Flight
Movement through the atmosphere or space, sustained by aerodynamic reaction or other forces. Animal flight includes gliding and flapping flight. Flapping flight in vertebrates was probably preceded by gliding; in insects it may have originated by leaping and gliding, by surface skimming on water, or (if small enough) by passive floating in the air. Flying insects show greater variation than flying vertebrates, and their flight spans a wider range of Reynolds numbers, which is the ratio of inertial forces to viscous forces in the flow. Flight of tiny insects is in the lower range of Reynolds numbers, where viscous forces are dominant, whereas large insects and vertebrates operate in the higher range, where inertial forces are important.
Flight is very expensive in terms of energy cost per unit time. However, flying animals can travel several times faster than nonflying ones, and the cost of carrying a unit of weight of an animal through a unit distance (cost of transport) is lower for flight than for running, but higher than for swimming.
The flight characteristics of large insects and vertebrates can be understood by aircraft aerodynamics. In steady level flight, an animal and an aircraft must generate forces that support weight against gravity and provide propulsive thrust against drag forces. The forces acting on an airfoil (the shape of the cross section of the wings) moving through the air depend upon the flow pattern around it. Because of the asymmetric profile on an airfoil, the air flowing over the upper surface travels farther and flows faster than air passing underneath. According to Bernoulli's principle, pressure falls when speed rises in a moving fluid, resulting in a pressure difference between the upper and lower sides of the airfoil (illus. a). This pressure difference has to disappear gradually toward the wingtips, and some air will flow upward around the wingtips. The air moves downward behind the wing as trailing vortices (illus. b), and the reaction of this momentum flow is experienced by the wing as lift. The stronger the vortex, the greater the lift generated, but with some energy loss to drag. The lift force is responsible for weight support (its vertical component) and thrust or drag (its horizontal component). See also Aerodynamic force; Airfoil; Bernoulli's theorem.
Aerodynamics of flight. (a) Airflow around a typical wing profile. (b) Pressure distribution around a typical wing profile. (c) The difference in pressure disappears toward the wingtips as trailing vortices in the wake. (After M. Brooke and T. Birkhead, eds., Cambridge Encyclopedia of Ornithology, Cambridge University Press, 1991)
Aerodynamics of flight. (a) Airflow around a typical wing profile. (b) Pressure distribution around a typical wing profile. (c) The difference in pressure disappears toward the wingtips as trailing vortices in the wake. (After M. Brooke and T. Birkhead, eds., Cambridge Encyclopedia of Ornithology, Cambridge University Press, 1991)
Energy must be expended to generate the trailing vortices in the wake and to overcome friction on the wing and body surfaces. These energy losses are experienced as drag forces, which act parallel to the direction of movement of the airfoil. Since drag is a retarding force, the animal must either descend (glide) through the air at such an angle that a component of its weight balances the drag, or do mechanical work with its flight muscles (flap its wings) to overcome it. The rate at which this work is done is the mechanical power required to fly, and it equals speed times drag (measured in watts). The flight muscles also produce heat when they contract, so the total metabolic power expenditure is the sum of this heat loss and the mechanical power. The metabolic power is estimated to be four to five times the mechanical power, and is dependent on the size of the animal. See also Work.
Compared with active flight, gliding flight is very inexpensive, and is found in a wide range of animals, such as squirrels, marsupials, lizards, frogs, fishes, and even one snake. It is the main component in soaring flight used by many birds and some bats, when the animals use thermals or updrafts. Gliding in birds costs only two to three times the basal metabolic rate, because the flight muscles do not perform any mechanical work other than for stability and control of movements, and produce mostly static forces to keep the wings down on the horizontal plane, opposing the aerodynamic force.
When gliding, the wings of the animal leave behind a continuous vortex sheet that rolls up into a pair of vortex tubes (wingtip vortices), as in fixed-wing airplanes (illus. b). The lift produced balances the animal's weight, but potential energy must be used to overcome the drag and the animal loses height. An animal gliding at steady speed descends at an angle to the horizontal (glide angle), established by the ratio of lift to drag (glide ratio). The best glide ratios in birds range from 10:1 to 15:1 for vultures and birds of prey and reach 23:1 in the wandering albatross, whereas modern gliders can achieve 45:1. An animal cannot glide more slowly than its stalling speed, which in birds can be reduced by splaying the wingtip primaries, or by spreading the alula (a digit of the wing) at the wrist, or both. An animal can increase its gliding speed by flexing the wings and reducing the wing area.
