OverviewOverview Brief discussion of the 4 forces acting on a Brief discussion of the 4 forces acting on a

planeplane Brief definition of the 4 forcesBrief definition of the 4 forces

WeightWeight DragDrag ThrustThrust LiftLift

How lift is developedHow lift is developed Two Perspectives on how lift is createdTwo Perspectives on how lift is created

DemonstrationsDemonstrations Factors that affect liftFactors that affect lift

ForcesForces ForceForce – a push or a pull acting– a push or a pull acting

on a body.on a body. As a plane flies it is in the As a plane flies it is in the

center of 4 forces.center of 4 forces. Weight, lift, drag and Weight, lift, drag and

thrustthrust Two natural forces being Two natural forces being

exerted on planeexerted on plane Weight and dragWeight and drag

A pilot needs to A pilot needs to overcome weight and overcome weight and drag to achieve flightdrag to achieve flight

Two forces a pilot needs to Two forces a pilot needs to create to overcome weight create to overcome weight and dragand drag Lift and thrustLift and thrust

Lift & thrust are required Lift & thrust are required to keep the airplane in the to keep the airplane in the airair

Lift

Weight

Drag Thrust

WeightWeight WeightWeight is defined as the downward force of is defined as the downward force of

gravitygravity Force is always directed toward the Force is always directed toward the

center of the earthcenter of the earth Weight is distributed throughout the planeWeight is distributed throughout the plane The magnitude of the weight depends on The magnitude of the weight depends on

the mass of the plane plus the fuel, the the mass of the plane plus the fuel, the people and baggagepeople and baggage

A pilot must overcome weight by lift to get A pilot must overcome weight by lift to get the plane in the air the plane in the air

DragDrag DragDrag is a resistance force created by is a resistance force created by

the plane’s movement through the airthe plane’s movement through the air The force of the air pushes against the The force of the air pushes against the

plane, therefore slowing the plane downplane, therefore slowing the plane down The magnitude of drag depends on the The magnitude of drag depends on the

shape, air quality and velocityshape, air quality and velocity Drag increases as air speed increasesDrag increases as air speed increases

A pilot must overcome drag with thrust to A pilot must overcome drag with thrust to gain speedgain speed

ThrustThrust ThrustThrust is is defined as the forward defined as the forward

push that gets the plane into the airpush that gets the plane into the air Thrust is artificially created and used to Thrust is artificially created and used to

overcome drag and to sustain liftovercome drag and to sustain lift This force is provided by the propeller or jet This force is provided by the propeller or jet

engineengine

Thrust is also used to accelerate and Thrust is also used to accelerate and gain altitudegain altitude

LiftLift LiftLift is the upward force on a plane is the upward force on a plane

Various parts of a plane help to achieve liftVarious parts of a plane help to achieve lift But most of the lift is created by the But most of the lift is created by the

wingswings The magnitude of lift depends on the shape, The magnitude of lift depends on the shape,

size and velocitysize and velocity For example, the faster the plane goes the For example, the faster the plane goes the

greater the liftgreater the lift The lift that is produced by the wings must The lift that is produced by the wings must

be greater than the weight of plane to leave be greater than the weight of plane to leave the groundthe ground

Two PerspectivesTwo Perspectives Two explanations to help understand how lift is Two explanations to help understand how lift is

createdcreated Both contribute to creating liftBoth contribute to creating lift Bernoulli’s PrincipleBernoulli’s Principle

Largely depends on the shape of the wingLargely depends on the shape of the wing Concentrates on speeds and pressures in the airstreamConcentrates on speeds and pressures in the airstream Involves pressure imbalancesInvolves pressure imbalances

Newtonian ExplanationNewtonian Explanation Largely depends on the tilt of the wingLargely depends on the tilt of the wing Concentrates on the acceleration of the passing Concentrates on the acceleration of the passing

airstreamairstream Involves the deflection of the air streamInvolves the deflection of the air stream

Important Concepts - AirImportant Concepts - Air Principal concept in aerodynamics is the Principal concept in aerodynamics is the

idea that idea that air is a fluidair is a fluid Air has mass, therefore it has weightAir has mass, therefore it has weight

Because it has weight, it exerts pressureBecause it has weight, it exerts pressure Air flows and behaves in a similar manner to Air flows and behaves in a similar manner to

other liquidsother liquids Air has molecules which are constantly movingAir has molecules which are constantly moving

Lift can exist only in the presence of a Lift can exist only in the presence of a moving fluidmoving fluid Faster moving fluids exert less force on Faster moving fluids exert less force on

surfaces they are flowing alongsurfaces they are flowing along

As an airplane moves forward, the As an airplane moves forward, the airflow splits up into two separate airflow splits up into two separate flowsflows

Before We Begin…

copyright 2006 Kevin Bailey

Bernoulli’s Principle Bernoulli’s Principle DefinedDefined

Bernoulli’s PrincipleBernoulli’s Principle states that states that when the speed of a moving fluid when the speed of a moving fluid increases, the pressure decreases increases, the pressure decreases and when the speed of a moving and when the speed of a moving fluid decreases, the pressure fluid decreases, the pressure increases.increases.

