Principles of flight
Pigeon like other birds can fly using the motion of air, a science called aerodynamics. A bird flies on the principle of indirect movement. It moves the air, which by its displacement, moves the bird. Air displaced by the beating of wings, sets up currents that keep the animal aloft and move forward, resulting in flight.
A plane surface moved through the air in a direction inclined at an angle to its plane is known as an aerofoil. Unlike the fixed wings of an airplane, the wings of a bird function both as an airfoil (lifting surface) and as a propeller for forward motion. The avian wing is admirably suited for these functions, consisting of a light, flexible airfoil. The primaries, inserted on the hand bones, do most of the propelling when a bird flaps its wings, and the secondaries along the arm provide lift.
Pigeon also has the ability to alter the area and shape of its wings and their positions with respect to the body. These changes in area and shape cause corresponding changes in velocity and lift that allow a bird to maneuver, change direction, land, and take off. Moreover, a bird’s wing is not a solid structure like a conventional airfoil such as an airplane wing, but allows some air to flow through and between the feathers.
Obviously, the aerodynamic properties of a Pigeon’s wing in flight-even in nonflapping flight- are vastly more complex than those of a fixed wing on an airplane or glider. Nevertheless, it is instructive to consider a bird’s wing in terms of the basic performance of fixed airfoil. Although a bird’s wing actually moves forward through the air it is easier to think of the wing as stationary with the air flowing past. The flow of air produces a force, which is usually called the reaction. It can be resolved into two components: the lift, which is a vertical force equal to or greater than the weight of the bird, and the drag, which is a backward force opposed to the bird’s forward motion and to the movement of its wing’s through the air.
When the leading end of a symmetrically streamlined body cleaves the air, it thrusts the air equally upward and downward, reducing the air pressure equally on the dorsal and ventral surfaces. No lift results from such a condition. There are two ways to modify this system to generate lift. One is to increase the angle of attack of the airfoil, and the other is to bend its surface. Either change increases lift at the cost of increasing drag.
When the contour of the dorsal surface of the wing is convex and the ventral surface is concave (a cambered airfoil), the air pressure against the two surfaces is unequal because the air has to move faster over the dorsal convex surface relative to the ventral concave surface. The result is reduced pressure over the wing, or lift. When the lift equals or exceeds the bird’s body weight, the bird becomes airborne. The chamber of the wing varies in birds with different flight characteristics; it also changes along the length of the wing. Camber is greatest close to the body and decreases toward the wing tip. This change in camber is one reason that the proximal part of the wing generates greater lift than the distal part.
If the leading edge of the wing is tilted up so that the angle of attack is increased, the result is increased lift- up to an angle of about 15 degrees, the stalling angle. This lift results more from a decrease in pressure over the dorsal surface than from an increase in pressure below the airfoil. If the smooth flow of air over the wing becomes disrupted, the air flow begins to separate from the wing because of the increased air turbulence over the wing. The wing is then stalled. Stalling can be prevented or delayed by the use of slots or auxiliary airfoils on the leading edge of the main wing. The slots help to restore a smooth flow of air over the wing at high angles of attack and at slow speeds.
The bird’s alula has the effect, particularly during landing or takeoff . Also the primaries act as a series of independent, overlapping airfoils, each tending to smooth out the flow of air over the one behind.
Another characteristic of an airfoil has to do with wing-tip vortexes (Any spiral motion with closed streamlines is vortex flow.)- eddies (a current or trend, as of opinion or events, running counter to the main current) of air resulting from outward flow of air from under the wing and inward flow from over it. This is induced drag. One way to reduce the effect of these wing-tip eddies and their drag is to lengthen the wing, so that the tip vortex disturbances are widely separated and there is proportionately more wing area where induced drag is greatest. The ratio of length to width is called aspect ratio.
Wing loading is another important consideration. This is the mass of the bird divided by the wing area. The lighter the loading, the less power is needed to sustain flight.
