1. INTRODUCTION

1.1 BIOMIMETICS

Biomimetic refers to human-made processes, substances, devices, or systems that imitate nature. The art and science of designing and building biomimetic apparatus is called biomimetics. The term itself is derived from bios, meaning life, and mimesis, meaning to imitate. This new science represents the study and imitation of nature’s methods, designs, and processes. While some of its basic configurations and designs can be copied, many ideas from nature are best adapted when they serve as inspiration for human-made capabilities. Nature has always served as a model for mimicking and inspiration for humans in their desire to improve their life.

Now-a-days biomimetics is of special interest to researchers in nanotechnology, robotics, artificial intelligence (AI), the medical industry, and the military. These can be used in remote observation of hazardous environments inaccessible to ground vehicles. These type of operations can be carried out using biomimetic MAVs.

1.2 EVOLUTION OF FLIGHT

Perhaps it is one of the most debated topics that how and why flight evolved. Since flight evolved millions of years ago in all of the groups that are capable of flight today, we can't observe the changes in behavior and much of the morphology that the evolution of flight involves. We do have the fossil record, though, and it is fairly good for the three main groups that evolved true flight namely Pterosaurian flight, Avian flight and Chiropteran flight [1].

1. Pterosaurian FlightThey were the first in vertebrates to evolve flight. E.g. - flying archosaurian reptiles. After the discovery of its fossils in the 18th century it was thought that the Pterosaurs might be a failed experiment of nature in flight or simply they were gliders. But the work did by UC Berkeley’s Dr. Kevin Padian shows that Pterosaurs were definitely proficient flyers i.e. why it lasted for 140 million years.

Figure 1.2.1 Wing structure of Pterosaur [1]

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Pterosaurs had kneeled sternum for the attachment of flight wings, a short and stout humerus and hollow but strong limb and skull bones. They were having a modified epidermal structure acting as wing supporting fibres and perhaps provided insulation too [1].

2. Chiropteran FlightThese are the second most diverse group of mammals surviving till day and the only mammals to evolve true flight. They have various flight adaptations as echolocation, reduced radius, large humerus and ulna and a high metabolic rate.

Figure 1.2.2 Wing structure of Bat [1]

Bat wing is very uniquely made of membrane supported by the arm and greatly elongated fingers of the hand, which support the distal parts of the wing where the thrust is produced.

3. Avian FlightThis is the most diverse group of flyers ever to evolve. The earliest known bird is Archaeopteryx. It was a true flyer but not as skilled as modern birds. Its sternum was flat and later modification of wrist bones were also not present.

Figure 1.2.3 Wing structure of bird [1]

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Birds have flight adaptations very much similar to that of Pterosaurs i.e. hollow but strong bones, keeled sterna for flight muscle attachment, short and stout humerus and feathers (analogous to pterosaurs using fibres)

Figure 1.2.4 Generalized bird pectoral girdle [1]

However, unlike the pterosaur wing, the bird wing (shown above) is primarily supported by an elongated radius, ulna and modified wrist bones (the carpometacarpus). Among other features, birds have a fused clavicle (collarbone) called the facula (wishbone), which serves as a brace during the flight stroke [1].

Figure 1.2.5 Comparison of bone structure of Pterosaurs, bird, Bat, and humans [1]

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1.3 ANATOMY

The sternum, or breastbone, bears a prominent keel where the flight muscles attach. The furcula, a fused clavicle (collarbone), serves as a brace during the flight stroke; it's visible in the pictures above as a large Y-shaped bone ahead of the sternum. The clavicle is also found in non-avian dromaeosaurian dinosaurs, and was probably co-opted in function from the dromaeosaurian function of providing a brace for the shoulder girdle while holding prey. Crucial for bird flight is a canal formed by the articulation of the humerus (forewing bone), the scapula (shoulder blade), and the coracoid (bone connecting the sternum itself to the humerus). Through this canal, the foramen triosseum or triosseal canal, runs the tendon of the supracoracoideus muscle, which attaches to the sternum and the dorsal side of the humerus, and lifts the wing upwards in flight. The powerful downstroke of the wing is powered by the large pectoralis muscle, which also attach to the sternal keel.

