HOW Birds Fly Aerodynamic Analysis of Flying

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Part 2 –

Introduction 02

How to get the air to move over a wing?! 03

Basic aerodynamics 04

Generating Thrust 05

Generating Drag 07

Generating Lift 08

Take-off & Landing 10

Gliding flight in birds 11

Flapping the wings 12

References 16

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First of all; there’s what the birds and planes do to lift off the ground

and fly:

Blowing fast moving air above

the wing lowering the air

pressure above the wing.

Now, the air pressure under the

wing is higher and creates lift.

Lift does exactly what it sounds

like; it lifts the objects off the

ground when everything is just

right.

Then; to make the air moves faster, we should blow air over the top

to create the lower pressure.

Both birds and planes can’t do this. Instead; the wings of both are

shaped so that the air flew over the top of the wing a longer distance

and so it has to speed up as it goes over the top of the wing. This

creates the pressure difference above and under the wing.

(Streamlined shape wings).

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How to get the air to move over a wing?!

Getting the air to move over and under the wing also requires the

wing to be moving. This is called Thrust. Thrust is created when birds

flap their wings using there strong breasts muscles, while planes uses

their engines to generate thrust.

For both Birds and planes, thrust is the other part of creating lift and

the ability to fly.

Then, the shape of the wing and the ability to move it through the air

are the two things needed for flight.

Birds use their strong breast muscles to flap their wings and give

them the thrust to move through the air and fly.

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Basic aerodynamics

When a wing moves forward through still air, the air exerts a force on the wing.

At a certain position for any wing, when the wing is approximately parallel to

the air stream, no lift is generated; the force is entirely drag force. This is known

as ‘parasitic drag’, and other parts of the bird’s body will also make their own

contribution to the total parasitic drag. For the moment we shall ignore this

force.

Parasitic drag is drag that results when an object is moved through a fluid medium

(in the case of aerodynamics, a gaseous medium, more specifically, the atmosphere). Parasitic

drag is a combination of form drag, skin friction drag and interference drag.

The line down the center of the wing in this position is known as the ‘line of

zero lift’.

Now, if the wing is inclined to the air stream at an angle of attack α (measured

with respect to the line of zero lift), a large force appears on the wing, F.

This force is resolved into two components: the lift FL (which is at right angles

to the flow of air) and the drag (known as ‘induced drag’) FID.

It is usual to express the lift FL in terms of a dimensionless constant called the

aerodynamic coefficient of lift, CL, by dividing the

force FL by the term (1/2 AW ρ v2). This takes into

account the area AW of the wing, the density ρ of

the air (equal to 1.3 kgm−3

at sea level) and the

velocity of the wing through the air, v.

Hence

Note: CL is constant; it varies critically with angle of attack. Within the laminar region

up to about, say, 15◦, CL is roughly proportional to α. Beyond the 15◦ the wing stalls and the lift drops rapidly. A graph of

(CL) against α, though the precise details will depend crucially on the shape and construction of the wing.

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Generating thrust:

When a bird in cruising flight flaps its wings, the flow of air over the wing is no

longer horizontal. On the down stroke, the relative velocity between the air and

the wing is inclined upwards from below, while on the upstroke, the velocity of

the air is angled from above. Now, assuming

for the moment that the wing is symmetrical

and has zero angle of attack, it can be seen

from figure (4) that, on both the down stroke

and on the upstroke, the force of lift on the

wing (which is, by definition, at right angles

to the flow of air) is inclined forwards and has

a significant forward component.

This is the origin of the thrust produced by a

flapping wing. It is equally clear that, on

average, this wing will generate no lift, but

that is because the angle of attack is zero. I have, for simplicity, omitted the

induced drag here. We shall consider the drag forces later.

All that we require is that there shall be a net forward component of the lift.

Now what is the answer to the objection that, if the Strouhal angle is 22◦ or

more, the wing will stall on the down stroke? We must remember that a bird’s

wing rotates about the shoulder and that the Strouhal angle is measured at the

wingtip.

The angle of attack will therefore vary along the

wing from zero at the shoulder to a maximum of

22◦ at the tip. Most of the wing is still within the

acceptable range of attack angles. The problem

does, however, indicate that a vigorously flapping

wing is probably going to generate less lift and less

thrust than the theory would suggest. We can now

visualize the lift forces acting on a bird’s wing

during the down-stroke (figure 5).

Figure (4)

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As we move from the shoulder to the wing, the lift

increases in size and inclines further and further

forward. Since the angle of attack at the wing tip is

equal to the strouhal angle, the angle of attack α at a

distance r from the wing root can be obtained by a

certain equation.

In order to calculate the total forward thrust, we are

going to have to integrate along the length of the

wing from 0 to the semi-span of the wing for each

wing. We must also remember that the resultant of

the lift and drag forces inclines forward at an angle

of α/2, not α.

WING SPAN AND SEMI SPAN

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Generating Drag:

Drag is a force exerted on an object moving through a fluid; it is always

oriented in the direction of relative fluid flow (try running against a high wind

and you'll feel the drag pushing you back in the direction of relative fluid flow).

