Lift augmentation

LIFT AUGMENTATION

DEVICES

BY:SUHAIL AHMED

Contents of Presentation

1. Purpose of High Lift Devices2. Flaps Trailing Edge Flaps - Plain Flaps

- Split Flaps - Slotted and Multiple Slotted Flaps - Fowler Flaps - Comparison of Trailing Edge flaps - Cl max and stalling angles - Drag, Lift/Drag Ratio - Pitching and centre of pressure moment - Change of Downwash

- Overall Pitch change

Contents of Presentation Leading Edge High Lift Devices

- Leading Edge Flaps- Kruger Flaps- Variable Camber Leading Edge Flaps- Effects of Leading Edge Flaps on Lift- Leading Edge Slots/Slats- Automatic Slots- Disadvantages of Slots- Drag and Pitching moment

Trailing + Leading Edge Devices together

Purpose of High Lift Devices To Reduce take off and landing distance Reduction in Stalling Speeds Lift Augmentation by increase in camber and related aspects If we revisit the formula: L = 1/2 ρ V2 x S x CL, we can

see that we have several ways to increase the lift for a given wing. Speed (V), wing area (S) and the coefficient lift (CL) can be varied for given angle of attack (AOA) to change lift. The use of flaps will change the wing area and coefficient lift.

Flaps – A hinged portion of leading of trailing edge of the wing.

Trailing Edge Flaps1. Plain Flaps

Simple Construction Good increase in Cl max High Drag, Mainly used on low speed aircrafts

Cl max and Stalling Angle

Effective angle of attack increases with extension of trailing edge flaps

2. Split Flaps

• The flaps are part of lower surface of trailing edge.• The upper surface contour being un affected when flap is lowered • or deployed.• The split flaps give some more increase in lift as compare to plain flaps at• low angle of attack . Split flaps gives slightly more lift at high angle of attack as upper camber is not increased and separation is delayed.

3. Slotted and Multiple Slotted Flaps

• A Slotted flaps have a gap or slot in them to allow faster-moving air from the lower surface to flow over the upper surface.

• Re-energies the boundary layer. And it delays the separation of airflow on upper surface.

• Slotted flaps gives a bigger increase in Clmax than plain or split flap and much less drag

• Slotted flaps have more complex construction.

4. Fowler Flaps

Fowler Flaps(Contd.)

combined effect of increase in camber and surface area

the fowler flaps give the greatest increase in lift.

It gives least drag because of the slots and reduction in thickness: Chord ratio.

However, the change in pitching moment is greatest.

Comparison of Trailing Edge Flaps

Aerodynamic effects of flaps Increase in Drag due to trailing edge flaps Lift/Drag Ratio Pitching Moment Centre of Pressure Movement. Change of Downwash

Increase in Drag due to trailing edge flaps

Lift/Drag Ratio

LIFT/DRAG RATIO

This ratio (L/D) is also reduced when full flaps are extended. Normally, flap settings between 0 - 25° will noticeably increase lift more than drag, ideal for take off. Flap settings beyond 25° will increase drag much more than lift, ideal for landing and steep approaches to runways when there are obstacles in the approach path.

Pitching MomentCentre of Pressure Movement.

Flaps extension will cause the CP to move aft thus pitching the aircraft nose down.

Flaps retraction will move the CP forward thus pitching the aircraft nose up.

Change of Downwash

Tail plane effective angle of attach is determined by the downwash of the wing. Lowering flaps increases downwash thus decreasing tail plane angle of attack. This has a nose up pitching moment.

Leading Edge High Lift Devices

Two types:

Leading Edge FlapsLeading Edge Slats and Slots

Leading Edge Flaps(They Increase the camber of the leading edge)

Kruger Flaps

They are fitted at lower surface of wing leading edge, rotated about their forward edge as shown in the fig.

Variable camber leading edge flap

They give better efficiency than Kruger Flaps by giving better leading edge profile.

Effect of Leading edge Flaps on Lift

Leading Edge Slot

It is a gap between the lower and upper surface of the leading edge of the wing. It could be a fixed or a moving part (Slat).

When Slats are employed, they re-energize the boundary layer

Effect of Slats/Slots on Lift

Automatic SlatsOn some aircrafts slots are not controlled by the pilot, but operate automatically their movement depends on change in pressure occur around the leading edge at low angle of attack the pressure around the stagnation point keep the slat in closed position. At high angle of attacks the stagnation point moves underneath the leading edge and suction pressure occur on the upper surface of slat this pressure cause slat to move forward and create the slot

Disadvantages of a Slat/Slot

Much higher angle of attack is required to benefit from the increased Cl max by the use of slat/slot. Visibility could hamper by increased nose up pitch and landing could be difficult.

Drag and Pitching Moment

Compare to trailing edge devices, the drag and pitching moment is small.

Trailing Edge + Leading Edge DevicesMost large aircraft employ both

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