The Work of Wings

7/25/2019 The Work of Wings

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The Work of Wings

Have you noticed the curved shape of a bird's wing? An airplane's wing

is curved also. A wing is designed for flight. It has a special shape calledan airfoil. Airfoil shapes can be found on wings, fans and propellers. The

airfoil shape provides a lifting force when air flows around it. An airfoil

has a thicker; rounded leading edge front end! and a very thin trailing

edge or back end!. In between the leading and trailing edge it is curved

both on the top and botto" surfaces. The top surface usually has a

greater curve or hu"p! than the botto" surface. #hen a surface is

curved we say it has ca"ber.

An airfoil takes advantage of $ernoulli's %rinciple. &ince the top surface

of the wing has "ore ca"ber than the botto" surface, the air flows

faster over the top of the wing than it does underneath. This "eans thatthere is less air pressure above the wing than there is beneath the wing.

The difference in air pressure above and below the wing causes lift.


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How "uch lift does a wing "ake?

The a"ount of lift depends on these things

(. the wing's airfoil shape

). si*e area! and shape of the wing

+. angle of attack

. density of the air

-. speed of flight

The wing's airfoil shape:

An airfoil shape is used to give the greatest lift possible to an airplane. A

flat plate held at the proper angle of attack does generate lift, but also

generates a lot of drag. &ir eorge /ayley and 0tto 1ilienthal during the

(233's showed that curved surfaces generate "ore lift and less drag than

flat surfaces. 4arly research also showed that a round leading edge and a


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sharp, flat trailing edge add to a wing's ability to generate "ore lift and

less drag.

1et's construct step5by5step an airfoil section.

A. The length of the airfoil section is deter"ined by placing the leadingand trailing edges their desired distance apart. This length is called the

chord line.

$. Add curvature with the ca"ber line. The a"ount of curvature isdeter"ined by the ca"ber line. This curvature greatly helps generate lift.

/. Add thickness above the ca"ber line. The a"ount of thickness that is

added will depend on the a"ount of strength needed in the wing and the

speed the airplane will usually fly.


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6. Add the sa"e a"ount of thickness below the ca"ber line.

4. 7ow you have an airfoil shape.

6ifferent airfoil shapes generate different a"ounts of lift and drag. If an

airplane is being designed to fly at low speed 3 5 (33 "ph!, it will havea different airfoil shape than an airplane designed to fly at supersonic

speed 893 5 +,-33 "ph!. That's because the air flows in slightly

different ways at different speeds and at different altitudes. In general,

low to "ediu" speed airplanes have airfoils with "ore thickness and

ca"ber.

$ecause the airplane is not "oving through the air very fast the wing

needs to generate as "uch lift as possible at a slower speed. The air

density at lower altitudes is greater. :ore "olecules in the air generate"ore lift than fewer "olecules in the sa"e a"ount of air. reater

ca"ber gives greater lift at slower speeds. At faster speeds supersonic!

and at higher altitudes airfoil shapes need to be thinner. That's because

when flying close to or at the speed of sound a shock wave for"s at the

nose of the airplane. 7A&A researchers discovered that a thin airfoil

delays the for"ation of the shock wave. This reduces drag that is caused

as the airplane "oves through the shock wave.

6uring the (3's, the 7ational Advisory /o""ittee for Aeronautics

7A/A! did research on different airfoil shapes. Their investigations

gave results that are still used today to influence the design of new

aircraft.


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Size (area) and shape of the wing:

#hen engineers design a new airplane, the si*e and shape of the wings

are very i"portant to efficient flight. #ings provide the "a


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the wingtip drag will be for this wing. The greater the nu"ber for aspect

ratio, the less the wingtip drag.

1et's look at an e=a"ple. Take two wings with the sa"e a"ount of area

let's say (33 s>uare units!, but with different lengths and widths.

7ow figure the aspect ratio for each wing. The wing with the greater

>uotient will have less wingtip drag.

4=peri"ents have shown that a wing built with a greater aspect ratiotends to create less drag than a wing built with a lesser aspect ratio even

when their area re"ains the sa"e.

1ong slender wings like those on a sailplane are called high aspect

ratio wings, and are "uch "ore efficient at "aking lift without very


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"uch drag. 1ow aspect ratio wings like on a fighter airplane have "uch

"ore of this type of drag.

The shape of a wing greatly influences the perfor"ance of an airplane.

The speed of an airplane, its "aneuverability, its handling >ualities, all

are very dependent on the shape of the wings. There are, for our

purposes here, + basic wing types that are used on "odern airplanes

straight, sweep and delta.

The straight wing is found "ostly on s"all, low5speed airplanes.

eneral Aviation airplanes often have straight wings. &ailplanes also use

a straight wing design. These wings give the "ost efficient lift at low

speeds, but are not very good for high speed flight approaching the

speed of sound.

The swept wing forward swept or sweptback! is the wing design of

choice for "ost "odern high speed airplanes. The swept wing design

creates less drag, but is so"ewhat "ore unstable for flight at low speeds.

A high sweep wing delays the for"ation of shock waves on the airplane


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as it nears the speed of sound. How "uch sweep a wing design is given

depends upon the purpose for which the airplane is designed to be used.

A co""ercial


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for"s between the chord line and the onco"ing airstrea". This angle is

called the angle of attack.

As long as the airflow can "ove s"oothly over and under the wing, the

lift will increase along with the angle of attack. At a certain point

though, the angle of attack is so great that the s"ooth or attached!

airflow cannot follow the shape on the upper side of the wing. The

airflow will then stop following the shape of the wing. The airflow will

spread out and away fro" the wing's surface. This is called airflow

separation.

4very wing has a particular angle of attack for certain speeds at which

the airflow separates fro" the wing's surface. This point is called the

stall angle. #hen an airplane's wing reaches the stall angle, the wing

stops generating lift. 4=ploring Aero ani"ation fro" 1ift seg"ent!

Density of the air:

Air density is "easured by how tightly co"pressed the "olecules are.

Air "olecules in the lower layers of the at"osphere are closer together


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than the air "olecules in the upper at"osphere. #hen there are "ore

"olecules in the air greater density!, it is easier to generate lift. @ewer

"olecules in the air "ake it "ore difficult to generate lift. That's why it

is easier to fly airplanes in the layer of at"osphere closest to the 4arth's

surface. There are "ore "olecules closest to the 4arth's surface.

Speed of flight:

There is another kind of drag that has to do with co"pressing air

"olecules in the at"osphere. #hen flying close to the speed of sound or

at the speed of sound :ach (!, the airflow around an aircraft acts

differently than at slower speeds. As the aircraft "oves through the air it

"akes pressure waves. These pressure waves strea" out away fro" the

aircraft at the speed of sound. This wave acts


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The energy lost in the process of co"pressing the airflow through these

shock waves is called wave drag. This reduces lift on the airplane.

The Work of Wings

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