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lift force
- 1. LIFT LIFT COEFFICIENTAND EQUATION Lift depends on the following variables The major factor of which is dynamic pressure. Aerofoil pressure and AoA det the pressure distribution. Any Aerodynamic force is the product of these 3 major factors;dynamic press , coeff of force , surface area. FORCE= DYN PRESS x FORCE COEFF x SURFACE AREA
- 2. LIFT LIFT COEFFICIENTAND EQUATION LIFT is the result of pressure differential between upper and lower surfaces for a straight and level flight Lift formula given as: TAS & AIR DENSITY For a given IAS, TAS will vary with altitude owing to air density changes In ISA density at 40,000ft is a quarter of MSL. To maintain constant lift as the density reduces, all other elements of the equation should be changed. If density drops, the TAS must rise, in this case 2x MSL value
- 3. TAS &AIR DENSITY Since the density has dropped by four folds at 40,000ft, we need to double out airspeed to cater for this. Maintaining a constant IAS will maintain constant lift. SPEED AND AoA If speed rises, Cl must be reduced to maintain the same total lift, usually by reducing AOA previous value by reducing alpha to keep lift constant.
- 4. As allother variables cannot be changed we can have: If speed is increased in level flight by 30% from the min level flight Speed (Vs). The new Cl can be calculated as the percentage of Cl max. speed increase of 30% above can be written as 1.3 giving 1/1.69 =0.59 = 59%
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- 6. DENSITY COEFFICIENT ANDALTITUDE VELOCITY used in the dynamic equation speed of the aircraft relative to the that is moving TAS IAS = dynamic pressure At a given AoA a constant Dynamic pressure must be maintained to maintain the required lift. Flying at an alt other than the sea level. With increase in alt == decrease in density the TAS increases, if the IAS/dynamic press is maintained
- 7. DENSITY COEFFICIENT ANDALTITUDE Pressure altitude – height above 1013.25mba/hPa Density altitude is pressure altitude corrected for temperature deviation.
- 8. LIFT CURVE Thecurve plots Cl against α. This is the lift curve of a symmetrical aerofoil as it starts at zero lift in 0 AoA. Cl inc with AoA upto a max, Cl max, at the critical angle, above which lift decreases sharply, in a condition known as stall. To maintain a constant lift force any change in dynamic pressure must be accompanied by an adjustment in AoA.
- 9. LIFT CURVE Mindynamic press V is det by Cl max, which occurs at a specific AoA, the critical angle 14o-16o. for an increase in weight a greater speed is required to maintain lift at a given AOA. The greater the weight the higher the speed required for level flight.
- 10. LIFT CURVE Liftformula manipulation Can be used to obtain data that can be used in the lift formula, eg: find the stall speed; given mass-60000kg gravity-9.81m/s2 density-1.225kg/m3 wing area-105m2 Ans =150kts This formula can be used to obtain speed for any Cl
- 11. LIFT/DRAG RATIO Liftcurve of different sections Thickness-if the thickness of an aerofoil is increased the Clmax increases. The red line represents thickness of 6% the black line is 12%. The blue represents a cambered aerofoil At 00 AoA the aerofoil is producing lift and zero lift is achieved at -40 For a cambered aerofoil.
- 12. LIFT/DRAG RATIO thegreater the Clmax the lower the Vs (min flight speed) But the thickness required for the low Vs creates more form drag and large twisting moment at high speed. Low Vs leads to low efficient cruise speed coz of excessive drag. Therefore it is better to use aerofoil that is efficient at high speed which is able to increase camber at low speed when needed eg on approach ---- use of flaps.
- 13. LIFT/DRAG RATIO Dragaerodynamic force that acts parallel to n in the direction of the RAF Drag is the product of dynamic pressure, drag coefficient and surface area. Drag coefficient, Cd, is the ratio of drag per unit wing area to dynamic pressure. Drag curve is shown The efficiency of the production of lift can be gauged from studying the ratio between lift & drag. A high L/D ratio is more efficient ie more lift than drag is produced The l/d curve is shown below
- 14. The optimum AoAfor L/Dmax is about 40. l/d decreases until Clmax L/Dmax occurs at a specific AoA the A/C will prod -uce the least Possible drag For the lift required Any other angle of attack results in a lower L/D ratio which will increase drag against lift required. To maintain the 40 AoA any speed can be used depending on the weight LIFT/DRAG RATIO
- 15. LIFT/DRAG RATIO Changesin weight in any given configuration or airframe contamination state, or at speeds below mach 0.4, will not change L/Dmax. Different types of aircrafts L/Dmax.
- 16. LIFT/DRAG RATIO Changesin weight in any given configuration or airframe contamination state, or at speeds below mach 0.4, will not change L/Dmax. Different types of aircrafts L/Dmax.
