LIFT AUGMENTATION DEVICES

LIFT AUGMENTATION

1. INTRODUCTION

2. NEED FOR LIFT AUGMENTATION

3. FLAPS

4. SLATS AND SLOTS

5. BOUNDARY LAYER CONTROL

6. FLIGHT SAFETY ASPECTS

7. CONCLUSION

L = CL 1/2 PV2 S = W

V = 2W/PCLSV STALL = 2W / PSCL (MAX)

IF W INCREASES V STALL INCREASES

CL(MAX) INCREASES V STALL INCREASES

OR

S (SURFACE AREA)

1. BOUNDARY LAYER CONTROL2. CHANGE OF CAMBER CL (MAX)

3. AREA CHANGE BY EXTENSION OF SURFACE

CL MAX AUGMENTATION

1. BY CHANGING THE GEOMETRY OF AEROFOIL

a ) TRAILING EDGE FLAPS

b ) LEADING EDGE FLAPS

c ) SLATS AND SLOTS

2. BY CONTROLLING THE BEHAVIOUR OF BOUNDARY LAYER

a ) SUCTION

b ) BLOWING

c ) VORTEX GENERATORS

FLAPS

FLAPS ARE SMALL AUXILIRY AEROFOIL ATTACHED TO MAIN AEROFOIL AT THE REAR OR FRONT. THE OPERATION OF FLAP IS TO VARY THE CAMBER, AREA OR BOTH OF THE WING. THIS CAN BE ACHIEVED BY DEFLECTING, EXTENDING OR BOTH.

EFFECT OF FLAP :

Increase CL MAX

Reduce Alpha FOR GIVEN CL (TE ONLY)

Reduce V STALL

VSF = VS Cl max (clean)

Cl max (dirty)

