aerodynamicsi

8/6/2019 AerodynamicsI

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Aerodynamics I

A study guide on aerodynamics for the

Piper Archer


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Introduction

The purpose of this pilot briefing is to discuss

the basic aerodynamics of the Piper Archer.

Please use the following references:

- Pilots Handbook of Aeronautical

Knowledge

- Flight Theory for Pilots


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Basics The atmosphere

These fundamental basics first must be

acknowledged:

Air is a fluid. It can be compressed &

expanded

The atmosphere is composed of

78% nitrogen

21% oxygen

1% other gasses


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Basics Production of Lift

The production of lift is explained through thetheories of Newton and Bernoulli.

Newton is famous for stating three laws ofmotion and force.

Bernoulli is famous for explaining how the

pressure of a moving fluid (liquid or gas)varies with its speed of motion.

(PHAK pg. 2-3)


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Newtons Laws of motion:

Law 1 A body at rest will remain at rest. A body in motionwill remain in motion

For an airplane, inertia keeps it moving.

In contrast, if it is out sitting on the ramp, it will remain on the rampuntil an outside force causes it to move.

Law 2 Force is equal to mass times acceleration (F=MA)

The force that Newton is referencing, is the force that overcomesinertia, which was mentioned in Law 1. This force could be achange in direction or speed. For example, an applied force couldcause acceleration.


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Law 3 For ever action there is an equal and

opposite reaction In an airplane, the propeller moves and pushes back

the air; consequently, the air pushes the propeller(and thus the airplane) in the opposite direction

forward.

In other words, an airplane takes

a bite of air with the propeller,

throwing air back behind over theaircraft. This is the action. The

airplane reacts to the propulsion

of air, by moving forward.

(1)


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Bernoullis principle of

Pressure:

An increase in the speed or

movement or flow will cause

a decrease in the fluids

pressure.

- Example: the Venturi

tube

Low

Pressure

(2)


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Consider when you blow air over

the top of a piece of paper. The

paper rises, displaying the effects

of lift.

The rise of the piece of paper is

due to the low pressure that was

created between the air stream,

and the original paper position.

The velocity of the air increasedabove the paper, thus the

pressure decreased, causing lift.

(3)


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Bernoullis principle:

Air going over a wing. Notice the shape of the wing creates a

Venturi. The low pressure develops on top of the airfoil.


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Basics Air as a fluid

Because air is a fluid, it utilizes the properties of the

Coanda effect. This is the tendency for a fluid to

follow the object along its flow path.

http://www.youtube.com/watch?v=AvLwqRCbGKY

http://www.youtube.com/watch?v=S-SAQtODAQw

The way the water causes the object to move, is thesame concept as the boy blowing over a piece of

paper, causing the paper to move upward.


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Aerodynamics of a wing

Wing construction plays an important role in

the aerodynamics of lift that were just

discussed.

Some terms to be familiar with: Camber This is the curvature of the wing.

Notice how the wing is thickest at the

middle. It then thins out at the trailing

edge. This creates curvature to the

wing.

The curve of the wing means that the molecules of air traveling

on top of the airfoil have a faster velocity than the molecules of

air traveling underneath. According to the Bernoulli, this

difference in velocity is what contributes to the pressure

differential above (LOW) and below (HIGH) the wing.

(4)


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Aerodynamics of a wing

Dihedral The upward angle that exists

between the wings and the fuselage.

Chord Line The exact line from the

leading edge to the trailing edge ofthe wing.

(5)

(6)


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Aerodynamics

Now that the basics of lift & wing

characteristics are understood, weight and

balance must be examined to insure safe

flight.

The next slides will explore

the weight & balance of the

Piper Archer

(7)


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Aerodynamics of Weight & Balance

Prior to every flight, the weight and balance is calculated for that particular day.

It is known that weight x arm = moment. But, what does that mean?

Weight = the actual weight of the

object/person in pounds.

Arm = the distance from a datum, to the applied force (PHAK 3-9)

The datum in the Piper Archer is right at the tip of the nose of the plane. Forexample, the arm for calculating fuel is 95 inches. This means that the fuel tanksare located 95 inches aft of the datum. The applied force is the location of thefuel tanks.

Moment = the product of the weight multiplied by the arm


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To calculate center of gravity, divide the total moment by thetotal weight.

Center of Gravity (CG) is the center point where all the weight

acts through.

The C.G. range for the Piper Archer is 82 inches to 93 inches.

Where is this located?

Answer: Right beneath your feet, as you sit in the pilots seat.

