Definition: Sustained and powered airborne travel by birds, insects, or mammals through the use of wings.
Significance: Animal flight, particularly that of birds, is important to humans, who first learned and dreamt of flight by studying flying animals. The study of animal flight remains a source of information for understanding and design of flying vehicles.
History
Animals have been flying for millions of years. The first flying animals were insects, which appeared approximately 350 million years ago. From that time, flight evolved separately among three other kinds of animals. There are four types of animals capable of flight: insects, birds, bats,and pterosaurs, the last of which are extinct. Each of these groups developed the ability to fly independently, and, in many cases, different species in each group separately evolved the ability to fly. Additionally, some mammals and birds developed the ability to glide but not to fly.
Unlike aircraft, which gain lift with wings that are either fixed or rotating, animals almost universally accomplish flight by flapping their wings. The flapping motion provides not only lift but also thrust and is referred to as ornithoptic propulsion. Both animals and manufactured aircraft using this method of achieving flight are commonly called ornithopters.
Basis of Animal Flight
The same aerodynamic laws that apply to man-made aircraft also apply to animals, and animal flight is divided into three categories, based on how it is attained. Gliding animals do not fly but trade potential energy (height) for kinetic energy (speed) to remain aloft. Gliding is only useful for small distances. Flying animals use their wings to generate both lift and thrust to remain in the air. Soaring animals, a cross between gliders and fliers, usually use wing movement only for takeoffs and landings, generally relying on subtle changes in wing geometry, thermals, and prevailing winds to gain altitude. Many large birds soar rather than fly. For an animal to remain in level and steady flight, the lift that it generates with its wings must be equal to its weight, whereas the thrust it creates must be equal to its aerodynamic drag. All flying animals generate both lift and thrust by the same method: flapping their wings.
In flapping, or ornithoptic, flight, the wing must produce lift and thrust at the same time. However, lift and thrust does not have to be produced constantly. During a single wing beat, lift and thrust vary. As long as the average lift and drag over the period of the wing motion are equal to the drag and weight, respectively, this will keep the animal in level and steady flight over time.
Aerodynamics
The primary difference in the aerodynamics of aircraft and animal flight is the slower speed and smaller size of flying animals, compared to that of manufactured aircraft. This difference is characterized by a parameter called the Reynolds number, which measures the effect of aerodynamic inertial forces compared to aerodynamic viscous, or frictional, forces. The lower the Reynolds number is, the more important the effects of fluid viscosity, or friction, become. The Reynolds value of most aircraft, whether general aviation craft, commercial airliners, or fighters, is in the millions. For birds and insects, however, the Reynolds value is usually 100,000 or less and is sometimes even less than 1,000 for very small insects. For flying objects with a Reynolds value greater than 100,000, thick, curved, or cambered, airfoils work best, whereas those with Reynolds values of less than 100,000 tend to work best with thinner, flatter airfoils.
This difference is demonstrated by examining the value of the lift-to-drag ratio as a function of Reynolds number for a number of given airfoils. Most fat airfoils have a higher lift-to-drag ratio at high Reynolds numbers, whereas thin airfoils have a higher lift-to-drag ratio at low Reynolds numbers. This fact was originally discovered during World War I by the Germans, who determined that fat wings worked better on their faster biplane fighters. Likewise, as an animal’s speed and size increases, the shape of its wings changes to reflect the increase in Reynolds number. Thus, large or fast birds, such as pigeons and falcons, have wing cross sections that look surprisingly similar to those of modern aircraft wings.
Another important aspect of speed or Reynolds number is how the roughness of a wing affects flight efficiency. The faster an object flies, the smoother the wing needs to be for maximum lift and minimum drag. At low Reynolds numbers, however, the lift dramatically drops for smooth wings, whereas it does not for rough wings. Thus, smooth manufactured wings do not operate as efficiently as rough wings, whether the animal wings are roughened by feathers, scales, or fur. Rough animal wings are most efficient for low speeds. The motion of the feathers and fur allows animals to sense when their wings are about to stall.
