question 22

What is the lift force perpendicular to and the drag force parallel to

Answer 22

The lift and drag force are perpendicular and parallel to the RELATIVE WIND the object experiences

Question 23

Why would we change the pitch angle

Answer 23

To get a positive angle of attack, especially when we have the propeller moving forward and hits upon oncoming wind ( relative wind will be at an angle)

Question 24

What is a consequence when we have a large pitch angle

Answer 24

the torque opposing the blade motion increases

Question 25

What are the 3 main drag components on a flying animal

Answer 25

1. profile drag: skin friction and pressure drag on the lift producing areas
2. parasitic drag: skin friction on non-lift producing areas and pressure drag and interference drag
3. induced drag: drag due to lift

Question 26

How can we minimize parasitic drag

Answer 26

1. streamline the body

2. reduce any protruding surfaces

Question 27

Explain how induced drag occurs

Answer 27

1. increased downwash occurs behind the trailing edge of the wing due to wingtip vortices
   2/ downwash causes the relative wind to be pushed down at the trailing edge
2. the downwash tilts the effective relative wind downwards . the new effective relative wind is half-way between the angle of wind in the downwash and the oncoming relative wind
   4.lift remains perpendicular to the effective relative wind and is tipped backwards
3. effective lift now directs more force backwards = induced drag
   ** the induced downward tipping of the lift force = induced drag

Question 28

What are wingtip vortices

Answer 28

A vortex that develops at the wingtip due to the difference in pressure , with the top wing having low pressure and bottom wing having higher pressure.

Question 29

What is the aspect ratio

Answer 29

AR= width/height = span/chord = span^2/ planform area

- indicates how long and narrow or short and stubby

Question 30

How is aspect ratio related to induced drag

Answer 30

the smaller the aspect ratio the more it will suffer from induced drag ( by % of wing will increase for shorter AR)

Question 31

What are the advantages of slotted primary feathers

Answer 31

reduction of induced drag by breaking the wingtip vortices such that there would be less downwash and hence less tipping of the induced drag

Question 32

What is wing loading?

Answer 32

weight of animal / total wing area =wing loading

• amount of lift needed to be produced per unit area of wing
• low wing loading means you can fly at lower speeds

Question 33

Why does a swan with a high wing loading need more speed

Answer 33

because it has a high wing loading more lift force needs to be generated and we can generate more lift by increasing the speed