How the Feathers Work During Wing Flapping

8/14/2019 How the Feathers Work During Wing Flapping

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How the feathers work during wing flapping?

I wanted to make an animation of a wing flapping mechanism. While doing some

research about these kinds of mechanisms, I stumbled upon

the TreeSwallowProjectswebsite. On the website they mention that the feathers ona wing were opening during the upward stroke of the wing to reduce drag. This

may be a well known concept to some, but was new to me. The swallow picture

you see here shows this phenomenon clearly and I want to thank hris !ates for

gi"ing me the permission to use this image in this animation. Please take a good

look at this image as it plays a "ery important role in the following e#planation.

The wings on a bird are in fact a set of flaps which open when the wing rises and

closes when the wing descends. This means there is less pressure on the wingduring the upward stroke than in the downward stroke. The $uill, or the shaft, of a

feather does not always lie in the center, but is often well to one side with a row of

feathers arranged so that they o"erlap and turn somewhat on their shaft. I wanted tomake this animation to show the flap action of feathers. %t first, I thought this

would be an easy task. Since I had my wing flapping mechanism, as you see here, I

thought the only change re$uired was to add feathers on it, and show how the

feathers were acting during the upward stroke.

&ere is the animation of the wing flapping with flight feathers superimposedon it. I based my animation on the figures shown below from 'ilienthal(s book,

)irdflight, page *+*. %s you can see flaps are opening on the upstroke to reduce the
http://www.treeswallowprojects.com/http://www.treeswallowprojects.com/http://www.treeswallowprojects.com/


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drag. The mo"ement of the feather flapping is e#aggerated to show you how thefeathers operate.

)ut there is a slight problem here. If you really look at the photo of the swallow,

you can clearly see the gaps between flight feathers. &owe"er when "iewing the

blue bird shown in this mechanism below-, you cannot see between the feathers.

ither I misinterpreted 'ilienthal(s drawing or he o"erlooked some subtle errors inthe artist rendering of the condor wing arrangement. I did not see anything wrong


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with this arrangement before doing the animation, but the error manifested itselfonce I completed the animation.

%pparently, something was not right here. &owe"er, the feathers could also

be arranged differently than what was shown in the fictitious blue bird here, andthat is the case for all li"ing birds which are represented with red bird animation

here. /otice that now you can see in between the feathers.

%ll the li"ing birds( feather flap mechanism works just like the red bird shown here.

What is the difference between these two birds0 There are two major differences.


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The first difference is that the shaft of the feather, represented by green circles, isalways away from the bird(s body for li"ing red bird, and the opposite for the

fictitious blue bird. The second difference is that, the tip flight feather of the red

bird is always fully "isible if we look under the bird, and the other feathers always

o"erlap one another. 1or the blue bird, the feather closest the bird body is fully"isible and the rest is o"erlapped. I call the red bird feather arrangement a positi"e

arrangement and the blue birds a negati"e arrangement. %lso if the shaft of thefeather is away from the bird, it will be called a positi"e feather and will be shown

as a red feather. The blue birds flight feather is such that, the shaft of the feather isalways closer the bird body. These kinds of feathers will be called negati"e

feathers. It will be clear soon why I use the term positi"e and negati"e for feathers

and feather arrangements.

If the feathers were acting as a flap, then why don.t we see birds in nature that

are like the blue bird0 Since in both cases the flaps are opened in upstroke, thereshould be a 2+32+ chance for both red and blue birds. &owe"er we do not see birds

like the blue bird. 4aybe it.s because all birds, by luck, descended from one type of

bird and that is why we see only birds like red one in nature. This would be "ery

simplistic e#planation. Or maybe there is more in this than we imagined. %s'ilienthal mentioned in his book, in nature any design is not an accidental one. If

nature selects one design o"er the other, than there must be a solid reason for that.

'et,s start digging into this subtle difference and see why nature selected thered bird o"er the blue one. %s you can see from these animations, when the feathers

are opened in up stroke, the air is pushed toward under the bird.s body for the redbird, and away from the body for the blue bird. This is a "ital difference, and here is

why.