TYPES OF FLAPS

PLAIN FLAP

1. ZERO LIFT AOA REDUCES

2. CONSTANT INCREMENT IN CL WITH AOA

3. SLIGHT INCREASE IN EFFECTIVCE AOA

4. INCREASES THE ONSET OF STALL DUE TO EARLY SEPERATION OVER INCREASED CAMBER AT THE REAR.

5. CP SHIFTS BACK

7. L/D RATIO GOES DOWN.

SPLIT FLAP

1. REDUCTION IN ZERO LIFT AOA

2. PERFORMANCE AT HIGH AOA BETTER THAN PLAIN FLAP

3. SEPERATION EFFECTS LESS MARKED AS UPPER SURFACE

NOT HIGHLY CAMBERED

4. INCREASED DRAG DUE TO LARGE WAKE

5. STALLING AOA LESS THAN PLAIN WING BUT MORE THAN CORRESPONDING VALUE FOR A PLAIN FLAP

6. PITCHING MOMENT IS LESS MARKED

SLOTTED FLAP

1. GIVES EFFECT OF PLAIN FLAP + BOUNDARY LAYER CONTROL

2. CL MAX HIGHER THAN PLAIN OR SPLIT FLAP

3. DRAG INCREMENT IS LESS DUE TO PREVENTION OF SEPERATION

4. CHANGE IN PITCHING MOMENT LARGE

5. REDUCED AOA FOR CL MAX

6. LARGE TWISTING MOMENTS

FOWLER FLAP

1. LIFT INCREMENT DUE TO INCREASED CAMBER, RE- ENERGISED BOUNDARY LAYER AND INCREASE IN EFFECTIVE WING AREA

2. REDUCTION IN EFFECTIVE THICKNESS - CHORD RATIO WHICH TENDS TO MAKE THE WING STALL EARLIER

3. CHANGE IN PITCHING MOMENT IS LARGE

4. DRAG INCREMENT IS LESS DUE TO SLOT EFFECT AND REDUCED THICKNESS - CHORD RATIO

5. ANGLE OF ATTACK FOR CL MAX LEAST

6. MAXIMUM TWISTING MOMENT

GENERAL EFFECT OF FLAPS

1. PERMITS STEEPER APPROACH

2. LOWER APPROACH SPEEDS

3. RAPID DECELERATION DURING FLOAT

4. NOSE DOWN PITCHING MOMENT)

5. NO CHANGE IN AERODYNAMIC CENTRE

6. TWISTING MOMENT ON WING INCREASES

7. ZERO LIFT DRAG INCREASES

8. LDD INCREASES

9. TIP STALLING MAY INCREASE OR DECREASE IN CASE OF SWEPT WINGS.

FACTORS AFFECTING EFFECTIVENESS OF FLAPS

1. AREA OF THE WING

- SIZE OF THE FLAP

- SPAN WISE LOAD DISTRIBUTION

2. THICKNESS OF THE WING

- MORE EFFECTIVE BECAUSE OF EFFECTIVE INCREAMENT IN CAMBER

3. SWEEP BACK

- LESS EFFECTIVE DUE REDUCTION IN

FRONTAL AREA

FLAP MANAGEMENTRETRACTION1. RETRACT AT SUFFICIENT SPD TO CATER FOR DROP IN CL

MAX

2. RETRIMMING TO BALANCE NOSE UP PITCHING MOMENT

3. REDUCTION IN DRAG. INCREASED ACCELERATION

4. REQUIRES INCREASE IN AOA TO MAINTAIN CL TO PREVENT SINKING

EXTENSION :1. RETRIMMING TO BALANCE NOSE DOWN MOMENT

2. DRAG INCREASES- HIGHER POWER REQUIRED

3. AOA FOR A CL IS LESS . . AOA SHOULD BE REDUCED

4. SHOULD BE LOWERED WITHIN SPEED LIMIT OTHERWISE STRUCTURAL DAMAGE MAY RESULT.

PRESSURE DISTRIBUTION WITH SLAT

SLATS AND SLOTS

1. ENERGY ADDED TO BOUNDARY LAYER ON TOP SURFACE . . SEPERATION DELAYED. STALLING AOA

2. STALLING SPEEDS ARE REDUCED

3. EFFECT ON PITCHING MOMENT SMALL

4. DRAG AT HIGH AOA REDUCED

5. IMPROVES LOW SPEED HDLG CHARACTIRISTICS

EFECT OF SLAT ON CL

1. INSIGNIFICANT INCREASE IN CL AT LOW AOA

2. STALLING AOA IS HIGH - SEPN & RE-ENG BL

3. CL MAX 70 % MORE THAN FLAPS.

DISADVANTAGES

1. REDUCED DRAG AT LOW SPEEDS NOT DESIRABLE

2. FIXED SLAT GIVES HIGH DRAG AT LOW AOA

3. CL MAX AOA VERY HIGH . . T/O & LDG ATTITUDE VERY HIGH.

4. STALL IS VISCIOUS - LARGE ROLLING MOMENT OR WING DROP

BOUNDARY LAYER CONTROL- BASED ON METHODS BY CONTROLING BEHAVIOR OF

BOUNDARY LAYER WITH NO CHANGE IN WING GEOMETRY

- AIM TO DELAY OR PREVENT SEPN OF BL

BOUNDARY LAYER BLOWING- RE-ENERGISES BL AND PREVENTS SEPN

- ADDL AIR AT INCREASED VELO GIVES DIRECT LIFT INCREMENT AT ALL AOA

- BLOWING SLOT NEAR LE AFFECTS THE WHOLE OF UPPER SURFACE

- IN BLOWN FLAP TO REENERGIZE BL

DIS ADVANTAGES- LARGE AMOUNT OF POWER IS NEEDED

- COMPLEX WING DESIGN

BOUNDARY LAYER SUCTION

THE PRINCIPLE OF SUCTION IS TO REMOVE THE SLOWLY

MOVING AIR IN THE BOUNDARY LAYER. . . THE FLOW IS

LAMINAR OVER ENTIRE WING .SUCTION HOLES ARE

CREATED ALONG THE CHORD TO PREVENT SEPERATION

OVER THE ENTIRE AEROFOIL. AN ADDED ADVANTAGE IS

THAT SKIN FRICTION DRAG REDUCES AS FLOW BECOME

LAMINAR.

DISADVANTAGES

1. INCREASED SEPARATION IF AOA > DESIGN VALUE

2. PRONE TO FOD DUE TO SUCTION

3. A SEPARATE SUCTION PUMP REQUIRED

4. COMPLEX DESIGN OF THE WING

VORTEX GENERATORSWORKS ON THE PRINCIPLE OF A TURBULANT

BOUNDARY LAYER HAVING HIGHER ENERGY AND

THE SEPERATION IS DELAYED

- VORTEX GENERATORS CAN BE SMALL METAL PROJECTIONS

OR SMALL JETS OF AIR EJECTED NORMAL TO THE

WING SURFACE

DIS ADVANTAGES- EXTRA DRAG FROM PROJECTION & FROM

TURBULANT BL

- CAN BE EMPLOYED ONLY ON SUB SONIC AEROFOILS

JET FLAP

LARGE QUANTITIES OF AIR IS BLOWN AT HIGHER

VELOCITIES FROM THE WING TRAILING EDGE AS A

PLANE JET AT AN ANGLE TO THE MAIN STREAM.

THE JET IS BLOWN OVER A SMALL FLAP, WHOSE

DEFLECTION ANGLE CAN BE CHANGED TO VARY

THE JET ANGLE.

CL MAX INCREMENT IS ACHIEVED BY

- AN INCREASE IN EFFECTIVE CHORD OF THE

FLAP.

- MAGNIFICATION OF THE VETICAL COMPONENT

OF THE JET REACTION

EXTERNAL FLOW SYSTEM EMPLOYING ENGINE

EXHAUST AS A FLATTENED JET SHEET (BY A

DEFLECTER PLATE ) CAN ALSO BE USED TO BLOW

OVER WINGS OR TOWARDS THE GAP OF A SLOTTED

FLAP TO CONTROL THE BOUNDARY LAYER.

JET BLOWING

DISADVANTAGES :

1. REQUIREMENT OF LARGE AMOUNT OF AIR AT HIGH VELOCITY ENTILES USAGE OF ADDITIONAL ENGINES

2. WING PARTS WILL BE EXPOSED TO HIGH TEMPARATURES

3. COMPLEX WING DESIGN SPECIALLY FOR INTERNAL FLOW SYSTEMS.