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Figure1.3.1 Anatomy of bird (Pigeon) [2]

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2. LITERATURE SURVEY

2.1 WING STRUCTURE

In the course of evolution some reptiles developed feathers and conquered the air for locomotion. The evolution of feathers and the transformation of the forelimb from a device for terrestrial locomotion into a wing were explicit features that split archosaurs, respectively the class aves, from other taxa. Due to different birds' needs, a great variability of wing planforms has evolved. Wings can be long or short, narrow or broad, thick or thin. A wing is a flying device or a kind of thin plate that generates lifting force or lift perpendicular to its moving direction. Although all animals that fly by gliding have one or more wings, the wing size, configuration, and material vary greatly from species to species. Because the gliding performance is strongly dependent not only on the wing area (or wing loading) but also on the wing configuration, each has a characteristic flying mode matched to its living environment and way of life.

The wings of any flying animal must generate lift and thrust to support the animal’s weight and drive the body forward against drag. The wing structure is, therefore, designed to bear the aerodynamic force and moment without diminishing its performance during either gliding or beating flight. Although there are a few exceptions, such as the penguin’s wing, wings are usually flexible and foldable.

In comparing the lifting or thrusting surface between birds and other flying creatures such as mammalia (including bats and flying squirrels), insects, flying reptiles, fish, and seeds, the most important morphological differences are in the construction of the wings. Specifically, in contrast with a bird’s wing, which consists of many feathers that can slide over each other, the wings of insects, mammals, fish, and seeds are made of a membrane reinforced with skeleton [3].

The external configuration of a bird’s wing can be seen in Figure 2.1.1. The wing is shaped by bony structures, muscles, and plumage consisting of 1) primary feathers or “primaries,” 2) secondary feathers or “secondaries,” 3) “tertiaries,” 4) humeral and auxiliary feathers such as “scapulars,” 5) “wing coverts,” and 6) “bastard wings” or “alulae.” A bird’s wing would thus seem to be more resistant to damage than a membranous wing like the bats.

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Figure 2.1.1 Birds wing structures (modified from Berger 1961) [3].

For good performance, a bird’s body is shaped into a streamlined form, and the airfoil or wing section is also streamlined in the manner of an airplane wing because the Reynolds number of the wing tip is usually of the order of 105. The thickness and camber of the wing section increase from tip to root. The bird can alter the wing camber to some degree, either actively by adjusting the muscle, the tendon, or passively by aeroelastic action of the feathers.

At the shoulder joint and two other joints (elbow and wrist), the wing can make “beating” consisting of “feathering” (pitch change), “flapping” (out-plane), and “lagging” (in-plane) motions in some restricted conditions. The shoulder is involved in all motions, whereas the elbow is mostly used to shorten the wing by folding it compactly in the lagging direction or in the shape of the letter Z (Whitfield and Orr 1978). On the contrary, the wrist joint appears to be responsible for all additional motions of the hand or outer wing, by which the outer wing attains a widened angle of attack as shown in Figure 2.1.2.[3]

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Figure 2.1.2 Additional feathering of outer wing in a flying gull. [3]

Musculature - The most important muscles for flight are two pairs of muscles that run between the upper arm and the keel. The larger pair is called pectoralis major, which, in some species, is divided into slow and fast fibers. It provides the powerful downstroke of the wing when it contracts. The upstroke requires far less power and is achieved by contraction of the second pair of flight muscles, the smaller pectoralis minor. This second pair of muscles runs between the humerus and the keel and lies between the pectoralis major and sternum. They are not attached directly to the humerus, but terminate in a tendon, which runs through a hole between the bones of the pectoral girdle to the upper side of the humerus (Perrins and Cameron 1976).

Because ample space for the pectoralis major is necessary for obtaining the powerful downstroke of the wing, the high-wing configuration cannot be avoided. From the aerodynamic-performance point of view, the wing should be attached to the body where the body width is at its maximum, in middle-wing configuration. This would enable the wing surface to extend at right angles to the body surface so that adverse interference such as flow separation between the wing and the body could be avoided. It is interesting to find that many birds have “gull-type wings,” that is, wings attached normally at the upper side of the body and with a dihedral at the wing root and capable of reducing the angle by making a downward deflection outside the elbow.

