Thursday, July 14, 2005

Hawk Aerodynamics

Anne, a reader of this website, saw a picture of Charlotte on Lincoln's website and hoped that John Blakeman would comment on it. I forwarded her query, and he sent a long response. Below is the first half. I'll post the remainder tomorrow.

Dear Marie,
There is presently on Lincoln's site an excellent photograph of
Charlotte flying (dated 6.11.05). [It] is great because you can see the curvature of her wing tops, the leading edge, & it makes the physics of 'lift' so clear. Everyone is surely focused on those eyasses and their amazing development but could you ask John Blakeman (at some point when he has a moment) to address the specifics of her wings and flight? I'm particularly interested in the wing tips with separated bending feathers. I've read that they are anti-stall at low speeds (like a canard?) but I don't understand how this works.
Thanks much,

Anne, and everyone,
Some comments on the red-tail’s wings and flight feathers.
The very first thing to understand about a soaring red-tail is that its wings are not locked or set into position, although they appear to be. When soaring, the bird appears to be flying with great ease. That it is, as it isn't flexing its strong thoracic flight muscles, consuming energy. But it's wings and feathers aren't locked into position, either. The bird is presumed to have pressure- and position-sensing nerves at the base of all of the large flight feathers, continuously sending flight information to the brain. If a gust of wind strikes the bird's extended left wing, the brain (or spinal cord) instantly responds with signals to feather-position muscles, telling them to properly reposition.
If possible, with binoculars or a spotting scope, try to close in on a slowly soaring red-tail. You will note thousands of tiny, rippling wing feather readjustments. For a red-tail (and other raptors, too), staying in the air requires continuous, microscopic tweaking of feather positions, accomplished by hundreds of small muscles attached to the flight feathers.
We've all folded a sheet of paper into a glider and given it a successful toss. But if we were to make any sort of model red-tail and give it a toss, it would spiral to a quick crash. Flying for a red-tail is a markedly active endeavor. Whether just soaring with set wings (but not set feathers), or alternately in muscular flapping flight, the bird is actively adjusting feather positions and attitudes (angles of attack).
Right now, as we watch the two Trump Parc eyasses begin to extend and pump their wings, keep all of this in mind. Of course, the primary purpose of these exercises is to strengthen the large flight muscles connecting the wings with the breastbone. But the birds are also beginning to develop and refine their feather control nerve reflexes. When an eyass jumps into the air for the first time, it has to have some literal feeling of where its feathers are and its ability to effectively readjust them. When you see the clumsiness of the eyass's first flights, laugh if you will (I do). But understand that the bird is still learning new nerve reflexes. You excited your parents when, for the first time, you stood up and staggered off for a few erect steps. Just as humans must minutely adjust leg and torso muscles merely to stand, red-tails must do the same to wing and tail muscles merely to soar.
About the long, end-of-the-wing primary feathers. Yes, these finger-like, projecting feathers are spread apart when the hawk soars. In a dive or in rapid flight, they are pulled together to form an extended single wing surface. In the spread, open position, each primary feather acts individually like the long wings of an albatross. The open-fingered primaries extend the effective lengths of the wings, yielding great lift. But because a soaring bird moves slowly through the air, the outer wing surface would stall out if the air stream had to pass over the entire outer wing surface. Instead, it can pass slowly and individually over each feather finger without stalling. It's just good, adaptive aerodynamics.
The extended primaries also assist when the hawk is landing. As it slows to take a perch, the bird has virtually no forward air speed, and therefore very little lift. Without the extended, high-lift outer wing feathers, these big birds would tend to crash when landing.
One other set of feathers to note – but usually only for an instant when landing or taking off – are the alulas (“AL-u-lahs,” singular “AL-u-la”), the small, short feathers that lay over the “wrist” of the wing, where a thumb would attach if birds had one. In normal flight, either flapping or soaring, the alulas are held flat on the top of the wing. But when great lift is required under stall conditions, usually when landing and taking off, the alulas are thrust up to smoothly direct air back over the top of the slowly moving wing.
John A. Blakeman

[more Blakeman on aerodynamics tomorrow]

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