The Theory of Lift

8/3/2019 The Theory of Lift

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The Theory of LiftThe Theory of Lift


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What is Lift?What is Lift?

AnAn aerodynamicaerodynamic force that overcomes anforce that overcomes an

objects weightobjects weight

AerodynamicsAerodynamics the study of air in motionthe study of air in motion


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Historys First FliersHistorys First Fliers

LighterLighter--thanthan--Air FlightAir Flight

Montgolfier BrothersMontgolfier Brothers

First flight in June 1783First flight in June 1783

First manned flight inFirst manned flight inSeptemberSeptember

Air DensityAir Density

Mass of air in a givenMass of air in a given

volumevolume

AerostaticAerostatic These aircraft rise withoutThese aircraft rise without

movement of themovement of the

surrounding airsurrounding air


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First Fliers (cont.)First Fliers (cont.)

HeavierHeavier--thanthan--Air FlightAir Flight Wright BrothersWright Brothers

December 17, 1903December 17, 1903

11stst manned, sustained,manned, sustained,controlled, and poweredcontrolled, and poweredflightflight

AirplaneAirplane FAR 1.1FAR 1.1 an enginean engine--drivendriven

fixedfixed--wing aircraft heavierwing aircraft heavier

than air, that is supported inthan air, that is supported inflight by theflight by the dynamicdynamicreaction of the airreaction of the air against itsagainst itswingswings


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Daniel BernoulliDaniel Bernoulli

Bernoulli studied theBernoulli studied the

pressure of movingpressure of moving

fluidsfluids

His study led physiciansHis study led physicians

of the time to measureof the time to measure

blood pressure byblood pressure by

puncturing an arterypuncturing an artery

with a glass tubewith a glass tube Static PressureStatic Pressure

Dynamic PressureDynamic Pressure


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Bernoullis EquationBernoullis Equation

PressurePressure force exerted on an areaforce exerted on an area

Static PressureStatic Pressure pressure exerted by still airpressure exerted by still air

The weight of a column of air on a certain areaThe weight of a column of air on a certain area

Dynamic PressureDynamic Pressure pressure exerted by moving airpressure exerted by moving air

Additional pressure you feel when holding your hand out of theAdditional pressure you feel when holding your hand out of the

window of a moving carwindow of a moving car

Bernoullis Principle:Bernoullis Principle:

Static Pressure + Dynamic Pressure = ConstantStatic Pressure + Dynamic Pressure = Constant


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ExamplesExamples

Static Pressure = 10, Dynamic Pressure = 4Static Pressure = 10, Dynamic Pressure = 4

Total Pressure = 14Total Pressure = 14

If the velocity of the flow were increased:If the velocity of the flow were increased:

Dynamic Pressure = 8Dynamic Pressure = 8

What is the static pressure?What is the static pressure?

Static Pressure = 6Static Pressure = 6


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ExperimentsExperiments

1.1. Lifting PaperLifting Paper

2.2. Attracting DiscAttracting Disc

3.3. Bernoullis Tractor BeamBernoullis Tractor Beam


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ContinuityContinuity

If a static pressure decrease is caused byIf a static pressure decrease is caused by

an increase in velocity, what causes thean increase in velocity, what causes the

increase in velocity?increase in velocity?

Law ofContinuityLaw ofContinuity

Density * Area * Velocity = ConstantDensity * Area * Velocity = Constant


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Venturi TubesVenturi Tubes

A tube in which the area isA tube in which the area isdecreased in a particular portiondecreased in a particular portion

This increases velocityThis increases velocity

The static pressure dropsThe static pressure drops

An opening is located at theAn opening is located at thepoint where the area (and staticpoint where the area (and staticpressure) is smallestpressure) is smallest

This opening experiences theThis opening experiences thelower pressurelower pressure

A flow of air is induced from highA flow of air is induced from highpressure to low pressurepressure to low pressure

What does any of this have toWhat does any of this have todo with lift?do with lift?