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- 18. EFFECT OF LIFTON OTHER FACTORS 1. Weight to maintain a given AoA when weight decreases the airspeed required to maintain the AoA decreases. The heavier the aircraft is the higher its Stall speed will be. The stall speed will be considerably higher at to than on ldg
- 19. EFFECT OF LIFTON OTHER FACTORS 2. Condition of the surface Leading edge roughness can considerably reduce Clmax and thereby increasing stall speed This is due to reduction in acceleration over the rough surface=== drop in the pressure differential. Aft of 20% chord, roughness will have little effect on Clmax or the lift curve slope. Frost, snow and even rain water can increase roughness. Ldg edge icing will cause an unknown increase in stalling speed.
- 20. EFFECT OF LIFTON OTHER FACTORS 2. Condition of the surface
- 21. EFFECT OF LIFTON OTHER FACTORS 3. Flight at High Lift Conditions High lift devices like flaps greatly increase Clmax thereby reducing the min flight speed/(Vs) and thus allow shorter field lengths for T/O n LDG
- 22. 3D AIRFLOW wing terminology WingArea- the plan Surface area of the wing. Though portion may be covered by fuselage* Wing Span-tip-tip dist. Avg Chord(c)-the geometric avg.
- 23. 3D AIRFLOW: WINGTERMINOLOGY
- 24. WING TIP VORTICES Lateral flow over a wing is created by the pressure differential between upper and lower surfaces. spanwise flow is induced towards the tips on the lower surface and towards the roots on the upper surface. vortices are formed by the crossover of the two spanwise flow at the trailing edge, and is strongest at the wing tip.
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- 26. WING TIP VORTICES Induced Downwash The vertical velocities created by the vortices cause downwash which results in a reduction in the effective AOA. This continues with increase in vortex strength Without vortices, the lift would be normal to the free stream airflow, but with the modified airflow, the lift tilts back creating induced drag which increases with vortex strength.
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- 28. WAKE TURBULENCE Thehigh rotational wingtip vortices can be hazardous and are called wake turbulence. Vortex generation commences as the nose wheel leaves the ground till landing. Exist in every ac including helicopters . Are maximum in a heavy aircraft in clean configuration at low speed Weight-the higher the weight the stronger the vortex Wingspan-shorter influences interference and weakening the 2 vortex Airspeed- The lower the speed, the stronger the vortex Configuration- Vortex strength for a given speed and weight is greatest when ‘clean’ Attitude-the higher the AoA, the stronger the vortices
- 29. WAKE TURBULENCE Vorticeswill stay about ¾ of the wingspan apart, and sink 500-1000ft Helicopter produces stronger vortices than an aeroplane of the same weight & speed. Vortex of a large aircrsfts extend upto 9nm in the air In still air, vortices will sink to a height 2 wingspan above the ground then move at above 5kts. A light crosswind may drift a vortex across a parallel rwy, appch and climb out, creating a hazard to following aircraft Vortex disperse with time that’s why we have minimum separation.(ops) Care should be exercised when approaching a heavier ac especially in light wind conditions. Always stay above the flight path of preceding ac. And rotate before it’s lift off point n land beyond its touchdown point
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- 33. GROUND EFFECT Isthe increased lift (force) and decreased aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface. When landing, ground effect can give the pilot the feeling that the aircraft is "floating". When taking off, ground effect may temporarily reduce the stall speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached. Why is there ground effect?? When an ac is flying at any height significantly higher than it’s own wingspan. The wing tip vortices is unaffected and the vortex will have norm effect of upwash before the wing and downwash after the wing When close to ground i.e. (when height is less than wing span). Vortex generation is inhibited—reduced upwash n downwash, which alters the effectiveness of the AoA on wing and tailplane—this is known as ground effect
- 34. GROUND EFFECT Isthe increased lift (force) and
- 35. GROUND EFFECT Inground effect lift will be increased and drag decreased & longitudinal stability and tailplane pitching moment affected. Large reduction of Cdi will result when the wing is very close to the ground. Low wing ac experience a higher ground effect than high wing ac. Low mounted tailplane will suffer decrease in downwash thereby having a nose down pitching moment while a high one will be unaffected.
- 36. GROUND EFFECT Increasingdownwash leads to decrease in tailplane AoA Decreasing downwash leads to increase in tailplane AoA
- 37. GROUND EFFECT Isthe increased lift (force) and decreased aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface. When landing, ground effect can give the pilot the feeling that the aircraft is "floating". When taking off, ground effect may temporarily reduce the stall speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached. Why is there ground effect?? When an ac is flying at any height significantly higher than it’s own wingspan. The wing tip vortices is unaffected and the vortex will have norm effect of upwash before the wing and downwash after the wing When close to ground i.e. (when height is less than wing span). Vortex generation is inhibited—reduced upwash n downwash, which alters the effectiveness of the AoA on wing and tailplane—this is known as ground effect
- 38. GROUND EFFECT Aircraftentering ground effect Aircraft leaving Ground effect vice versa
Editor's Notes
- #23 * The pressure carryover on these surfaceallows legitimate consideration of the entire plan area