Hovering flight represents the most expensive form of animal flapping flight. The essence of hovering is to produce a vertical force to balance body weight. The wake consists of a chain of vortex rings continuously shed during the wing strokes. In hummingbirds and insects, lift is produced during both the downstroke and upstroke of the wings (symmetrical hovering), and two vortex rings are produced during each wing stroke. In other hovering animals the wings are flexed during the upstroke (asymmetrical hovering), and the rings are produced during the downstrokes only. See also Aerodynamics; Aves.
Flight is very expensive in terms of energy cost per unit time. However, flying animals can travel several times faster than nonflying ones, and the cost of carrying a unit of weight of an animal through a unit distance (cost of transport) is lower for flight than for running, but higher than for swimming.
The flight characteristics of large insects and vertebrates can be understood by aircraft aerodynamics. In steady level flight, an animal and an aircraft must generate forces that support weight against gravity and provide propulsive thrust against drag forces. The forces acting on an airfoil (the shape of the cross section of the wings) moving through the air depend upon the flow pattern around it. Because of the asymmetric profile on an airfoil, the air flowing over the upper surface travels farther and flows faster than air passing underneath. According to Bernoulli's principle, pressure falls when speed rises in a moving fluid, resulting in a pressure difference between the upper and lower sides of the airfoil (illus. a). This pressure difference has to disappear gradually toward the wingtips, and some air will flow upward around the wingtips. The air moves downward behind the wing as trailing vortices (illus. b), and the reaction of this momentum flow is experienced by the wing as lift. The stronger the vortex, the greater the lift generated, but with some energy loss to drag. The lift force is responsible for weight support (its vertical component) and thrust or drag (its horizontal component). See also Aerodynamic force; Airfoil; Bernoulli's theorem.
Aerodynamics of flight. (a) Airflow around a typical wing profile. (b) Pressure distribution around a typical wing profile. (c) The difference in pressure disappears toward the wingtips as trailing vortices in the wake. (After M. Brooke and T. Birkhead, eds., Cambridge Encyclopedia of Ornithology, Cambridge University Press, 1991)
Aerodynamics of flight. (a) Airflow around a typical wing profile. (b) Pressure distribution around a typical wing profile. (c) The difference in pressure disappears toward the wingtips as trailing vortices in the wake. (After M. Brooke and T. Birkhead, eds., Cambridge Encyclopedia of Ornithology, Cambridge University Press, 1991)
Energy must be expended to generate the trailing vortices in the wake and to overcome friction on the wing and body surfaces. These energy losses are experienced as drag forces, which act parallel to the direction of movement of the airfoil. Since drag is a retarding force, the animal must either descend (glide) through the air at such an angle that a component of its weight balances the drag, or do mechanical work with its flight muscles (flap its wings) to overcome it. The rate at which this work is done is the mechanical power required to fly, and it equals speed times drag (measured in watts). The flight muscles also produce heat when they contract, so the total metabolic power expenditure is the sum of this heat loss and the mechanical power. The metabolic power is estimated to be four to five times the mechanical power, and is dependent on the size of the animal. See also Work.
Compared with active flight, gliding flight is very inexpensive, and is found in a wide range of animals, such as squirrels, marsupials, lizards, frogs, fishes, and even one snake. It is the main component in soaring flight used by many birds and some bats, when the animals use thermals or updrafts. Gliding in birds costs only two to three times the basal metabolic rate, because the flight muscles do not perform any mechanical work other than for stability and control of movements, and produce mostly static forces to keep the wings down on the horizontal plane, opposing the aerodynamic force.
When gliding, the wings of the animal leave behind a continuous vortex sheet that rolls up into a pair of vortex tubes (wingtip vortices), as in fixed-wing airplanes (illus. b). The lift produced balances the animal's weight, but potential energy must be used to overcome the drag and the animal loses height. An animal gliding at steady speed descends at an angle to the horizontal (glide angle), established by the ratio of lift to drag (glide ratio). The best glide ratios in birds range from 10:1 to 15:1 for vultures and birds of prey and reach 23:1 in the wandering albatross, whereas modern gliders can achieve 45:1. An animal cannot glide more slowly than its stalling speed, which in birds can be reduced by splaying the wingtip primaries, or by spreading the alula (a digit of the wing) at the wrist, or both. An animal can increase its gliding speed by flexing the wings and reducing the wing area.
Hovering flight represents the most expensive form of animal flapping flight. The essence of hovering is to produce a vertical force to balance body weight. The wake consists of a chain of vortex rings continuously shed during the wing strokes. In hummingbirds and insects, lift is produced during both the downstroke and upstroke of the wings (symmetrical hovering), and two vortex rings are produced during each wing stroke. In other hovering animals the wings are flexed during the upstroke (asymmetrical hovering), and the rings are produced during the downstrokes only. See also Aerodynamics; Aves.
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