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The AirfoilThe Airfoil

CrossCross--section of the typical wingsection of the typical wing


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Airfoil TerminologyAirfoil Terminology

Leading EdgeLeading Edge

Trailing EdgeTrailing Edge

Chordline

Chordline

CamberlineCamberline

CamberCamber

SymmetricalSymmetrical

ThicknessThickness

Relative WindRelative Wind Angle of AttackAngle of Attack

Stagnation PointStagnation Point


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Cambered AirfoilsCambered Airfoils

Zero Lift, negative angle ofZero Lift, negative angle of

attackattack

Positive Lift, zero angle ofPositive Lift, zero angle of

attackattack

Increased positive lift,Increased positive lift,

positive angle of attackpositive angle of attack


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Symmetric and Inverted AirfoilsSymmetric and Inverted Airfoils

SymmetricSymmetric

Zero angle of attack,Zero angle of attack,

zero liftzero lift

Positive angle of attack,Positive angle of attack,positive liftpositive lift

Inverted CamberedInverted Cambered

Higher angle of attackHigher angle of attack

is required to achieveis required to achievethe needed liftthe needed lift


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StallStall


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Newtonian LiftNewtonian Lift

DownwashDownwash thethedownward component ofdownward component ofvelocity imparted to the airvelocity imparted to the airleaving the trailing edge ofleaving the trailing edge of

the airfoilthe airfoil

A force had to be appliedA force had to be appliedto the air in order toto the air in order tochange its directionchange its direction

ByN

ewtons 3ByN

ewtons 3rdrd

Law, theLaw, thereactive force is on thereactive force is on thewing in the upwardwing in the upwarddirection, Lift!direction, Lift!


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Which Explanation?Which Explanation?

As we saw earlier throughAs we saw earlier throughexperimentation, increasesexperimentation, increasesin air velocity doesin air velocity doesdecrease pressuredecrease pressure

We can also plainly seeWe can also plainly seethat downwash existsthat downwash existsbehind the wing and mustbehind the wing and musthave a causehave a cause

Both descriptions are validBoth descriptions are valid


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33--Dimensional Aspects of LiftDimensional Aspects of Lift

We have only lookedWe have only looked

at 2at 2--dimensionaldimensional

airfoils.airfoils.

Wings also have aWings also have a

third dimension, span.third dimension, span.


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Span EffectsSpan Effects

During lift production, the pressureDuring lift production, the pressureunder the wing is greater than theunder the wing is greater than thepressure above the wing.pressure above the wing.

The higher pressure air tries to reachThe higher pressure air tries to reachthe lower pressure area.the lower pressure area.

The higher pressure air cant travel overThe higher pressure air cant travel over

the leading or trailing edges, but it canthe leading or trailing edges, but it canflow around the wingtips.flow around the wingtips.

This sets upThis sets up wingtip vorticeswingtip vortices.. Wingtip vortices trail behind the aircraft.Wingtip vortices trail behind the aircraft.

They are responsible for downwash.They are responsible for downwash. This bridges the gap between theThis bridges the gap between the

Bernoulli and Newtonian methods ofBernoulli and Newtonian methods ofexplaining liftexplaining lift

They are also known asThey are also known as wake turbulencewake turbulence The more lift that is being produced, theThe more lift that is being produced, the

greater the pressure differential betweengreater the pressure differential betweenthe upper and lower surfaces, the greaterthe upper and lower surfaces, the greaterthe strength of the vorticesthe strength of the vortices


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Airfoil DesignationsAirfoil Designations


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Calculation of LiftCalculation of Lift

Lift CoefficientLift Coefficient

Angle of AttackAngle of Attack


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High Lift DevicesHigh Lift Devices

Trailing Edge FlapsTrailing Edge Flaps

SpoilersSpoilers

Leading Edge Slots and SlatsLeading Edge Slots and Slats

Documents

The Theory of Lift