Whereas avian biomechanics are complex, insect bio-mechanics are relatively simple and easy to analyze. This simplicity lends itself well to duplication using modern mechanical technology. In the simplest of insect wings, wing motion is controlled by contraction of interior muscles. The motion in this case is indirect; other insect systems have a direct relationship between muscle movement and wing motion. Without examining complex muscle mechanics, however, one can quickly determine the limit to a flying animal’s size by examining the weight in relation to the size.
Scaling determines whether ornithoptic propulsion is efficient for a given weight and length scale. The length scale is a measure of an animal’s size, in either length or wingspan. A flier’s weight is proportional to the length scale cubed, whereas the wing area is proportional to length scale squared. This is known as the cube-square law. Thus, one can deduce that the wing loading (weight divided by wing area) is proportional to the length scale. As size increases, the wing loading must also increase. Eventually, the wing loading will be too great for the bones and muscles of an animal to withstand, and any animal above this size will be unable to fly.
The required power or energy input for a given weight can be determined from commonly known aerodynamic relations and can be shown to increase as the 7/2 power of the length scale. Thus, if the wingspan of an animal doubles, the power required to fly must increase by more than ten times. Based on muscle-mass arguments stating that the amount of energy available is related to the amount of muscle mass, the power available to flying animals can be shown to quadruple as the wingspan doubles. Thus, as the size of a flying animal increases, required power will soon overtake available power, not only limiting the animal’s maximum possible weight but also decreasing the animal’s ability to take off, climb, and hover. Hence, larger flying animals tend to use soaring as the primary flight mode instead of powered flapping.
The ratio of unsteady lift, derived from flapping, compared to steady lift, derived from forward motion, shows that flapping frequency can be directly related to the size and weight of a flying animal. Using the flapping frequency as an approximate measure of this ratio and comparing it with the flier’s length scale, it is shown that the frequency is inversely proportional to this length scale, which can also be related to the Reynolds number. Thus, as the Reynolds number increases, or as the speed or size of a flier increases, the frequency at which the wings flap decreases. Eventually, the flapping frequency will decrease to the point where the wings will be stationary, indicating that there is a limit to the efficiency of flapping as a flight mechanism, as size increases. On the contrary, as weight decreases, there is a limit below which flapping is a very efficient flight mode. This principle has direct applications to the development of manufactured microaerial vehicles: Instead of shrinking down conventional aircraft designs to a smaller scale, it may be more practical to design miniature aircraft that use flapping wings instead of fixed wings for lift generation and engine-propeller combinations for thrust generation. The efficiency of flapping flight for small birds may also be one reason why wings evolved over propellers for thrust generation. As the size of a flying animal decreases, the generation of unsteady lift becomes more important to its flight. This is especially true for insects that derive much of their lift from unsteady effects alone.
Bird Flight
Birds are by far the best-known animal fliers. There are more than 9,000 species of birds, of which only a handful, such as the penguin, kiwi, ostrich, and emu, are flightless.
Birds are characterized as warm-blooded, egg-laying vertebrates with feathered wings and strong hollow bones, many of which are fused together to increase strength and decrease weight. They have powerful muscles that allow for flight and require large amounts of food for energy. Birds evolved from dinosaurs approximately 150 million years ago.
Most birds appear to have evolved flight from ground-up gliding, used both to catch prey and to evade predators. Wings may also have developed as an aid to increase leaping distances and as a display to attract mates. Two scenarios for the evolution of flight include the ground-up scenario, in which running and leaping animals evolved wings, and the tree-down scenario, in which tree-dwelling creatures evolved wings to move from tree to tree. In either case, the ability to survive and gain access to unoccupied niches appear to be the greatest reasons for the development of bird flight.