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Think for a moment that the blue bird is on the ground, and a predator isapproaching. The bird instincti"ely flaps its wings to take off. 1irst it is pushing its

wings downward, it is also pushing the air under its wing downward, and it gains

slight altitude. )ut when the bird is in up stroke, the motion of the wing creates a

"acuum under the wing. Since the air under the wing has an outward inertia, due topre"ious down stroke, does not ha"e enough time to fill this gap. %t the same time

the air passing between the feathers going outward, blocking some of e#tra aircoming toward under the wing5 %s a result the bird will be sucked downward. This

will nullify all the altitude gain obtained during the wings down stroke and becauseit did not get enough air under the wing, the ne#t down stroke will be weaker. The

bird will frantically flap its wings, and with each down stroke it will go up slightly,

and fall down with each up stroke. So birds that are like the blue bird could only fly

if they were to jump from a branch off of a tree or from a cliff, taking off from the

ground would be ne#t to impossible for them./ow let(s consider the red bird. When the wings are coming down, whate"er

happened with the blue bird will be repeated for the red bird as well. &owe"er their

up stroke is $uite different from blue birds. While the wings are going up, the

opening in the feather will push the air under the bird.s body with a force that, notonly will it will super charge the "oid with the fresh air from abo"e, it will also gi"e

some upward force to the bird.s body and keep it from falling downward. %lso theair coming around and under the bird toward the "oid will be assisted by the air

coming through the feathers abo"e. I use the term super charge here, because this is

e#actly what it is, the power used during the upstroke by the bird is con"erted

sending more air atoms downward. This has two effects. 1irst, it gi"es an upward

push to the bird much like a &arrier jet. Second, it accumulates air under the wing

for ne#t downward stroke. %nd the ne#t downward stroke will take the bird higher.In short, for the red bird, there will be more air atoms to be pushed downward for

each down stroke than the blue bird. Since it is supercharging air under its wing,the red bird will take off from ground "ery efficiently. 6ue to the supercharging

effect we call the red bird.s feather arrangement a positi"e arrangement.

1eather arrangements also play a crucial role during cruising flight, asmentioned by 'ilienthal, both birds, blue and red, will benefit from the opening of

flaps because it will reduce the drag during the upstroke. %ssuming that both birdsare flying the same gi"en speed during a cruising flight one may assume that there

are no differences between the two birds. )oth birds will get fresh air under their

wings due to their forward speed. &owe"er, there is a slight difference between the

airflow generated by the upstroke of the wings.

When the blue birds feathers open during the upstroke outward mo"ing air

pushes the tip of the feathers downward and tilts it in such a way that it creates an

out and downward air mo"ement. &owe"er, with the red bird this process willcreate a merging air flow as shown in the figure. This is "ery important e"ent. %s

we know, when an object mo"es through the air with speed, it creates a low


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pressure region behind it, which causes drag that will reduce the speed of themo"ing object. Therefore the red bird.s air mo"ement will try to fill the gap behind

the bird, reducing the drag. Since the blue bird.s wings are pushing the air away

from the body, it will not fill the gap behind it. This will cause considerable drag on

the birdr(s mo"ement. %lso, the blue bird is trying to push the air outward, into stillair, which might show resistance to it, and cause the bird to use more force and

energy to push the air. On the other hand, the red birds wings are pushing the airtoward a ca"ity, which needs air, so that it can be filled. In short filling the "oid

behind a flying bird will reduce the drag and as a result energy consumption ofupward mo"ing wings will be reduced.

The figure gi"en abo"e shows a con"erging air flow for the red bird and di"erging

one for the blue one. Is it a coincidence that these con"erging lines on the feather

ha"e the same direction and almost the same angle with the barbs on the feather0 Is

this a freak accident, or are barbs assisting the airflow to be pushed toward the

ca"ity behind the bird0 I ha"e a small hunch that this is the case.


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I thank 6r. )ret from the 7ni"ersity of 4ontana, 6i"ision of )iological Sciences,for gi"ing me permission to use the hummingbird picture in a wind tunnel shown

abo"e. %s you see in this picture the feathers open in the upward stoke and theairflow con"erges toward the back of the hummingbird.

This figure shows four possible combinations of feather arrangements, positi"efeather arrangement, negati"e feather arrangement, positi"e feather arrangement

and negati"e feather arrangement. Only the red bird has the capability of taking off

of the ground and ha"e efficient flight capability. The blue bird has the ability to


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glide but would fly inefficiently. The other combinations are not suitable for flightand or gliding.

%s you can see, ha"ing two birds identical in their looks, but with "ery

different feather arrangements leads to a different path in the e"olution of birds. Ifin nature, there were birds like the blue bird we showed here today they could not

compete with the red birds.

The red bird is a master piece of the nature, because nature threw e"erythingin its arsenal to make perfect flying machine.

How the Feathers Work During Wing Flapping

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