How Birds Fly

C172 — N66213— North Perry Hollywood, FL > Tamiami (first leg) — Night ops, Pilotage, Radio Nav, night Landing — 0.7 hrs

Well after that near miss with the big bird couple days ago, I got to being curious about just how the hell do birds fly anyway??

We all presume "they flap their wings and take off" is the simple story, but that's not even the tip of the iceberg really.

The aerodynamic shape, pneumatic bones, how they move their wings is like swimming in the air, and their tail is a multi-purpose stabilator that changes shape, rotation and angle of attack on demand. It really is fascinating.

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Here's some excerpts from some decent concise articles I found and revised slightly that cover pretty well just how birds fly.

How Birds Fly

Gliding:

When a bird is gliding, it flies the same way as an airplane. As the wings move through the air (blue lines), the special airfoil shape of the wings causes the air pressure above the wings to be lower than the pressure underneath. The difference in pressure is lift, a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.

Flapping:

Flapping flight uses the same principle, but the movement of the wings is more complicated.

There are three important motions in addition to the bird's forward motion:

1. Flapping

2. Twisting

3. Folding 

Flapping Twisting Folding

By flapping its wings down, together with the forward motion of the body, a bird can tilt the lift of its wings forward for propulsion. Notice that the outer part of the wing moves down much farther than the inner part close to the body. Twisting allows each part of the wing to keep the necessary angle relative to the airflow. If part of the wing is angled lower than the airflow, there might not be enough lift. If part of the wing is pointed too high, there could be a lot of drag. The wings are flexible, so they twist automatically.

Wing folding isn't essential -- ornithopters fly without it -- but it helps birds fly with less effort. This is helpful when you think about what happens during the upstroke. Because the wing is going up, the lift vector points backward, especially in the outer portion of the wing. The upstroke actually slows the bird down! By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.

All motionsAll Motions:

Utilizing only the wings, birds can do a lot to maneuver in the air. They do indeed use their tails for flight control, but they also move their wings forward and backward for balance. To make a turn, they can twist the wings or apply more power on one side. For slow flight, birds can flap their wings almost forward and backward instead of vertically; the upstroke and downstroke produce lift without forward body motion.
Since flapping wings are subject to unsteady flows (they not only move but accelerate through the air) they can produce more lift than fixed wings and are resistant to stalling.

The Tail

The wings are not the only lifting surfaces found on birds, the tail also plays an important role in flight. Without their tail feathers, flying would be a pretty difficult chore. Obviously tail types vary greatly within the birds and some tails are used for display as well as for flight, but like the wing, their shape is often influenced by their lifestyle.

Tails are actually integral to bird flight. But if a bird’s wings produce the lift, where do the tails come in? Well, its tail feathers are analogous to the rudders of ships and boats. They help them steer and maneuver while flying, as well as provide stability as they take-off and land. By twisting its tail, the bird can change its direction mid-flight. Birds can use their tail feathers to help pitch their bodies and adjust their altitude, similar to rudders on airplanes.

Tails are analogous to rudders. To help the bird slow down, the tail flares out downward, creating more drag and decrease the bird’s velocity. The tail also helps the bird balance when it is perched on a branch. And while the bird is soaring, it can spread out its tail feathers behind it to create additional lift and stability.

Tail is used only at low speed to get additional maneuverability, lift and drag. It's used as airplanes' air-brake and flaps for landings. At high speed it is furled and acts as a drag reducer 

The wings of a bird generally lie slightly ahead of the centre of gravity, this means that when a bird flies its posterior trails in the airflow behind it. The tail provides not only the lift required to buoy up the weight of the body but it also helps in flight control, unfortunately it also adds to drag at higher speeds. The tail allows the wing design in birds to be tailored for efficient cruising and high speed flight and under these conditions it is furled to provide minimal drag. At lower speeds, however, or during maneuvers the tail can be quickly unfurled to reduce induced drag (see later) from the wings and provide a surface for enhanced steering, lift or braking. The tail is suspected to play an important part in maintaining balance and stability in flight and it would seem that it is required to generate lift at low speed when the interaction between the wings and the tail can also most effectively reduce drag.

The retrices, or tail feathers generally number 10-12, but are found to range between 8 and 24. These are normally straight and bilaterally paired and the bases covered by coverts to produce a smooth surface for airflow. When you look at a birds tail you see the retrices which are controlled by the tail muscles that allow for the various movements that are required for precision flying.

It isn’t just the front of a bird that has to be aerodynamic though , its at the rear end where the greatest energy savings are to be had. It is here that air bleeds off the body and the potential for a great deal of energy sapping turbulence is produced. Most birds have a tail that helps the flow of air leave the body and can be used to increase lift if required. When a bird is flying quickly it furls its tail and tries to emulate the water droplet, which you’ll remember narrows to a fine point at the back and is the shape that is imposed on the fluid by the forces induced by the passage of the air over it. That is why swallows, frigatebirds and an number of other fliers have sharply forked tails and why falcons and nightjars have very narrow ones.

 

 

Refs:
www.n6iap.com/ornithopter/howbirdsfly.html
www2.unil.ch/biomapper/opengl/BirdFlight.html