Plumage - A bird’s plumage provides insulation, protection from water, streamlining, and camouflage.

Primary feathers - In flying birds, 9 to 12 primaries are attached to the bones of the hand. The primaries are capable of various independent movements and form an

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outer wing with the triangular surface of the manus, accounting for 30–40% of the whole area of the wing.

Secondary feathers - The secondaries, numbering from six in some hummingbirds up to 20 in land-soaring birds, and more in sea-soaring birds, are attached to the ulna of the forearm, parallel to one another. They are controlled not individually but in small groups by the motion of joints and by an elastic membrane running from the first primary back to the elbow (Storer 1948).

Tertiaries - The feathers rising from the upper-arm bone or humerus are known as tertiaries or tertials. They are considered extensions of the secondaries and close the gap between the active wing and the body. In most birds these feathers are few in number, but in those species in which the upper-arm bone is long, such as gulls, herons, or albatrosses, they are fully developed.

Scapular feathers - These feathers are found on a bird’s shoulders and constitute a separate group for “tailoring” at the connection of the body and wing. In soaring flight they fulfill the role of an aircraft’s wing fairing.

Coverts - The humeral groups of feathers are covered on the outside by a triple row of small coverts and on the inside by one or two rows of finer feathers that are easily lifted. These coverts play an important role in fairing the profile of the wing.At a high angle of attack, flow separation, which results in a loss of lift or “stall” of the wing, can probably be sensed by the upward deflection of the coverts.

Bastard wing or alula - The feathers of the bastard wing or alula are thumb quills attached to the first digit of the manus and are capable of independent movement affected by a special system of muscular connections. It has been reported by Storer (1948) that some birds cannot take off or land without them.

Structure of feathers - The vane of a feather is, as shown in Figure 2.1.3, made up of parallel rows of barbs projecting obliquely from either side of a shaft. The bare end of the shaft, the quill, and the distal portion, the rachis, are corneous tubes, the material of which, keratin, has a specific gravity of only 1.15 g/cm3. It has been reported that the modulus of elasticity and the tensile strength of keratin are E = 9.0 × 103 MPa or 920 kgf/mm2 and σB = 3.5 × 10MPa or 36 kgf/mm2, respectively (Hertel 1966).

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Figure 2.1.3 Structure of a feather. [3]

The quill is a hollow elliptical tube with an approximately constant wall thickness. The rachis has a rectangular section filled with foam material and can more easily accommodate bending distortion than the quill, specifically in directions normal to the vane. To maintain aerodynamic smoothness on the upper surface of the vane, most of the rachis projects downward from the vane surface. . Its shape is almost square at the tip of the vane, oblong in the middle part, and elliptical with a little dimple at the bottom near the root. The barbs are flattened (about 0.08 mm wide) and are very flexible with respect to bending toward either end of the feather, but are fairly stiff and rigid against bending up or down. A row of very fine fibers or “barbules” (length=120 μm, diameter = 3–5μm) runs along either side of these barbs. Barbs and barbules are interlocked with microscopic hooked “barbicels” (length = 20 μm and diameter = 1.5μm) and make the surface of the vane. Thus, the wing, with these feathers and coverts, does not have a completely smooth surface but a somewhat rough or grooved surface having what can be called “riblets.” The riblets are devices to reduce drag.

Tail wing - A bird’s tail wing is made up of “rectrices” and can be regarded as a stabilizer with variable area and angle of attack and also as an organ that creates moment for the control of rotation about the horizontal and vertical axes. Whenever the tail wing shares the lift, the lateral tilt of the lifting surface generates a lateral force in the direction of tilt and thus induces yawing moment [3].

2.2 WING SHAPE COMPARISION

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Figure 2.2.1 Wing structure of Hen (Evolution of true flight) [4]

The shape of the wing is important in determining the flight capabilities of a bird. Different shapes correspond to different trade-offs between advantages such as speed, low energy use, and maneuverability. Two important parameters are the aspect ratio and wing loading. Aspect ratio is the ratio of wingspan to the mean of its chord (or the square of the wingspan divided by wing area). Wing loading is the ratio of weight to wing area.

