Ch 9 - 3D Flow

Question 1

Difference between 2D and 3D Flow

Answer 1

3D flow has spanwise flow

Question 2

The two main factors that Influence Flow Direction (spanwise flow)

Answer 2

Inertia of the mass of the air in the flow

Pressure differential (the driving force) of the phenomenon

Question 3

Wing Tip Vortices

Answer 3

Caused by the spa wise flow (pressure differential and inertia of mass flow)

Vortices go down and out (unless there is a crosswind small in which case it stays still - Very dangerous) A large crosswind dissipates it

Question 4

Formation of Wing Tip Vortices

Answer 4

Inertia (high speed= high inertia)
The size of the driving force (pressure differential)
- Time the driving force has to act on the air mass

High speed = relatively small wing tip vortices ( time decreased)

Low speed = worse when slow as the wing is being worked hard

The longer the chord, the greater the driving force for the vortex
(tip chord)

Rectangular wings have larger vortices
Tapered wings have smaller vortices

Question 5

Vortex Intensity Reduced by:

Answer 5

Reduced if:

• Aspect ratio increases
• Amount of lift produced decreases
• Aircraft speed increases
• Flaps decrease wing tip vortices as the pressure differential decreases at the tip

Question 6

Other Effects of Tip Vortices

Answer 6

2 Effects:

Downwash: Increased downwash = increased induced drag

Drag increases

The stronger the tip vortex, the greater the amount of drag and downwashh

Question 7

Trailing Edge Vortices

Answer 7

Caused by the change in angle of the airflow over the wings caused by the spanwise flow on the upper and lower surfaces.

Mix and cause trailing edge vortices

Question 8

Vortices and Downwash

Answer 8

Vortices suppress upwash and increase downwash

Question 9

Downwash Sheet

Answer 9

Rectangular wing has increased vortices which mean increased downwash (towards the tip especially) because of the stronger vortices combining with the wingtip vortices (and trailing edge)

Question 10

The Effective Airflow - Lift and Induced Drag

Answer 10

EAF - Effective airflow, induced by downwash.

• Result of RAF being affected by downwash
• Airflow inclined down to the wing which reduces AoA
• Lift then becomes 90’ (perpendicular) to the EAF

Induced drag - depends on the lift

Slower = induced drag increased due to the increased AoA
= Increased vortices = Increased downwash = Increased induced AoA = Lift induced drag

Question 11

AoA Definitions

Answer 11

2D -> AoA is the angle between the chord line and the RAF

3D-> AoA is the angle between the aircraft’s longitudinal axis and the RAF

Question 12

Induced AoA

Answer 12

Angle between the EAF and the RAF

Larger when there is a lower TAS and when the vortices are stronger and producing a greater downwash

Effective AoA - Angle between the EAF and the section chord line of the wing

• Determines the pressure distribution and the production of dynamic force
• May vary along the wing due to vortices and washout

Question 13

AoA

Answer 13

Is the sum of the induced AoA and the effective AoA

Question 14

Change with Effective AoA with Span

Answer 14

Tip:

• Greater induced drag than the root
• Greater down was
• More vortices
• Less lift produced

Root:
- Works harder as it has a larger AoA - smaller induced AoA
- Less downwash
- Less Vortices
More lift = Larger AoA
Root will stall first. (For a rectangular wing)

Question 15

Induced Drag

Answer 15

A result of Dow was generated by tip vortices

Question 16

Induced angle of attack

Answer 16

A result of downwash due to tip vortices