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Flight of the Falcon

Falcons are the most beautiful creatures I know of. They are superbly adapted to their lifestyles, and were it not for DDT they'd be very successful. (Fortunately, their numbers have rebounded since DDT was banned, and they have reappeared in areas where they had vanished.)

There's at least one which lives near me; sometimes I see it when I'm out for walks. And I always stop and watch as long as I can, even if all it's doing is sitting on a lamp-pole for ten minutes. That's ten minutes of wealth beyond calculation as far as I'm concerned.

Recently I became interested in how they fly. There's a lot more to it than just flapping wings. When a falcon is flying, power and lift come from the wings, but control comes from the tail.

The wings are certainly the most obvious flight adaptation of any bird. Birds are the only flying creatures with a two-surface wing, which means they're the only ones capable of gliding. (They take advantage of what humans call the Bernoulli Principle.) Bats, on the other hand, must flap continuously when in the air except in rare conditions because bats have single-surface wings which inherently have a terrible glide ratio (which is to say that they lose a lot of altitude for each unit horizontal distance moved). Birds, on the other hand, are so superbly streamlined and have such efficient wings that some birds can glide for hours at a time with almost no flapping simply by gaining energy from thermals. Pterosaurs seem to have been more similiar to bats in this regard (though that's not certain). No insect can glide at all.

Birds also have feathers, not a particularly profound observation. But feathers are an incredible design, being enormously versatile, extremely strong and very light. The large feathers on a bird's wing are hinged, so that when the bird pulls its wings up the feathers open to let air through, and when the bird pushes its wings down the feathers close again to resist the air. This massively improves the efficiency of powered flight. (This is another thing bats don't have since the bat wing membrane is solid at all times. On the other hand, a bat's wing weighs less per unit area.)

The bone and muscle supporting a bird's wing is on the leading edge, and this too is an advantage. The muscles which control flapping push and pull the bone, and since it's off center it means that as the wing flaps it warps. On the down stroke the bone pulls the rest of the wing down, so the leading edge of the wing is lower than the trailing edge. This makes it a wedge which forces air backwards in addition to down. On the up stroke, the leading edge is higher than the trailing edge, and again this forces air backwards. So it actually gains energy on each stroke of the wings, though it gets more from the downstroke.

A bird can also change the shape of its wing if need be; a falcon going into a serious dive will nearly fold its wings at the beginning to increase the speed of the dive.

But as superbly designed as the wings are, it's the tail where the greatest wizardry happens.

The tail of a falcon is made of feathers. Archeopteryx had a long bony tail (betraying its close relationship to its recent reptilian forebears) but modern birds have dispensed with that, since feathers work just as well and weigh a lot less.

A bird can spread and contract its tail, angle it up or down, and twist it either direction. All of these are critically important and it does them all constantly while in flight.

When the tail is spread, it presents more surface area and thus has more effect on air flow. It also increases drag. Generally, a bird will spread its tail during maneuvers but keep it contracted in a level glide.

I need some words from aeronautical engineering: A bird is pitched up if its head is above its tail. A bird is rolled right (starboard) if its left wingtip is above its right wingtip. Yaw is the compass heading.

A falcon controls its flight by using its tail to create forces on the bird's body. Since the tail is well away from the center of gravity, the effect of these forces is to try to alter the pitch or roll of the bird.

As long as the tail is away from the nominal airflow then it interferes with the air flow and thus generates force on the bird's body.

The tail is used to stabilize powered flight. The wings of a bird are attached slightly forward of its center of gravity. When a bird flaps its wings down, this not only pushes the bird up and forward, but it also tries to pitch the bird upwards because of the off-center force. During the time that the wing is moving down, a bird will angle its tail down. This generates a balancing upward force on the tail, which keeps the bird level. Without this, the bird would stall.

If a bird wants to dive, it angles its tail down without flapping, or more than normal while flapping. This forces the tail up, pitching the bird down, and changing the attack angle of the wing -- and the bird begins to drop, gaining speed. (If it needs to shed that speed, it will cup its wings, deliberately making them stall by increasing turbulence. Airliners do the same thing with the structures on the front of the wing used during landing. They're sometimes called "air brakes" but a better term is "spoiler" because they spoil the normal efficiency of the wing. The increased turbulence this causes is why it gets louder inside the jet while the spoilers are extended.)

If at the bottom of the dive the bird wants to angle back up into a climb, it will raise its tail. This forces the tail down, pitching the bird up again.

Turns are even more interesting. If the bird wants to change yaw, it doesn't have a direct way to do that since it has no rudder. Instead, it will roll into a bank in the direction it wants the yaw to change, effectively turning pitch partially into yaw through angled flight. So, here in detail is how a bird performs a starboard turn:

1. Warp the tail to roll to the right. The right side of the tail angles up and the left side down. This causes downward force on the right side and upward force on the left side. While the tail is warped a constant amount, the roll angle will continuously change. (If held too long, the roll angle will become extreme, the wings will lose lift, and the bird will tumble from the sky. Kids, don't try this at home!)
2. Once the desired roll angle is achieved, the tail unwarps and rises. This causes a constant change of pitch, counteracted by the lesser efficiency of the wings in the turn, but also causes a change in yaw because of angled flight.
3. Once the correct yaw is achieved, the roll is reversed. The left side of the tail is raised and the right lowered; this rolls the bird port.
4. When level flight is again achieved, the tail comes back to true

A bird can perform very complicated maneuvers by making complex tail motions. For instance, if it twists its tail to lower the port (left) side, and also lowers the whole tail, the effect is a rolling dive to the starboard (right).

But the piece de resistance is in turbulence control. Just as we can feel when something brushes the hair on the back of an arm, bird can feel when individual feathers move. This is important during flight because it permits the bird to monitor the airflow over all parts of its body. If there is turbulence (usually a bad thing unless you're deliberately trying to shed energy) then it will make the feathers in that area ruffle. The bird will feel that and can flex to get rid of it by smoothing out the air flow.

No human flying machine currently has anything like that degree of control; as a result our flying machines have to compensate with vastly more power.

And birds can be made from simple and cheap material by unskilled labor, in large numbers, in a short amount of time. To a human engineer, observing birds is a very humbling experience.

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