Drag occurs because the fluid and the object exchange momentum when

impacting, creating a force opposing the motion of the object.

The surface area of the object exposed to the fluid flow is higher.

The object is moving faster (or the relative fluid flow is faster).

The fluid has more momentum, or inertia (the viscosity and density of the

fluid are high).

This is generally low for air relative to other fluids such as water. Trying to

walk in a strong wind will demonstrate drag for you. A dropped weight falls

faster through air than through honey largely because of drag forces.

So, air causing drag on the flying bird, not only on the wings but also the body

and the tails of the bird.

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Generating lift

Of course, in order to fly straight at level, the wing must also produce a net lift.

It does this by using a positive angle of attack, β. On the down-stroke, this angle

is added to the α term, which governs the size of the lift force, but it does not

change its angle with respect to the vertical.

Also, since the vertical component of the lift force depends on the cosine of α,

we shall ignore this factor as α is small.

There is, however, the problem of whether or not the wing generates negative

lift on the upstroke. Mathematically we have tacitly assumed that the relation

between the coefficient of lift and the angle of attack works both for angles in

excess of 15º and for negative angles as well.

This is simply not the case. It is true that immense amounts of lift (and thrust)

are generated on the down-stroke, but it is inevitable that, on the upstroke, the

highly cambered wing of a typical bird is going to stall, and the forces

generated, both thrust and the - probably negative - lift, are going to be greatly

reduced.

When a large bird such as a heron with relatively slow wing beats flies closely

past, you can actually hear the powerful thrust of the down beat, but the

upstroke is relatively quiet. Many large birds partially fold their wings during

the upstroke, reducing both the area of the wing and, even more importantly, the

vertical speed of the wingtip.

Crows using an asymmetrical wing beat: a long powerful down-stroke followed

by a quick upstroke. This is further evidence that, in many situations, the forces

during the upstroke are considerably smaller than those involved in the down-

stroke.

Many soaring birds like gulls and birds of prey do not simply flap their wings

up and down but use an elliptical motion which carries the wing backwards

during the down-stroke and forwards during the upstroke. This changes little on

the down-stroke but significantly decreases the (negative) angle of attack on the

upstroke, possibly even reducing it to zero.

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The upshot of this discussion is that, in many cases, we can probably assume

that, for vigorous flapping, the formula we have derived work reasonably well

for the down-stroke but that a bird typically gains neither lift nor thrust during

the upstroke. The mean thrust and lift will therefore be approximately halved,

i.e.,

Take-off and Landing:

Take-off is one of the most energetically demanding aspects of flight, as

the bird needs to generate enough airflow across the wing to create lift. With

small birds a jump up will be sufficient, while for larger birds this is not

possible. In this situation, birds need to take a run up in order to generate the

airflow to take off. Large birds take off by facing into the wind, or, if they can,

by perching on a branch or cliff so that all they need to do is drop off into the

air.

The bird crouches and unfolds its wings. It jumps while its wings bite into

the air in a strong down-stroke.

The curved shape of the wing and its particular movement make flight

possible.

In down-stroke, the wing-beats push and compress the air underneath the

wings, adding air pressure there. The bird is thus thrust forward and upward.

Landing is also a problem for large birds with high wing loadings. This

problem is dealt with in some species by aiming for a point below the intended

landing area (such as a nest on a cliff) then pulling up beforehand. If timed

correctly, the airspeed once the target is reached is virtually nil. Landing on

water is simpler, and the larger waterfowl species prefer to do so whenever

possible, landing into wind and using their feet as skids. In order to lose height

rapidly prior to landing, some large birds such as geese indulge in a rapid

alternating series of sideslips or even briefly turning upside down in a maneuver

termed as whiffling.

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Gliding Flight in birds:

When a bird is gliding, it doesn't have to do any work. But it can't stay in the air

forever! The wings are held out to the side of the body and do not flap. As the

wings move through the air, they are held at a

slight angle, which deflects the air gently

downward.

Pushing the air downward causes a reaction force

in the opposite direction. You will notice a

reaction force, any time you push against

anything! The reaction force is called lift. Lift is a

force that acts roughly perpendicular to the wing

surface and keeps the bird from falling.

There is also air resistance or drag on the body and

wings of the bird. This force would eventually

cause the bird to slow down, and then it wouldn't

have enough speed to fly. To make up for this, the

bird can lean forward a little and go into a shallow

dive. That way, the lift force produced by the

wings is angled forward slightly and helps the bird

speed up. Really what the bird is doing here is

giving up some height in exchange for increased

speed. (To put it another way, it is converting its

gravitational potential energy into kinetic energy.)

The bird must always lose altitude, relative to the

surrounding air, if it is to maintain the forward

speed that it needs to keep flying.

In gliding flight, a bird's wings deflect air downward, causing a lift force that holds the

bird up in the air.

By tilting forward and going into a slight dive, the bird can maintain forward speed.

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Flapping the Wings:

Flapping of the wings creates forward thrust, while the air rushing against the

wings maintains lift.

There are three main movements in flapping flights:

1. Flap: up and down movements of the wing about the

shoulder joint.

2. Pitch: Rotating the wing along the span to change the

angle of attack.

