When a bird flaps it's wings up, doesn't the pressure drop below?

[iVenky]

I am trying to understand how birds fly. If you consider a plane, using it's engine & streamline shape of the wings, it can create a low pressure region above the wings and high pressure region below and this makes it fly and the upward force is proportional to the speed.
Birds seem to flap its wings up and down. I can understand that when it flaps its wings down, it creates a high pressure region between the wings and low pressure above its body that provides the lift. Now, when it brings its wings back up, doesn't the pressure region swap? meaning, it would create a higher pressure region above. Then this should cancel the lift that it got before. I don't quite understand this mechanism of flying of bird. And how does it move forward? Thanks for your support :)

 

[Bandersnatch]

The wings don't just flap up and down like rigid paddles, but rotate to present low-resistance edge to the airflow during the recovery stage of a flap.
It's the same reason one is able to swim breaststroke despite the arms staying under water at all times.

There should be some gifs out there of slow-mo birds, that illustrate this.

 

[cjl]

In addition, the flapping is in many ways more related to propulsion than lift, at least for most birds (hummingbirds are quite different, of course, but they also flap very differently). They also gain lift through their forward speed, similarly to how airplanes do.

 

[A.T.]

I am trying to understand how birds fly.

Why don't you watch some slow motion videos of birds flapping? There are tons of it online.

 

[Lnewqban]

Bending and rotation happen simultaneously and/or alternately.
We have built flapping machines that get close to what most birds do.
The real creatures must be much more efficient, since all the energy for sustained flight has to come from food, which is scarce during long migrations.

Please, see:
http://ornithopter.de/english/principle.htm

 

[davenn]

Why don't you watch some slow motion videos of birds flapping? There are tons of it online.

Very cool videos, thanks

 

[iVenky]

The wings don't just flap up and down like rigid paddles, but rotate to present low-resistance edge to the airflow during the recovery stage of a flap.
It's the same reason one is able to swim breaststroke despite the arms staying under water at all times.

There should be some gifs out there of slow-mo birds, that illustrate this.

Ok, I think I got it, after this paper

https://www.epj-conferences.org/articles/epjconf/pdf/2016/09/epjconf_efm2016_01001.pdf

So rephrasing my question, the upward stroke cause a downward force acting on the bird which creates drag. But, by opening the feathers in between and by reducing the surface area of the upward stroke by bending the wings, it reduces the drag ( by letting the air flow between the feathers, there is less pressure difference on either side of the wings during the upward stroke). Over a complete cycle, there is a net non-zero thurst that's lifting the bird. Is my understanding, right?

In essence, it looks like thrust is similar to static friction while walking, drag is similar to sliding friction, right?

 

[Bandersnatch]

right?

Personally, I'm not seeing how the friction analogy is entirely apt here, but otherwise your understanding seems fine.

 

[A.T.]

In essence, it looks like thrust is similar to static friction while walking, drag is similar to sliding friction, right?

I also don't understand what you mean here. One analogy that makes some sense is:
drag ~= kinetic friction
lift ~= normal force

 

[sophiecentaur]

The real creatures must be much more efficient, since all the energy for sustained flight has to come from food

Flight is very demanding on Power. In general, people seem not to know about the entirely different respiratory system that birds use - no in-out-in-out in avian 'lungs'. The air takes a one way path through the gas exchangers. Be prepared for a surprise in this link. (Don't hold your breath!)
No surprise that there are so few species of bats.

 

[Lnewqban]

Flight is very demanding on Power. In general, people seem not to know about the entirely different respiratory system that birds use - no in-out-in-out in avian 'lungs'. The air takes a one way path through the gas exchangers. Be prepared for a surprise in this link. (Don't hold your breath!)
No surprise that there are so few species of bats.

This is extraordinarily interesting.
Thank you!

 

[sophiecentaur]

This is extraordinarily interesting.
Thank you!

I was gobsmacked when I learned it and relatively few people know about it.That seems to go for medics and biologists who I know. No wonder flightless birds do so well in locations where there are no serious predators; no 'overheads for transport'.

 

[gmax137]

I was gobsmacked

Me too, thanks for the link! I like to learn at least one new thing every day so this was a very good start early today!

 

[iVenky]

Very interesting link. Birds are the most fascinating creatures of all.

 

[rcgldr]

flapping is in many ways more related to propulsion than lift.

This seems to be the key point being missed. Birds flap their wings mostly forward and backward to hover, and up and down for forward flight. The shape of the wing produces a significant part of the lift during forward flight so the up and down motion is mostly for thrust. In the case of birds soaring (hawks, pelicans, seagulls, ...) the wings aren't moving. There is already a video of a humming bird hovering with the wings moving horizontally, but the much larger white tailed or black tailed kite can also truly hover. There are white tailed kites in my area.



The even larger Ospreys can truly hover for a few seconds (5 to 15 seconds) while looking for fish in water. They also dive into the water, completely submerged, resurface, and take off. There are Ospreys in my area, but I've only seen them a few times from a small cliff overlooking the ocean.