Lift Augmentation

Question 1

Purpose of lift augmentation devices

Answer 1

Aim to provide the advantages of high lift at low speeds without incurring the disadvantages of generating high drag at high speeds

Question 2

Examples of lift augmentation devices

Answer 2

Flaps (trailing edge and leading edge) and slats and slots

Question 3

Flaps

Answer 3

Operate by changing the effective camber of the aerofoil section, when flaps are lowered the increase in camber results in a greater pressure differential being generated above and below the wing

Question 4

Spoilers

Answer 4

They are situated on the upper surface of wings, they are hinged surfaces which when extended disturb the airflow over the upper lift-producing part of the wing, thereby decreasing lift and increasing drag

Question 5

Spoilers on gliders purpose

Answer 5

reduce airspeed and steepen descent path without increasing speed

Question 6

Spoilers on jet a/c

Answer 6

They are deployed after touchdown to dump the lift and increase the weight on the wheels (this improves the effectiveness of the wheel brakes, increasing the drag during roll out and improving directional control in strong wind).

Some jet a/c also utilise differential spoilers to augment the ailerons in the control of the a/c in roll. When the spoilers on 1 wing are deployed that wing loses lift and the a/c will roll in that direction

Question 7

Trailing edge flap types

Answer 7

• Simple/plain
• Slotted flap
• Split flap
• Fowler flap

Question 8

Simple/plain trailing edge flap

Answer 8

It is a basic hinged trailing edge flap surface

Question 9

Slotted trailing edge flap

Answer 9

A slot is opened up ahead of the flap when its lowered

Question 10

Split flap

Answer 10

Only the lower surface of the aerofoil section moves

Question 11

Fowler flap

Answer 11

Similar to the slotted flap but in addition to being deflected downward it also moves back

Question 12

Trailing edge flap types - comparing their ability to produce lift

Answer 12

• Lowest CL max = Simple flap (due to relatively early separation of the boundary layer)
• Second lowest CL max = Split flap (due to upper surface retaining its shape so early flow separation doesn’t occur)
• Second highest CL max = Slotted flap (due to air moving from high to low pressure through the slot accelerating around the nose of the flap and directed down over the upper surface of the flap, this provides a form of boundary layer control in which the boundary layer is reenergised delaying separation)
• Highest CL max = Fowler flap (due to in addition to having a slot the wing area is also increased giving a further increase in lift)

Question 13

Trailing edge flap types - comparing how much drag they generate

Answer 13

Highest - lowest:
- Spilt
- Simple
- Slotted
- Fowler

Question 14

What are the effects of trailing edge flaps

Answer 14

• CL increases
• CD increases
• L/D ratio decreases
• Stalling (geometric) AoA decreases
• CP moves rearward

Question 15

The effect of trailing edge flaps on CL

Answer 15

Flaps down increases CL

With the initial stages of flap lowered (20° - 30°), the increase in CL is large however there is less of an increase as further increments are lowered

Question 16

The effect of trailing edge flaps on CD

Answer 16

Whenever flaps are lowered drag is increased, this is an advantage on approach to land as there is:
- steeper approach angles with better forward visibility
- better obstacle clearance
- decreased landing distance

Question 17

The effect of trailing edge flaps on L/D ratio

Answer 17

When the flaps are lowered the increase in CD is proportionally greater than the increase in CL, the L/D ratio is thus reduced when flaps are extended

Question 18

The effect of trailing edge flaps on stalling/geometric AoA

Answer 18

Flap Lowered = lower AoA = lower stalling AoA.

The a/c will fly level and stall with a lower nose attitude

Question 19

The effect of trailing edge flaps on CP

Answer 19

When the flaps are lowered the shape of the pressure envelope around the wing changes, most of the increase in pressure differential occurs toward the rear of the aerofoil section, this causes the CP to move rearward, which can result in a nose down pitch as the flaps are travelling

Question 20

Leading edge flap effects

Answer 20

When extended it has the same effects as the trailing edge flap with the exception of the CP moving forward and there is an increase in the stalling AoA

• increase CD
• increase CL
• increase stalling AoA
• decrease L/D ratio
• CP moves forward

Question 21

Leading edge flap types

Answer 21

• Drooping flap (the whole of the leading edge surface moves forward and down)
• Krueger flap (the lower leading edge surface hinges downward from the nose)

Question 22

What is a slat

Answer 22

It is a small aerodynamic surface placed ahead of the main aerofoil to form a slot through which air can flow.

Question 23

Fixed vs automatic slats

Answer 23

Fixed slats generate a lot of drag at high speed so automatic slats are commonly used

Question 24

Automatic slats

Answer 24

• At low AoA the effective AoA of the slat is negative and is held in the closed position
• As the AoA of the wing in increased it is accompanied by an increase in upwash over the leading edge
• The effective AoA of the slat reaches a point where it begins to produce positive lift and the slat moves up and forward on its tracks, opening up the slot
• The slat is thus in operation when it’s needed for lift augmentation at high AoA and closed to avoid extra drag at low AoA

Question 25

Slats principle of operation

Answer 25

Air from below the wing accelerates through the slot and becomes directed tangentially back along the upper surface adding KE to the boundary layer

Question 26

Effects of slats

Answer 26

• Separation is delayed
• Stall is delayed and will occur at a higher AoA
• CL max is increased
• Pressure envelope over the upper surface is flattened out