3. Flex: Lengthening and shortening the wing.

Recall that the wings are angled slightly, which allows them to deflect the air

downward and produce lift. The slight angle of the wings is called the angle of

attack. If the angle of attack is too great, the wing will produce a lot of drag. If

the angle is too small, the wing won't produce enough lift. The best angle

depends on the shape of the wing, but it's usually just a few degrees! Notice that

what matters is the angle relative to the direction of travel, not relative to the

horizontal.

The wings flap with an up-and-down motion. This

may change in special situations, but we aren't going

to talk about those until later. When the wings move

up and down, they are also moving forward through

the air along with the rest of the bird. Close to the

body, there is very little up and down movement.

Farther out toward the wingtips, there is much more

vertical motion.

As the bird is flapping along, it needs to make sure it

has the correct angle of attack all along its wingspan.

Since the outer part of the wing moves up and down

more steeply than the inner part, the wing has to

twist, so that each part of the wing can maintain just

the right angle of attack. The wings are flexible, so

they twist automatically. This picture shows how the wing must twist in the

down-stroke, to keep each part of the wing aligned with the local direction of

travel.

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If thrust is produced in the down-stroke, you might be wondering what happens

in the upstroke.

Since the wing is travelling upward, shouldn't there be a lot of drag, tending to

slow the bird down?

To avoid this, the bird does two things:

The outer part of the wing points straight along its line of travel so it can

pass through the air with the least possible resistance. In other words, the

angle of attack is reduced to zero.

The bird partially folds its wings, which reduces the wingspan and

eliminates the draggy outer part of the wing. This is not strictly necessary

though, and most insects lack the capability.

The inner part of the wing is different.

There is little up-and-down movement

there, so that part of the wing continues

to provide lift and function more or less

as it would when gliding. Because only

the inner part of the wing produces lift in

the upstroke, the upstroke as a whole

offers less lift than the down-stroke.

As a result, the bird's body will bob up

and down slightly as the bird flies.

This control of the wing surface can

prevent flow separation and enhance lift to drag ratio.

The pressure of the air on top of its wings is less than the pressure below. When

the bird's wings move forward, the air must travel farther and faster over the

curved top surface of the wings than it does over the bottom surface. The

pressure on top of the wing is less than the pressure below, because of this

difference in the speed of the air.

Left: The wing feathers

overlap on the down-stroke

and push against the air. On

the upstroke, the feathers twist

open, allowing the air to pass

through and making it easier

to lift the wing.

Right: Air flowing over the top of a bird's wing decreases in pressure, but the air under the

wing maintains a constant pressure. This difference in air pressure helps lift the bird in flight.

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Dickinson [6] discovered that at the

onset of rotation, two vortices are

created.

One is a bound vortex attached to the

wing surface facing the direction of

rotation. The second vortex, termed

the ‘‘mirror vortex’’, is a free vortex

of equal and opposite strength to the bound vortex. This free vortex sheds from

the surface opposite the direction of rotation.

When the rotational motion ends, the bound vortex splits into two vortices that

shed from the leading and trailing edges.

Wing rotation created lift even at zero AOA and appeared to enhance

it for all other angles of attack tested.

If a wing translates through a series of vortices (such as a von Karman

street) created by the previous stroke, then it would encounter a high

velocity fluid stream that could speed the fluid velocity in the direction

of translation and add to lift production.

VORTICES ON WING SURFACE

VON KARMAN STREET

VORTEX

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And from here; it isn’t all about Birds Flying but a conclusion of the

aerodynamic and analysis of their property of flying.

And Allah who gave them this ability of flying and we notice that they are fully

designed as a perfect engineering design.

Attached below the references we laid onto for all of these information,

containing websites, published books by scientists or even biologists; because

these creatures aren’t include only engineering application but also science

explanations must be included.

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References:

[1] Ask a Biologist Website: Ask a Biologist began in 1997 in the School of

Life Sciences. It is designed as an educational resource for students, and their

teachers and parents. Ask A Biologist funded in part by the National Science

Foundation.

http://www.askabiologist.asu.edu

[2] The Physics of Flight by Oliver Linton.

[3] WIKIPEDIA. http://en.wikipedia.org/wiki/Main_Page

[4] Bird Flight Website. http://www.ornithopter.org/birdflight/

[5] Robinson Library. The Robinson ( A collector of books most of his life, and

his library currently contains over 3,000 books.) Library website was created as

a way to impart knowledge to anyone who seeks to broaden their world. The

idea is to provide information on as wide a variety of subjects as possible. And

its information are directly from books.

[6] Dickinson MH, Lehmann FO, Gotz KG. The active control of wing rotation by

Drosophila. J Exp Biol 1993;182:173–89.

[7] Liu H, Ellington CP, Kawachi K, Van den Berg C, Willmott AP. A computational

fluid dynamic study of hawkmoth hovering. J Exp Biol 1998;201:461–77.

[8] Van Den Berg C, Ellington CP. The three-dimensional leading-edge vortex of a

hovering model hawkmoth. Philos Trans R Soc London Ser B 1997;353:329–40.

Note: The reference name followed by its number

between brackets [].

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