Although it appears that modern birds evolved from dinosaurs, birds are not related to the now-extinct pterosaurs, or flying archosaurian reptiles. Pterosaurs were lizards and appeared to be proficient fliers with wing structures, similar to those of bats, that had an outstretched membrane over a thin upper limb. They had large heads that may have assisted their flight stability. Their wingspans ranged from a few inches to almost 40 feet. The pteranodon had a wing-span of up to 25 feet but weighed only 25 pounds. Due to their large sizes, most pterosaurs were probably soaring animals that relied on thermals to fly at high altitudes.
The oldest known bird is the archaeopteryx, named for the Greek “ancient wing,” which lived around 150 million years ago. It had a wingspan of approximately 18 inches and weighed about 1 pound. With its feathers and beak, it had similarities to modern birds, and with its teeth and clawed wings, it had similarities to dinosaurs.
There is a wide variety of flying birds, including the small hovering hummingbird, the swift falcon, and the lumbering condor. Each adopted a mode of flight suited to its evolutionary niche. There are several differences between flying and flightless birds that illustrate requirements for successful bird flight. Flightless birds tend to have shorter, symmetrical wings, whereas flying birds have long, cambered wings that produce substantially more lift. To keep their weight low, flying birds tend to have fewer feathers than their grounded counterparts. Flying birds also have longer tails, or keels, that aid in flight stability.
Birds occupy almost every low-speed flight niche known. They are adept fliers, using their wing muscles and feathers to control the distribution of lift over the wings.
This allows them to easily adjust to changes in ambient flight conditions, such as gusts or downdrafts. Their whole bodies are designed for flight. They have strong, hollow bones that minimize weight and withstand impacts. They have unique single-path pulmonary systems that constantly feed fresh oxygen to the lungs to maximize energy. They use their heads, tails, and feet to help control flight.
Variations across the bird species detail how well designed birds are for their particular niches. The hummingbird, for example, is well known for its ability to hover in one place in flight, beating its wings at an amazing 60 or more beats per second while feeding on the nectar of flowers. Although other birds, such as kestrels, terns, and gulls, can also hover, only hummingbirds can fly sideways and backward in hovering flight.
Insects
Insects are both the oldest and generally smallest of flying animals. The first winged insects appeared some 350 million years ago and were the first creatures to fly on Earth. There are one million species of insects, many of which fly. They range in size from barely visible to almost 1 foot in wingspan.
Insects are invertebrate arthropods with a hard exoskeleton and a three-part body consisting of head, thorax, and abdomen, three pairs of jointed legs, and two antennae. The legs and wings are attached to the thorax. Most winged insects have two sets of wings, fore and aft. Most flap their wings in synch, whereas a few, such as the dragonfly, flap their fore and aft wings asynchronously. In the former case, synchronous wing movement appears to be limited to approximately 200 beats per second, because the wing motion is related directly to the nerve inputs to the muscles. For asynchronously flapping winged insects, beat frequencies of more than 1,000 beats per second have been recorded, because the myogenic flight muscles used in asynchronous wing motion can contract more than once per nerve impulse.
Most insects cannot fly using the laws of conventional aerodynamics. Under these assumptions, lift is determined by the steady flow of air over a wing just as in aircraft flight. The wing areas of most insects are too small to obtain the required lift at their measured flight speeds, however. Much of their lift is instead derived from unsteady lift as described above. For insects, the clap-and-fling effect is used to generate the required lift. In this method, the wings are beaten together (clap) and rapidly pulled apart (fling). The air rushing in to fill the void develops a fast-moving vortex over the top of the wing that generates a large amount of unsteady lift. Insects must beat their wings rapidly and repeatedly to generate lift. Bees flap their wings more than 100 times per second. The common housefly beats its wings more than 20,000 times per minute, or about 300 times per second. A midge of the genus Forcipomyia has a measured wing-beat frequency of more than 1,000 beats per second.
