Lift Augmentation Flashcards
Purpose of lift augmentation devices
Aim to provide the advantages of high lift at low speeds without incurring the disadvantages of generating high drag at high speeds
Examples of lift augmentation devices
Flaps (trailing edge and leading edge) and slats and slots
Flaps
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
Spoilers
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
Spoilers on gliders purpose
reduce airspeed and steepen descent path without increasing speed
Spoilers on jet a/c
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
Trailing edge flap types
- Simple/plain
- Slotted flap
- Split flap
- Fowler flap
Simple/plain trailing edge flap
It is a basic hinged trailing edge flap surface
Slotted trailing edge flap
A slot is opened up ahead of the flap when its lowered
Split flap
Only the lower surface of the aerofoil section moves
Fowler flap
Similar to the slotted flap but in addition to being deflected downward it also moves back
Trailing edge flap types - comparing their ability to produce lift
- 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)
Trailing edge flap types - comparing how much drag they generate
Highest - lowest:
- Spilt
- Simple
- Slotted
- Fowler
What are the effects of trailing edge flaps
- CL increases
- CD increases
- L/D ratio decreases
- Stalling (geometric) AoA decreases
- CP moves rearward
The effect of trailing edge flaps on CL
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
The effect of trailing edge flaps on CD
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
The effect of trailing edge flaps on L/D ratio
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
The effect of trailing edge flaps on stalling/geometric AoA
Flap Lowered = lower AoA = lower stalling AoA.
The a/c will fly level and stall with a lower nose attitude
The effect of trailing edge flaps on CP
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
Leading edge flap effects
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
Leading edge flap types
- 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)
What is a slat
It is a small aerodynamic surface placed ahead of the main aerofoil to form a slot through which air can flow.
Fixed vs automatic slats
Fixed slats generate a lot of drag at high speed so automatic slats are commonly used
Automatic slats
- 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
Slats principle of operation
Air from below the wing accelerates through the slot and becomes directed tangentially back along the upper surface adding KE to the boundary layer
Effects of slats
- 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
