High Lift Devices (Chapter 8)

PRINCIPLES OF FLIGHT

TopicHigh Lift DevicesTime:3 HoursAim:To define and understand the high lift(Lift Augmentation) devices and to know the related aspects.Learning Objective number.: Teaching Aids:References:

High Lift Devices

By: Gurjit Singh Bal

By the end of this lesson you should be able to understand and to:

Recall Basic Lift AugmentationRecall Trailing Edge FlapsRecall Types Of Trailing Edge FlapsRecall Comparison of Different Types of trailing Edge FlapsRecall The Effect of Trailing Edge Flaps on the Stalling AngleRecall The Effect of Trailing Edge Flaps on the Stall SpeedRecall The Operation Of Trailing Edge FlapsRecall The Use Of Trailing Edge Flaps For Take-OffRecall The Effect of Raising The Trailing Edge FlapsRecall The Effect of Raising The Flaps In FlightRecall The Use Of trailing Edge Flaps During The Approach and Landing Recall High Lift Devices On Transport Category Aircraft Recall Leading Edge High Lift DevicesRecall The Effect Of Leading Edge Flaps On The Stalling AngleRecall The Operation of High Lift Devices On Transport Category Aircraft Recall The Protection Of High Lift Devices On Transport Category Aircraft

Scheme of PresentationPurpose of High Lift DevicesFlaps 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

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 Operation4. Asymmetry of High Lift Devices5. Flaps Load Relief System6. Choice of Flaps setting for take off and climb 7. Management of High Lift Devices- Flaps retraction after take off- Flaps extension before landing8 . Summary/Question?

Purpose of High Lift Devices To Reduce take off and landing distanceReduction in Stalling SpeedsLift Augmentation by increase in camber and related aspectsFlaps A hinged portion of leading of traling edge of the wing. Low speed a/c Trailing edge, High Speed a/c Leading and trailing

Trailing Edge Flaps

Plain Flaps

Simple ConstructionGood increase in Cl maxHigh 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

3. Slotted and Multiple Slotted Flaps

4. Fowler Flaps

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?)

Comparison of Traling Edge Flaps

Increase in Drag due to trailing edge flaps

Lift/Drag Ratio

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 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.)

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.

Leading Edge High Lift DevicesTwo types:Leading Edge FlapsLeading Edge Slats and Slots

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.

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.

Effect of Leading edge Flaps on Lift

Leading Edge SlotIt 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 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.

Trailing Edge + Leading Edge DevicesMost large aircraft employ both

Sequence of OperationThe 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.

Asymmetry of high lift devicesDeployment 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 SystemOn 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.

Choice of Flap setting for T/O, Climb And LandingTake 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:

Management of high lift devices

SummaryHigh Lift Devices are used to lower the landing and take off speeds.There are two type of high lift devices: - Trailing Edge and leading EdgeTrailing Edge flaps plain, slotted, split, fowlerLeading edge Kruger, Variable camberLeading 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.

Any Questions?