©2003 m. mitchell

Daniel Bernoulli18th century Swiss Scientist

Bernoulli’s PrincipleBernoulli’s Principle Air flowing around the wing experiences a change in Air flowing around the wing experiences a change in

speed and each change in speed is accompanied by a speed and each change in speed is accompanied by a change in pressurechange in pressure Airflow going Airflow going underunder the wing encounters a sloping surface the wing encounters a sloping surface

Slows airflow down and slow moving air maintains a higher Slows airflow down and slow moving air maintains a higher pressure on the bottom surfacepressure on the bottom surface

Airflow going Airflow going overover the wing encounters the up/down sloping the wing encounters the up/down sloping Slows the airflow down, then it speeds it up; with the faster Slows the airflow down, then it speeds it up; with the faster

moving air a lower pressure develops on the top surface moving air a lower pressure develops on the top surface Air going over must travel farther, so its average speed is Air going over must travel farther, so its average speed is

greatergreaterthan the speed of the air belowthan the speed of the air below

Result: A reduction in sidewise pressure which occurs at the top, Result: A reduction in sidewise pressure which occurs at the top, exerting a lifting force on the entire wingexerting a lifting force on the entire wing

Pressure imbalance produces an overall upward forcePressure imbalance produces an overall upward force

Conservation of EnergyConservation of Energy(Bernoulli’s Principle)(Bernoulli’s Principle)

Bernoulli principle derived from the Law of Conservation Bernoulli principle derived from the Law of Conservation of Energyof Energy A fluid under pressure has potential energy.A fluid under pressure has potential energy.

Energy can be stored in pressurized airEnergy can be stored in pressurized air The higher the pressure the greater the potential energyThe higher the pressure the greater the potential energy

Moving fluids have both potential energy and kinetic Moving fluids have both potential energy and kinetic energy. energy. Total energy must remain constant, so its potential energy Total energy must remain constant, so its potential energy

decreases, and which means its pressure decreases as welldecreases, and which means its pressure decreases as well When the air’s speed and motional energy increase, the When the air’s speed and motional energy increase, the

pressure and pressure energy must decrease to compensatepressure and pressure energy must decrease to compensate Speed increases over the wing because the airflow Speed increases over the wing because the airflow

converts some of its pressure energy into kinetic converts some of its pressure energy into kinetic energyenergy

Fast Moving Air; Low Air Pressure

Air travels farther

Slow Moving Air; High Air Pressure

airfoil

BERNOULLI’S PRINCIPLE DIAGRAM

Leading edge

Trailing edge

The distance traveled is the same. Equal distances in equal times means the air is traveling at same speed. There’s no net force=no lift.

The curved shape is a longer distance so the air is traveling faster. Equal distances traveled in equal times. No net force=no lift.

The air on top is traveling faster. It exerts less force. When 2 forces are combined they do not cancel each other out. Therefore there is some net force upward.

Shape of the Wing

Bernoulli’s Principle

Newtonian ViewNewtonian View Newton’s Third Law states that “for every Newton’s Third Law states that “for every

action there is always an equal but action there is always an equal but opposite reaction.”opposite reaction.”

Newton’s Third Law, is often called the Newton’s Third Law, is often called the Law of Conservation of Momentum, which Law of Conservation of Momentum, which states:states: When an object is given a certain momentum in When an object is given a certain momentum in

a given direction, some other body will receive a given direction, some other body will receive an equal momentum in the opposite directionan equal momentum in the opposite direction

This theory predicts that as the air stream This theory predicts that as the air stream passes by, it is deflected downward.passes by, it is deflected downward.

Both top and bottom surfaces of Both top and bottom surfaces of wing play important roles in deflectionwing play important roles in deflection

© Texte Olivier Esslinger 2003-2006

Newtonian View Newtonian View ExplainedExplained

As the airflow separates, they both experience two different As the airflow separates, they both experience two different accelerationsaccelerations Flow Flow underunder

encounters downward slope; airflow is deflected downward (action), and encounters downward slope; airflow is deflected downward (action), and the air stream reacts by pushing the wings up (reaction).the air stream reacts by pushing the wings up (reaction).