Modes of flight
Flapping flight involves a complex, screw-like motion of the wing, downwards and forwards then upwards and backwards, more rapid upwards than downwards. The action of the wings differs during take off or landing from that in sustained flight. In the flight of a pigeon at a medium speed of 10 m/s the wings beat at a frequency of about 5 Hz . The body is nearly horizontal. At the top of the down stroke the wings are fully extended and then move downwards through an arc of 90 degree (figs. 2a-c). Before the upstroke the elbow is first flexed and the wrist and the manus supnated. The primary feathers are then turned backwards (fig. 2d and 2e) producing a feathered ‘upstroke’ with a large positive pitch angle to the direction of locomotion and a thrust as the primaries are swung backwards. At the end of the upstoke the elbow and wrist are extended and the wrist pronated, spreading the primaries for the next downstroke. These feathers separate as the wing descends, an arrangement that increases their lift in the same way as the leading edge slots used in aeroplanes.
In this type of flight, bird holds their wings spread, motionless and glide for a considerable distance without flapping them. The gliding flight can be seen for a short time.
Soaring flight or sailing flight
Pigeon economize in the energy needed for flapping flight by making skilful use of the possibilities presented by movement of the air. Theoretically the bird can use three types of air movement: (1) ascending currents, (2) variations in the wind velocity at any level, (3) differences in wind velocity at different levels and as a result at high level, describes great circles without any movement of the wings. The bird rises without any loss of kinetic energy.
Take off and landing
At take off the bird has to acquire sufficient forward momentum to provide lift, and yet must leave the air sufficiently undisturbed for subsequent beats to be effective. In many birds the jump provided by the legs is adequate for the take off.
The first beats are usually very large, beginning with the wings above the back and held at such an angle as to produce a large forward component. The wings may be heard actually clapping together in pigeons.
In take off of a pigeon, the body is held vertically. The action of the wing on the down stroke is as in sustained flight but on the upstroke the wing is first adducted, folded and flexed and supinated at the wrist, by the actions of supracoracoideus and other muscles. A very rapid flick then follows, produced by upward and forward rotation of the humerus, extension of the wing and pronation of the humerus. These movements, produced largely by the triceps and other extensors, result in a forward component. After jumping the bird is thus able to acquire sufficient forward velocity to take off from flat ground.
Landing is also a delicate operation, especially since it often involves coming to rest suddenly on a branch. This is achieved by lowering and fanning out the tail, which thus acts as a flap, providing both lift and braking. The legs are then lowered; often one further wing stroke is given to bring the bird forward to drop onto the perch. The adjustment of braking in such a way as to prevent stalling involves a very special system of co-ordination.
Wing tips, slots (a narrow, elongated depression), and camber
A pointed wing tends to stall first at its tip and is therefore only suitable for fast fliers like pigeon. Such birds show great development of the hand feathers, producing a narrow wing.
The condition of the air around the wing is of first importance for the maintenance of lift; if there is not a smooth stream over the upper and under surfaces that air becomes turbulent, and the aerofoil stalls. This tends to happen either if the speed falls too low or if the angle of the wing relative to the line of motion increases above about 20 degree. Turbulence is mitigated, however, by the provision of openings, known as slots, which let through part of the air and provide the necessary smooth stream. The spaces that occur between the feathers, especially towards the wing tip, almost certainly function as slots. Probably the arrangement provides a series of such apertures, giving a very efficient high-lift device.
The shape of the wings have a very important influence on the air stream. In pigeon there is a thick leading edge and a thinner trailing edge. This arrangement directs the air stream over the upper surface of the wing in such a way as to provide an extra lift by creating a ‘suction zone’ of reduced pressure. However, high camber, like low aspect ratio, reduces the speed of the bird.
The feathers are held by an elaborate system of tendons and they are allowed to twist only when the wing is being raised and the barbs of the feathers themselves are so arranged that they open like the vanes of a blind when under pressure from above, but close when the pressure is from below.
The whole upward movement is usually faster than the downward one. Before the wing tip has reached its highest point the upper arm is already beginning to descend and in this way the line of flight is maintained almost straight and does not follow a wavy path.
In general the wing is a very liable system and regulates itself automatically with changes in the aerodynamical forces. This regulation is produced partly by feather plasticity and joint mobility, with participation of reflex muscular adjustments that are little understood.