The wings of most insects are less flexible than those of birds or bats. Most insects change direction and speed primarily by altering the motion and frequency of their wing beats. Pitch, yaw, and roll control involve changes in wing-beat amplitude on one wing with respect to the other, lateral wing twisting, or leg and abdominal movement. Some insects can twist their wings like those of a bird to control motion, such that a large area is projected on the down-stroke and a small area is projected on the upstroke. These traits give great maneuverability to most insect species.
The number of flying insect species is enormous. Typical insect flight speeds range from 15 miles per hour for bees to 1 mile per hour for mosquitoes, and even less for smaller insects. The fastest flying insect may be the tabanid, with a flight speed estimated at 90 miles per hour; it has been observed to execute Immelman maneuvers while in flight. The Australian dragonfly can reach 36 miles per hour over short distances, outrunning most horses.
Some dragonflies have wingspans of up to 11 inches, and some butterflies have wingspans of up to 10 inches. Dragonflies have two sets of high-aspect ratio wings, and butterflies have two pairs of large low-aspect ratio wings covered with colorful, iridescent scales in overlapping rows. Lepidoptera, as butterflies and moths are known, are the only insects that have scaly wings. Flight speeds vary among butterfly species. The poisonous varieties fly more slowly than nonpoisonous varieties, because they do not have to fly as quickly to evade predators. The fastest butterflies can fly at about 30 miles per hour or faster, whereas slow-flying butterflies fly about 5 miles per hour.
Mammals
Only one mammal is truly capable of powered flight: the bat, of the order Chiroptera, a word that means “hand-wing.” All other so-called flying mammalian species do not actually fly but rather glide. Other mammals that fly by gliding include the flying squirrel and the flying lemur, neither of which actually fly and the latter of which is not actually a lemur. Bats, however, like birds, do attain true powered flight.
Bats are vertebrates with fur that bear and nurse live young. Nocturnal animals, they are found in all regions of the world except for the North and South Poles. There are more than 900 different species of bats, ranging in wing-span from the 6-inch bumblebee bat to the 6-foot flying fox. Some bats migrate, whereas others hibernate.
Because the fossil record is limited, the origin and evolution of bats remain unknown. It is believed bats appeared around fifty million years ago. Bats are related to the co-lugo, or flying lemur, but their common link is a mystery. They probably evolved from arboreal ancestors related to primates that used gliding and climbing as separate means of locomotion. The fact that the earliest bats had tails supports this assertion.
Bats are divided into two suborders based upon their method of navigation. Those of the suborder Microchir-optera use a type of sonar called echolocation to navigate and search for prey. They send off high-pitched sounds beyond the range of most human hearing. These sounds echo off surroundings and other animals, and bats use the echoes to determine the size and distance of the object. Microchiropteras include the vampire bat, the only mammal to feed exclusively on blood. Bats of the suborder Megachiroptera, such as the fruit bat, use their sense of smell to find food. Both types of bats have poor eyesight.
Bats are agile fliers. The bat wing is a membrane stretched across elongated fingers of the hand which support the distal, thrust-producing portion of the wing. Bats can change the effective airfoil cross section of the wing by moving their fingers. The fingers are extremely flexible, much like those of humans, and allow a bat to create almost any desired airfoil shape. The uropatagium, a membrane stretched between the hind limbs, helps stabilize the bat during flight and is often used to capture prey. Because gliding animals incorporate their hind limbs into their wings, this membrane is believed to have evolved from gliding.
Gliding mammals include the flying squirrel and flying lemur. Flying squirrels have a fold of skin extending from the wrist of the front leg to the ankle of the hind leg that forms a wing like gliding surface when the limbs are extended. The tail serves as a control device during glides to steer and stabilize flight. Colugos, or flying lemurs, arboreal climbers and gliders with lateral skin membranes and large, webbed, clawed feet, are found in certain regions of the Pacific Rim. They resemble large flying squirrels. Like bats, they have a short tail, which is used for stability and is connected to the hind limbs by skin folds.