Air molecules impart some of their momentum to the wing, therefore Air molecules impart some of their momentum to the wing, therefore nudging wingnudging wing

Flow Flow overover travels up, over and down travels up, over and down Initially flow encounters upward sloping surface-pushes it upwardInitially flow encounters upward sloping surface-pushes it upward This upward force causes air to push downward on the leading portion of This upward force causes air to push downward on the leading portion of

wings top surfacewings top surface Top surface is curved, so it soon begins to slope downwardTop surface is curved, so it soon begins to slope downward Before airflow leaves trailing edge there is a slight downward Before airflow leaves trailing edge there is a slight downward

component to its motioncomponent to its motion This airflow must accelerate downward to stay in contact with surfaceThis airflow must accelerate downward to stay in contact with surface

In both cases, wing has made the air accelerate downward by In both cases, wing has made the air accelerate downward by pushing the air downward. pushing the air downward.

DownwashDownwash – downward velocity behind the wing (downward – downward velocity behind the wing (downward deflection of airflow)deflection of airflow)

UpwashUpwash – slight upward flow of air at leading edge – slight upward flow of air at leading edge

Downwash

NEWTONIAN’S VIEW DIAGRAM

Airfoil

Upwash

Wing gets a momentum downward from air. According to Law of Conservation of Momentum, the wing gets an upward momentum in the opposite direction equal to the downward momentum

Air is not just flowing from left to right but upward/downward

Experiment 1Experiment 1

Demonstrates Bernoulli’s PrincipleDemonstrates Bernoulli’s Principle1.1. Hold paper horizontally just below your lips (let Hold paper horizontally just below your lips (let

paper hang limp).paper hang limp).2.2. Blow hard over the top of the paper.Blow hard over the top of the paper.

What happens to the paper?What happens to the paper? Paper responds by moving up toward the air Paper responds by moving up toward the air

stream.stream.

Why does this happen?Why does this happen? Moving air above is at a lower pressure, Moving air above is at a lower pressure,

so paper is lifted up by higher pressureso paper is lifted up by higher pressurebelow it.below it.

copyright Terry Colon, 2006

Experiment 2Experiment 2

Cup full of waterCup full of water StrawStraw ScissorsScissorsResults:Results: Blowing over the straw will make the air Blowing over the straw will make the air

move faster over the top of the move faster over the top of the straw. The air straw. The air pressure above the pressure above the straw will decrease and the water straw will decrease and the water will go will go up the straw and squirt out.up the straw and squirt out.

ExplanationExplanation:: The difference in the air pressure The difference in the air pressure over the over the straw and the rest of cup straw and the rest of cup is what lifts the wateris what lifts the water

Factors Which Affect the Factors Which Affect the Amount of Lift CreatedAmount of Lift Created

SpeedSpeed The faster the wing moves through the air the more The faster the wing moves through the air the more

air is forced over and underair is forced over and under So a plane must maintain ample velocity to keep the So a plane must maintain ample velocity to keep the

upward lifting forceupward lifting force If it slows down too much—lift decreases—plane descendIf it slows down too much—lift decreases—plane descend

Density of airDensity of air The denser the air the more lift (colder air is more The denser the air the more lift (colder air is more

dense; air density changes with altitude)dense; air density changes with altitude) Planes climb better in winter. Planes climb better in winter.

Shape of wingShape of wing Asymmetrical Asymmetrical

Angle of attackAngle of attack (its tilt relative to the wind) (its tilt relative to the wind) Downside: increases dragDownside: increases drag

SourcesSources TextsTexts

Physics Made SimplePhysics Made Simple by Ira M. Freeman, 1990 by Ira M. Freeman, 1990 Inquiry Into PhysicsInquiry Into Physics by Vern J. Ostediek & Donald J. by Vern J. Ostediek & Donald J.

Bord, 1987Bord, 1987 WebsitesWebsites

www.howstuffworks.com/airplane.htm http://Howthingswork.virginia.edu/airplanes.htmlhttp://Howthingswork.virginia.edu/airplanes.html www.grc.nasa.gov/WWW/k-12/airplane/forces.htmlwww.grc.nasa.gov/WWW/k-12/airplane/forces.html www.allstar.fiu.edu/aero/airfly/vl3.htmwww.allstar.fiu.edu/aero/airfly/vl3.htm www.washington.edu/faculty/eberhardt/lift.htmwww.washington.edu/faculty/eberhardt/lift.htm www.av8n.com/how/htm/airfoils.htmlwww.av8n.com/how/htm/airfoils.html http://sln.fi.edu/flights/own2/forces:htmlhttp://sln.fi.edu/flights/own2/forces:html www.alphatrainer.com/handouts/ac61-23c.pdfwww.alphatrainer.com/handouts/ac61-23c.pdf