Most kinds of bird wing can be grouped into four types, with some falling between two of these types. These types of wings are elliptical wings, high speed wings, high aspect ratio wings and soaring wings with slots.

Figure 2.2.2 Comparison of different types of wings. [1]

1. Elliptical Wings

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Elliptical wings are short and rounded, having a low aspect ratio, allowing for tight maneuvering in confined spaces such as might be found in dense vegetation. As such they are common in forest raptors (such as Accipiter hawks), and many passerines, particularly non-migratory ones (migratory species have longer wings). They are also common in species that use a rapid takes off to evade predators, such as pheasants and partridges. [5]

Figure 2.2.3 Elliptical wings in birds, sparrow and crow in flight [6]

2. High Speed WingsHigh speed wings are short, pointed wings that when combined with a heavy wing loading and rapid win beats provide an energetically expensive high speed. This type of flight is used by the bird with the fastest wing speed, the peregrine falcon, as well as by most of the ducks. The same wing shape is used by the auks for a different purpose; auks use their wings to "fly" underwater. The peregrine falcon has the highest recorded dive speed of 242 mph (389 km/h). The fastest straight, powered flight is the spine-tailed swift at 105 mph (170 km/h).

Figure 2.2.4 High speed wings in birds, swift and falcon during its flight [6]

3. High Aspect Ratio WingsHigh aspect ratio wings, which usually have low wing loading and are far longer than they are wide, are used for slower flight, almost hovering as used by kestrels, terns and nightjars or alternatively by birds that specialize in soaring and gliding flight, particularly that used by seabirds, dynamic soaring, which use different wind speeds at different heights (wind shear) above the waves in the

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ocean to provide lift. Low speed flight is important for birds that plunge dive for fish.

Figure 2.2.5 High aspect ratio wings, albatross and gull during flight [6]

4. Soaring Wings With Deep SlotThese are the wings favored by the larger species of inland birds, such as eagles, vultures, pelicans, and storks. The slots at the end of the wings, between the primaries, reduce the induced drag and wingtip vortices by "capturing" the energy in air flowing from the lower to upper wing surface at the tips, whilst the shorter size of the wings aids in takeoff (high aspect ratio wings require a long taxi in order to get airborne) [4].

Figure 2.2.6 Slotted high lift wings, eagle and stork during flight [6]

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2.3 FLIGHT IN BIRDS

The flight of birds can be generally classified into non flapping flight (including gliding, soaring, or sailing) and flight by means of a flapping or beating of the wings. The latter type of flight will be discussed in depth in the following chapter.

The flight performance of birds is strongly related to their body configuration, specifically wing configuration and arrangement, in which there are clear differences between land birds and sea birds, depending on their ecology or way of life and the environmental conditions. Each bird is also able to alter its wing configuration in response to flight conditions. With this adaptive ability the bird can optimize performance in any flight condition and select the degree of stability and control or maneuverability, which are inversely related to each other.

Figure 2.3.1 a Axes of rotation. Mp, Mr and My are the pitch, roll and yaw moments about the transverse, longitudinal (median) and vertical axis, respectively, and Cpm' Crm' and Cym their respective moment coefficients. b Dihedral of the wings controlling roll moments, resulting in roll stability. A roll to the left (left wing down) would increase the lift L on the left wing and decrease the lift on the right wing because of increased angles of attack on the wing moving down and decreased angles of attack on the other wing. This force difference will lift the left wing again and restore the bird to the horizontal. Lv is the vertical lift force of one wing. c Partial retraction of the left wing decreasing left wing area and lift. L and L' are the lift forces and Lv and Lv’ are the vertical lift forces of the two wings. Positions of wings in the middle of a downstroke [12]

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Flight Modes

A bird can adopt different flight modes depending on its purpose in flying. It can adjust the configuration of its wings as well as their profile by either extending or folding them against the body. This allows for the optimal wing for each flight mode and each phase of a stroke movement. The following are typical types of flight observed in the sky (Storer 1948; Vinogradov 1951; Terres 1968): 1. Cruising flight

In steady level flight the wing acts to give a lifting force mainly at the inner part of the wing and a thrusting force at the outer part of the wing or oscillating manus. In this mode, the wing is almost fully extended to give the best performance for minimum power. Ducks, geese, swans, flamingos, storks, and cormorants always fly with their head and neck stretched out to the fullest extent. On the other hand, herons, egrets, and pelicans, though also long-necked birds, draw their head back till it rests almost on their shoulders.