The center of gravity is a point at which an airplane wouldbalance if it were suspended at that pointThe center of gravityis not necessarily a fixed point; its location depends on thedistribution of weight in the airplane. (PHAK 8-2).


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For the Piper Archer, the envelope that the C.G. mustremain within is only 11 inches.

When C.G. shift is calculated for weight loss during

flight (due to burning fuel, decreasing the weight inthe fuel tanks), it actually is only shifting a few inches.

It is uncommon for the C.G. to reach the forward or aftlimits of the envelope. But, it is essential to check it

each flight.

Well, perhaps you might, but you sure wont make it offthe ground!


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Characteristics of a forward CG:

Higher stall speed

Slower cruise speed

More stable Greater back elevator pressure required

Characteristics of an aft CG:

Lower stall speed Higher cruise speed

Less stable

(Page 5-23 Commercial Oral Exam Guide)


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Center of Pressure vs. Center of

Gravity

The balance of an airplane depends on the

center of gravity and the center of pressure.

But, what is the difference between the two?

The center of gravity is calculated for every

flight.

The center of pressure is not calculated.

The center of pressures (CP) is determined by

the design of the wing.


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Gravity

The center of pressure is dependent upon the

shape of the wing and the angle of attack.

So, what is angle of attack?

New term: Angle of Attack (AOA)

Angle of attack is the angular difference

between the chord line and the relative wind.

(8)(9)


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Center of pressure vs. Center of

Gravity

Knowing that the center of pressure moves

throughout the flight, what really is it?

Center of pressure is the point where the

resultant force crosses the chord line (PHAK2-7)

Now, what is the resultant force?

Resultant force is the average

between the force of lift and the

force of drag.

(11)


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Gravity

Tying it all together, it would make sense that the

center of pressure would move during flight.

This is because throughout the flight, your lift

varies (you climb, descend, level off) and your

drag varies (fly with/without flaps).

Remember, when you climb and descend, you

are increasing, or decreasing your angle of attack.

When the forces of lift and drag are constantly

changing, the center of pressure is constantly

changing.

(11)


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Drag

In the previous slide, drag was introduced into the

discussion.

In the Piper Archer, adjusting the position of the flaps,is an example of how the force of drag can be

varied. But, it is not the only way that drag can be

altered.

There are two main types of drag:

Parasitic Drag

Induced Drag


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Parasitic Drag

Parasitic drag is the most basic type of drag. It is broken downfarther into three subcategories:

Form Drag Results from the disrupting the airflow goingover the surface of the wing

Interference Drag This occurs at the intersection of aircurrents. For example, the wing root connected to thefuselage.

Skin Friction The basic friction that exists from air (a fluid)

flowing over an airfoil. Think of sliding a box on carpet vs. a tile floor

Which one has more friction?

Although a wing is much smoother than carpet in your home,friction still exists.


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Induced Drag Wingtip Vortices

Induced drag is a result of wingtip vorticies.

What are wingtip vortices?

This is the wake that is generated from the wingtips.They are counter-rotating vortices that are caused from

air spilling over the end of the wing. This pressure differential triggers the rollup of the

airflow aft of the wing resulting in swirling airmasses trailing downstream of the wingtips(PHAK 12-13).

The pressure difference the PHAK is referencing is theLow pressure above the wing, countered with a Highpressure below the wing.

http://www.youtube.com/watch?v=E1ESmvyAmOs


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Wingtip Vortices

Always land beyond an aircraft generating significant wingtip vortices.

Rotate prior to their rotation point. ALWAYS give yourself plenty of time to

avoid them. Remember to sidestep upwind.

Problem: Have you ever seen a Piper Archer out climb a 727? Probably Not.

So, what good will it do to rotate prior to their rotation point if you cant

remain high above their climb out path? You will eventually fly through them.

Time will solve this problem so that the vorticies can dissipate.

(12)


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Wingtip Vortices


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Wingtip Vortices

Imagine an infinite wingwould it have

wingtip vortices?

Answer: No. This is because an infinite wing

would not have wingtips, therefore it would not

develop wingtip vortices.

Wingtips generate induced drag. Therefore if

an infinite wing does not have wingtips, it

would not generate induced drag.


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Introduction to Stability

An important design characteristic to beaware of with the Piper Archer is stability.

What is stability? Stability is the tendency for the aircraft to

correct back to the original state.

There are two types of stability: Static

Dynamic


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Static Stability is the aircrafts initial response

following a disturbance.

Positive static stabilitymeans that initially, the

aircraft will return to its

original position. After

being disturbed, it

wants to go back.

Neutral static stabilitymeans that initially, the

aircraft will remain in a

new position after

being disturbed.

