# How do birds coordinate their turns?

I am building a plane that looks like an Eagle, so I was watching YouTube videos so I can see how to realistically fly it.

One thing I don't understand is how do birds like this coordinate their turns? The bird's tail is flat and flexes a little, but not very much. You can see the wings warping (Wright Brothers style) to go into and out of turns. But with no rudder, it seems like there's both no way to counter the adverse yaw; and with no vertical fin, nothing to bring the tail around while in the steady-state turn.

I was watching the wing tips and have a theory. It appears the inner wing tip feathers are more splayed out than the outer ones. I'm thinking that might increase drag on the inner wing tip, which both counters adverse yaw and brings the nose around during a steady state turn.

• There's a book called Miracle Of Flight that covers flight dynamics and mechanics up through the Reynolds Number range, from insects, through birds, to airplanes. Very well written with great photography. Sep 7, 2023 at 17:56
• @JohnK Thank you for the pointer! Looks like just the book to add to my frequently re-read books on the Wright Brothers as well as Stick and Rudder. Sep 7, 2023 at 19:19
• You might take a look at Prandtl wings, and research by Al Bowers. youtube.com/watch?v=RoT2upDbdUg Sep 8, 2023 at 18:04
• See also comments in chat room for this question -- chat.stackexchange.com/rooms/148369/… Sep 8, 2023 at 18:40
• worth reading anything by Adrian Thomas, he's done a lot of research in this area biology.ox.ac.uk/people/professor-adrian-thomas#tab-2779236 Sep 9, 2023 at 17:20

Birds coordinate their turn by rotating their tail along the roll axis. This tilts the lift vector of the tail and makes it work like a rudder.
The roll angle of the bird itself is controlled by wing warping.

You can see this in the video especially when the bird is operating in gliding configuration and the tail is providing positive (upward) lift. During gliding phases, rolling the tail (w.r.t. the body) to inside of the turn reduces slip / increases skid, and rolling to the outside of the turn increases slip / reduces skid). You can see constant small corrections of the tail roll angle, as well as the tail pitch angle to correct for pitch instability.

• Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. Sep 7, 2023 at 15:51
• You could say that the bird can create half a V tail whenever it needs to. Sep 7, 2023 at 17:54

You might take a look at Prandtl wings, and research by Al Bowers. The lift distribution on most bird wings are different than an most planes. This different lift distribution can mean that the tip of the wing is at such an AoA to actually get a "thrust" out of it -- this is actually what you would see with a winglet. (See 17:54 in the below video.) With the right lift distribution, instead of adverse yaw, you can get proverse yaw.

Birds can warp their wing to customize the effective angle of attack across the wingspan. This is similar to washout on a plane wing, except a bird can change this dynamically in flight. This can be (is) exploited to adjust the drag on each side to cause yaw without a "rudder" -- potentially by causing "thrust" at a wingtip by very heavily "washing out" the wing.

I highly recommend watching the following video of a talk by Al Bowers (NASA researcher) on this topic, he does a much better job of explaining it than I ever could:

• Another advantage that birds have over aircraft is pressure (i.e. touch) sensitivity over the whole wing or control surface. They have feeback about the response to small adjustments that a computer or human pilot does not.
– Liam
Sep 9, 2023 at 11:53
• Thank you, this is a great video. I did search but didn't find this great video before. Mar 12 at 15:56
• @Liam That's a great point. We have to use gross movements, whereas they can sense the smallest pressure and react with tiny movements that immediately counter it. Much like we manage to stand on our own two feet, so there must be lots of tiny adjustments going on. (A fact that I've become very aware of now that I'm getting older and my coordination isn't quite as good as it used to be!) Mar 12 at 15:58