Thursday, July 14, 2005

More on Hawk Aerodynamics: Blakeman

In yesterday's query to John Blakeman Anne mentioned an aeronautical term unknown to me: canard. Below, the Ohio biologist gives some informatioin about that term. Then he offers more thoughts about hawk flight. He writes:

From a Google dictionary:
canard \kuh-NAHRD\, noun:
1. An unfounded, false, or fabricated report or story.
2. A horizontal control and stabilizing surface mounted forward of the main wing of an aircraft.
3. An aircraft whose horizontal stabilizer is mounted forward of the main wing.
Because red-tails tend to move the outer feathers of their wings forward during soaring, the outer primaries act very much like canard wings.
The alulas, the smaller projecting feathers at the "wrist" also act as canard wings when the bird is landing, smoothing the flow of air over the wing at low speed, when it would otherwise break up and create drag and lose lift. Very complicated aereodynamics, so I will have to be careful in what and how I state things. I'll defer to any aeronautical engineer who weighs in on the subject. But the key matter is that the hawk is in continuous control and minute modification of its feather and wing attitudes. If we had a stiff, mounted red-tail in perfect soaring postion, even with perfect weight distribution, we couldn't toss the specimen off a tall building and expect it to glide with stability to a distant landing. The bird model would go immediately into a spiraling, uncontrolled crash. Not much different from the balance refinements of a ballarina. Looks easy and perfect. It's hard and

John A. Blakeman

One last note, sure to offend some, as it contradicts some sensible things we all learned back in science class. Most general science and physics books, at least older ones, have a cross-section diagram of the classic airfoil. The text states that as an airplane, or in our case, a red-tailed hawk, cuts horizontally through the air, the airstream passes cleanly and quickly along the flat underwing surface. But the air split off and sent over the arched top of the wing must shoot along faster to pass completely over this longer surface. Fluids passing fast over long surfaces reduce their pressure on those surfaces, so the pressure on the flat bottom of the wing is greater than in the faster air above. The wing is therefore thrust up, and the hawk or airplane stays in the sky.
Not really so, however. If airfoils really worked this way, how could an airplane (or hawk) ever fly upside down? Flight for both hawks and airplanes is accomplished primarily by a favorable angle of attack against the oncoming air. Air may be thin, but it's still a bit sticky and cohesive, just as water is. Tilt the wing into the moving air and the lower edge simply bounces off the viscous air, quite in the manner of skipping a stone across the pond.

So when you see the aesthetic airfoil configuration of a red-tailed hawk's wing, forget what your eighth-grade science teacher tried to teach you. The hawk's airfoil wing is shaped that way to accommodate bones, muscles, and feathers, not to cause air to shoot faster over the top of the wing.

Nonetheless, the dynamic shaping of a red-tail's wings when soaring is one of the most beautiful aspects in all of nature. I am privileged to see all of this as my falconry red-tail, Savanna II, flies in my close presence. I get to feel the shwoosh of her air as she takes off from my fist. When landing, I see her extended allulas and primaries and feel the final downthrust of wind as she artfully collapses on my gloved fist. I've experienced this many thousands of times, but it's always marvelous.

It's one thing to ponder the wings of some early Renaissance Italian painter's angel in a great museum, or to head off to the museum's sculptures of antiquity and find a marble eagle. But none of these match the provided high art of red-tails in life – now even in the heart of Manhattan. Give thanks, all. Our birds are special.


John A. Blakeman