Negative static stabilitymeans that initially, the

aircraft will continue

away from its original

state, after being

disturbed.

(14)


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Dynamic stability is the aircrafts response over

a period of time.

(A) Is an example of positive dynamic

stability. Over time, the aircraft desires tocorrect back to the original state.

(B) Is an example of neutral dynamic

stability. Over time, the aircraft will

continually find a new position.

(C) is an example of negative dynamic

stability. Over time, the aircraft will

continue away from the original state.

(15)


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So, what types of stability does the Piper

Archer have?

Knowing that the Archer is used as a training

aircraft, it has positive static stability, and

positive dynamic stability.

This means that initially, and over time, theaircraft wants to return to the original state.


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When discussing stability, the words

maneuverability and controllability might

also be included in the conversation.

It is important to understand the differencebetween them:

Maneuverability - The ability to change

attitude and withstand stresses

Controllability - The aircrafts response to

pilot imputs

Maneuverability, controllability, and stability are each unique design

characteristics, dont mistake them for the same thing!


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Concepts to understand

It is important to understand some base aerodynamic

concepts about the Piper Archer. The first is the idea

of ground effect. So, what is it exactly?

Ground effect- Fly an airplane just clear of theground (or water) at a slightly slower airspeed than

that required to sustain level flight at higher altitudes

(PHAK 3-7).

Ground effect alters:

Upwash

Downwash

Wingtip vorticies


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According to the definition on the previous slide,

being in ground effect allows you to maintain level

flight at slower airspeeds than you normally would, if

you were up at altitude.

Why?

When the aircraft is that close to the ground, there is a

reduction of induced drag. Because the effects of

aircraft upwash, downwash, and wingtip vortices are

altered, the inherent drag due to lift, is decreased. When the aircraft generates less drag, consequently

the airspeed, when operating in ground effect, can be

reduced.


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When you enter ground effect, the following phenomena occur:

On entering ground effect:

1. Induced drag is decreased

2. Nose-down pitching moments occur3.The airspeed indicator reads low

Upon leaving ground effect:

1. Induced drag is increased

2. Nose-up pitching moments occur3The airspeed will read higher (correctly)

Page 72 Flight Theory for Pilots


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According to the diagram, in ground effect, less thrust is required to maintain

any given velocity, compared with the thrust required out of ground effect.Because less thrust is required, also displays the correlation as to why you

can maintain level flight at a slower airspeed.

Therefore, the wing will require a lower angle of attack in ground effect to

produce the same lift coefficient or, if a constant angle of attack is

maintained, an increase in lift coefficient will result (PHAK 3-7). (16)


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Where exactly are the limits of the ground effectregion?

The maximum altitude that an aircraft can experienceground effect depends on the wing span of that

specific airplane. Generally, the distance of about the wing span,

determines the region of ground effect.

The wingspan of the Piper Archer is 35.5 feet.

To estimate the altitude you will enter/exit ground

effect, take 35.5 divided by 2 = appx. 18 About 18 feet, above field elevation, is the upper limit

that the aircraft will experience the phenomenaassociated with ground effect.


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Another concept to be familiar with is the idea

of adverse yaw.

Adverse Yaw You change the

camber (shape) of the wing with

the ailerons when executing a

turn. The upward wing has

more lift than the lower wing. In

adverse yaw, the aircraft tends

to slip towards the upward wing

due to the differential of lift.

In a turn, an increaseinlift results

in an increaseindrag. The moredrag on the upward wing causes a

shift/twist around the vertical axis

resulting in an uncontrolled turn.

Adverse Yaw is one explanation for why a rudder is essential. (17)


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Stalls are an important maneuver in the

fundamentals of flight training. But,

understanding the aerodynamics behind a

stall is equally important.

When does an airplane stall?

Whenitexceeds thecritical angle of attack.

Remember from previous slides that angle of

attack is the angle between the chord line and the

relative wind.


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Lift is developed when air is flowing over the surface

of a wing. When a stall occurs, airflow over the top of

the wing is disturbed. The airstream is no longer

smooth, and the production of lift is reduced.

A stall occurs first at the wing root, then works out

toward the tip. This design characteristic is so that

you still maintain aileron control as long as possible.

http://www.youtube.com/watch?v=9eoboZNL9R8


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Referencing the definition of when a stall will occur,note that there is not an airspeed associated with it.

It does NOT matter what speed your aircraft is flying

at. A stall will ALWAYS occur when you exceed thecritical angle of attack.

