Aerodynamics: High Lift Devices.

1. Topic High Lift Devices
Time: 3 Hours
Aim: To define and understand the high lift(Lift Augmentation)
devices and to know the related aspects.
Learning Objective
number.:
Teaching Aids:
References:

3. By the end of this lesson you should be able to
understand and to:
 Recall Basic Lift Augmentation
 Recall Trailing Edge Flaps
 Recall Types Of Trailing Edge Flaps
 Recall Comparison of Different Types of trailing Edge Flaps
 Recall The Effect of Trailing Edge Flaps on the Stalling Angle
 Recall The Effect of Trailing Edge Flaps on the Stall Speed
 Recall The Operation Of Trailing Edge Flaps
 Recall The Use Of Trailing Edge Flaps For Take-Off
 Recall The Effect of Raising The Trailing Edge Flaps
 Recall The Effect of Raising The Flaps In Flight
 Recall The Use Of trailing Edge Flaps During The Approach and Landing
 Recall High Lift Devices On Transport Category Aircraft
 Recall Leading Edge High Lift Devices
 Recall The Effect Of Leading Edge Flaps On The Stalling Angle
 Recall The Operation of High Lift Devices On Transport Category Aircraft
 Recall The Protection Of High Lift Devices On Transport Category Aircraft

4. Scheme of Presentation
1. Purpose of High Lift Devices
2. Flaps
A. 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
- Aircraft Attitude with Flaps Lowered

5. Scheme of Presentation(Contd.)
B. 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
3. Trailing + Leading Edge Devices together
- Sequence of Operation
4. Asymmetry of High Lift Devices
5. Flaps Load Relief System
6. Choice of Flaps setting for take off and climb
7. Management of High Lift Devices
- Flaps retraction after take off
- Flaps extension before landing
8 . Summary/Question?

6.  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
 Flaps – A hinged portion of leading of traling edge of the
wing.
Low speed a/c– Trailing edge, High Speed a/c – Leading
and trailing

7. Trailing Edge Flaps
1. Plain Flaps
Simple Construction
Good increase in Cl max
High Drag, Mainly used on low speed aircrafts

8. Cl max and Stalling Angle
Effective angle of attack increases with
extension of trailing edge flaps

9. 2. Split Flaps

10. 3. Slotted and Multiple Slotted Flaps

11. 4. Fowler Flaps

12. Fowler Flaps(Contd.)
Because of the 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 – Guess why? (could it be the rearward
extension of the chord?)

13. Comparison of Traling Edge Flaps

14. Increase in Drag due to trailing edge flaps

15. Lift/Drag Ratio

16. Pitching Moment
Centre 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
• Tailplane effective angle of attach is determined by the
downwash of the wing. Lowering flaps increases downwash
thus decreasing tailplane angle of attack. This has a nose up
pitching moment.
Overall effect will be determined from the dominant effect from
the above mentioned.
(Illustrated in the next fig.)

18. Overall attitude when lowering the flaps.
 When the flaps are lowered, the lift force increases as the
value of Cl increases. So when the flaps are lowered, the
airplane starts to climb. To arrest this, the value of Cl has to
be lowered by decreasing the angle of attack which is done
by lowering the nose down.

19. Leading Edge High Lift Devices
Two types:
Leading Edge Flaps
Leading Edge Slats and Slots

20. Leading Edge Flaps
(They Increase the camber of the leading edge)
Kruger Flaps
They are at lower surface of wing
leading edge, rotated about their
forward edge as shown in the fig.

21. Variable camber leading edge flap
They give better efficiency than Kruger Flaps by giving better leading edge
profile.
Thus, to promote root stall first, Kruger flaps are used inboard and variable
camber flaps are used outboard.

22. Effect of Leading edge Flaps on Lift

23. 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

24. Effect of Slats/Slots on Lift

25. Automatic Slats
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.

26. Trailing Edge + Leading Edge
Devices
Most large aircraft employ both

27. Sequence of Operation
The sequence of operation is critical. Lowering the trailing
edge flap increases both upwash and downwash. At high
angle of attacks, especially for large aircraft, the increased
upwash could stall the airplane. Leading edge device
therefore be deployed before the trailing edge flaps is
lowered to take advantage of the ability of the leading edge
flaps to increase the angle of attack for Cl max.. When the
flaps are retracted, the trailing edge flaps should be
retracted first.

28. Asymmetry of high lift devices
Deployment of high lift devices can cause large change in lift force.
Asymmetry of deployment could cause a severe roll control problem.
Rudder might help in roll stability to some extend. Thus, in modern
aircrafts, the deflection of two sides is compared and if it is
asymmetrical, flaps movement will be prevented by automated
systems.
Flaps Load Relief System
On Large aircraft, flaps are prevented to be deployed beyond certain high
speed (Vfe) to prevent damage to the flaps. A Flap load relief system
will sense the flap load and prevent the flaps deployment. If the flaps
selection is made but the speed is too high, the flaps would extend as
soon as the speed drops below Vfe.

29. Choice of Flap setting for T/O, Climb And
Landing
Take Off: Lowest take-off speed will be possible at large flaps deflection
but this gives large increase in drag as well which will make
acceleration to take off speed difficult. So a compromise is made and a
lower flap setting is selected.
Climb: Climb gradient is reduced by flaps. So if climb gradient is limited,
a lesser flap angle could be used to compensate but the take off
distance required will increase.
Landing: landing distance will depend on touchdown speed and
deceleration. The lowest touchdown speed will be given by the highest
Cl max obtained by a large flap deflection. Large flaps deflection will
also give good drag which helps in deceleration. Leading edge devices
are also used to obtain high CL max.
Illustrated in the next figure:

31. Management of high lift devices

32. Summary
 High Lift Devices are used to lower the landing and
take off speeds.
 There are two type of high lift devices:
- Trailing Edge and leading Edge
 Trailing Edge flaps – plain, slotted, split, fowler
 Leading edge – Kruger, Variable camber
 Leading edge flaps are deployed before than the
trailing edge and retracted later than the trailing edge
flaps.
 Proper Management of the high lift devices is
important.

33. Any
Questions?