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| Origin of
Bird Flight Exactly how birds acquired the ability to fly has baffled scientists for years. Archaeopteryx provided a starting point for speculation. Built like a dinosaur, but with wings, scientists guessed at how a hypothetical ancestor might have taken flight. Some scientists support the arboreal hypothesis and suggest that the ancestors of Archaeopteryx lived in trees and glided into flapping flight. But others argue that the claws of Archaeopteryx weren't suited to climbing. So, others support the cursorial hypothesis and suggest that these ancestors used their long, powerful legs to run fast with their arms outstretched, and were at some point lifted up by air currents and carried into flapping flight. Studying living animals can throw light on their evolutionary past. Ken Dial of the Flight Lab at the University of Montana noticed the ability of gamebird chicks to escape danger by scrambling up vertical surfaces. The chicks first run very fast, flapping their immature, partially feathered wings, frantically creating enough momentum to run up a vertical surface to safety. Could this survival instinct be the origin of flight? Bird Flight Flight requires lift, which results when an air stream passing over wing must travel further (and faster) than the air stream passing under a wing. The slower moving air under the wing 'pushes' against the bottom of the wing with greater force than the faster moving air above the wing & this generates lift. The lift generated is influenced by a bird's velocity Why does the slower moving air generate more pressure against the wing than the faster moving air? In calm air, the molecules are moving randomly in all directions. However, when air begins to move, most (but not all) molecules are moving in the same direction. The faster the air moves, the greater the number of air molecules moving in the same direction. So, air moving a bit slower will have more molecules moving in other directions. In the case of a wing, because air under the wing is moving a bit slower than air over the wing, more air molecules will be striking the bottom of the wing than will be striking the top of the wing. This is called the Bernoulli effect & this creates lift! Wings also provide lift through Newton's Third Law of Motion which states that for every action there is an equal and opposite reaction. As the wing moves though the air, the lower surface of the wing deflects some of the air downward. As Newton's Third Law of Motion explains, an additional force is generated. The deflected airflow underneath the wing is the action. The reaction is that the wing moves in the opposite direction (in this case, upwards). This means that the development of low pressure above the wing (Bernoulli's Principle) and the wing's reaction to the deflected air underneath it (Newton's third Law) both contribute to the total lift force generated. When the curvature over the top becomes greater by increasing the angle of attack), the air moves even faster over the top of the wing and more lift is generated. Eventually, however, if the angle of attack becomes too great, the flow separates off the wing and less lift is generated. The result is stalling. For birds, the optimum angle of attack is typically about 3 - 5 degrees. Birds also tend to stall at low speeds because slower moving air may not move smoothly over the wing. Of course, a wing moving through the air is opposed by friction & this is called drag. The two main types of drag acting on birds are pressure (or induced) drag & friction (or profile) drag. Induced drag occurs when the air flow separates from the surface of a wing, while friction drag is due to the friction between the air and bird moving through the air. Friction drag is minimized by a wing's thin leading edge (wings 'slice' through the air). Induced drag occurs at low speeds and at higher speeds as, at wing tips, air moves from the area of high pressure (under the wing) to the area of low pressure (top of the wing). As wings move through the air, this curling action causes spirals (vortices) of air which can disrupt the smooth flow of air over a wing (and reduce lift). The amount of drag varies with a bird's mass (increased mass = increased friction drag) & speed (increased speed = increased induced drag at the wing tips), and with a wing's surface area & shape. As described below, some wing shapes help to reduce induced drag. Wing shapes vary substantially among birds. A convenient way to describe the shape of a wing is by its aspect ratio - the ratio of length to width. Among bird wings, aspect ratios vary from about 1.5 to as high as about 18. Elliptical (or 'rapid takeoff') wings have relatively low aspect ratios, while high speed wings & soaring wings have high aspect ratios. The long (or soaring) wings of birds with very high aspect ratios, like albatrosses, generate lots of lift, while the narrow, pointed shape helps reduce drag while gliding (because the small area of the pointed tip minimizes pressure differences and, therefore, turbulence at the wing tip). High speed wings, like those of falcons, swallows & swifts, have relatively high aspect ratios. These narrow, tapering wings can be flapped rapidly to generate lots of speed with minimal drag (because, again, the small area of the pointed tip minimizes pressure differences and turbulence at the wing tip). High-lift wings have lower aspect ratios & there are spaces between the feathers at the end of the wing. These 'slots' help reduce drag at slow speeds because the separated tip feathers act as 'winglets' and spread vorticity both horizontally and vertically. Wings with low aspect ratios (elliptical wings), like those of many songbirds, woodpeckers, pheasants & quail, permit sharp turns while flying among trees & shrubs. Another important factor that influences a bird's flying ability is wing loading - the weight (or mass) of a bird divided by wing area (grams/total wing area in square centimeters). Birds with low wing loading need less power to sustain flight. Birds considered to be the 'best' flyers, such as swallows & swifts, have lower wing loading values than other birds.
The high
wing loading of birds like grebes, loons, and
swans means that it's more difficult for them to
generate sufficient lift to take-off. That's why
these birds often run along the surface of a lake
for some distance before taking flight. They must
generate enough speed to generate enough lift to
get their relatively heavy bodies into the air!
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