The speed is irrelevant for determining when a stallwill happen. But, the manufacture does provide uswith a respective airspeed that a stall will usuallyoccur. It is incorrect to believe that flying at this speedwill produce a stall. Rather, the critical angle of attackwill normally be exceeded at those approximatespeeds.


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In the Piper Archer there are two airspeedsassociated with stalls:

Vs and Vso

Vs is the stall speed without flaps (Cleanconfiguration)

Vs = 50 knots

Vso is the stall speed with flaps (DirtyConfiguration)

Vso = 45 knots

Stall speeds are based upon 1G, wings level, unaccelerated flight.


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Why would there be a difference in stallspeed with or without flaps?

First recognize the purpose of flaps. Flapsare designed to maintain lift at slowairspeeds.

When flaps are added, the camber of the wing

is changed. Because the chord line hasincreased, it is now easier for lift to develop.

This is why a stall the respective stall speed isslower with an airplane configured with flaps.


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The topics addressed in this briefing arecritical to develop a solid understanding aboutthe basic aerodynamics regarding the Piper

Archer.

If you have any questions about topicscovered in this presentation feel free to

contact a RMC

flight instructor, or professor.

FLY SAFE!


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Resources Used

Pilots Handbook of Aeronautical Knowledge

FAA-H-8083-25, 2003, U.S. department of

Transportation Federal Aviation

Administration. Flight Theory for Pilots, Charles Dole, Fourth

Edition, Jeppesen-Sanderson Training

Products

Commercial Oral Exam Guide, Michael D

Hayes, Sixth Edition, ASA


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Resources Used Continued

(1) http://www.sciencetoymaker.org/balloon/images/newton.gif

(2) http://www.neam.co.uk/MathsFolder/venturi.gif

(3) http://www.coolscienceexperimentsforkids.com/wordpress/wp-content/uploads/2009/02/paper_lifted_by_air.jpg

(4) http://www.mansfieldct.org/schools/MMS/staff/hand/flightpressureonawing_files/image007.jpg

(5) http://knoxmodelairplane.com/_borders/clip_image004.jpg

(6)http://www.researchsupporttechnologies.com/boomerang_site/Boomerang%20aerodynamics3_files/angle%20of%20attack.jpg

(7) http://www.cartoonstock.com/lowres/wpa1258l.jpg

(8)http://images.google.com/imgres?imgurl=http://content.answers.com/main/content/img/oxford/Oxford_Sports/0199

210896.angle-of-attack.1.jpg&imgrefurl=http://www.answers.com/topic/angle-of-attack&usg=__WyvMspHN9_L4SDhMIfsK5gf9VnI=&h=544&w=518&sz=55&hl=en&start=7&itbs=1&tbnid=uHQI8iAeKLy5JM:&tbnh=133&tbnw=127&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1

(9) http://images.google.com/imgres?imgurl=http://resources.yesican-science.ca/100_years/images/attack1.png&imgrefurl=http://resources.yesican-science.ca/100_years/lift.html&usg=__UblbFSnzuShPfuFAcedC8Uf-2w0=&h=416&w=300&sz=22&hl=en&start=15&itbs=1&tbnid=3kW88gsx18m_TM:&tbnh=125&tbnw=90&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1

(10)http://images.google.com/imgres?imgurl=http://www.free-online-private-pilot-ground-school.com/images/CP_angle_of_attack.gif&imgrefurl=http://www.free-online-private-pilot-ground-school.com/aerodynamics.html&usg=__Q3CvhSjlrmiFNsP_HDI5Hn3jnTY=&h=658&w=589&sz=12&hl=en&start=1&it

bs=1&tbnid=0W

c1faGAVIEQGM:&tbnh=138&tbnw=124&prev=/images%3Fq%3Dangle%2Bof%2Battack%26hl%3Den%26gbv%3D2%26tbs%3Disch:1 (11)http://2.bp.blogspot.com/_fX9doSZqagk/SNcYVKrM9OI/AAAAAAAAAvk/BP8gYVm3gXU/s200/Figure+2-

9+Force+vectors+on+an+airfoil.jpg

(12) http://www.paragonair.com/public/docs/FAA-Handbooks/8083-25_AC61-23C_PHAK/_61-23C_Fig_06-18.jpg

(13) http://www.atlasaviation.com/AviationLibrary/wake%20turbulence/accov.jpg

(14) http://www.flightlearnings.com/backup/wp-content/uploads/2009/08/4-18.jpg

(15) http://www.free-online-private-pilot-ground-school.com/images/damped_undamped.gif (16) http://www.free-online-private-pilot-ground-school.com/images/ground_effect_drag_lift.gif

(17) aviatorthings.com

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