Figure 2.3.2 Cruising flight [7]

2. Gliding or soaring flight Gliding is performed without supplying any beating energy to the wings other than for maneuvering action; therefore, either the flight altitude or the flight speed gradually decreases during flight in calm air. The lost potential or kinetic energy is equal to that consumed by the drag of the body in forward motion. Gliding for a long duration or great distance is called “soaring” (Cone 1962a).

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Figure2.3.3 (a) Gliding flight: a) buzzard, Buteo buteo (courtesy of M. Tanaka 1976) and

b) Albatross, Diomedea albatrus (courtesy of Asahi News Paper).Soaring Flight – Soaring is a phenomenon shown by the birds in which they can maintain their flight without wing flapping using the rising air currents.

Figure 2.3.3 (b) Soaring flight [7]

3. Diving flight A steep descent is executed with immobile and partly drawn wings to control speed and direction with the help of the tail. This flight mode is used for preying and sometimes for pinpoint landing by small birds, such as the lark.

Figure 2.3.4 Diving flight [8]

4. Bounding flight Small birds employ an undulating form of flight called “bounding flight” in which, with wings periodically or intermittently folded, they fly like an arrow, first losing height and then swooping up again.

5. Hovering flight Before touching down on a tree or on land, many birds can temporarily remain in one place even in calm air by raising their body and beating their wings in what is called the “avian stroke.” This is illustrated in Fig 2.3.5. On the other hand, the hummingbird is able to stay at one point in the air for a prolonged period by beating its wings (mostly the manus) almost horizontally in what is called the

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“insect stroke,” shown in Figure 2.3.5 (b), at an exceptionally high rate of more than 20 strokes per second.

Figure 2.3.5 Hovering flight (sketched from Ruppell 1977): a) avian stroke and b) insect stroke

6. TakeoffFor flight, birds must acquire enough speed with respect to the air to utilize the aerodynamic force. Because a bird does not have forward velocity at takeoff, it needs some lift assistance before gaining speed for normal flight.

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Figure 2.3.6 Takeoff flight [8]

7. Landing In landing, birds must drive their wings to get a large drag force to kill their speed and to maintain the lift against their weight at low speed. As shown in Figure 2.3.7 in a highly lifted wing, stall is prevented by extension of the alulae, which act as “vortex generator,” wide spreading of the primaries, and use of the ligament to form a concave wing section that is tailored by the coverts on the underside of the wing in the manner of a “Krueger flap”. This change in section configuration is important. In addition, the wings elevated in a “V shape,” increase the maximum aerodynamic coefficients by reducing the aspect ratio.

Figure 2.3.7 Landing flight (about to strike) of red-shouldered hawk (Buteo lineatus)(From Gilliard 1967). [1]

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Either of the high-lift devices just mentioned will prevent flow separation of the wing at high angle of attack and increase the maximum lift at slow flight speed. The effect on the tail surface cannot be ignored either. The spread tail forms an auxiliary surface behind and slightly below the main wings, like the flap of an airplane wing and not only prevents flow separation of the main wing, but also produces a positive lift. Unlike the landing of the artificial airplane, the positive lift on the tail surface is obtainable for compensating the head-up moment generated from a slightly forward shift of the main wing.

The legs of the landing bird are fully extended to increase drag and reduce speed and to obtain an adequate stroke to cushion the impact at touchdown. In water birds, touchdown on water is accomplished either on the breast or on the feet. Breast landing is sometimes observed in diving birds. Most water birds land of their feet. They stretch their webbed feet forward and slide on them as though on water skis.

Many birds can reduce their flying speed by assuming an ascending flight path or can convert their kinetic energy to potential energy during the landing approach by changing their flight attitude, wing span, and wing area.

3. MECHANISMS

3.1 CHORD- WISE DRIVING MECHANISM

To develop the chord-wise mechanism the slow motion pictures of birds are taken and the points are being marked depending on the angle of movements of the wings. This gives the pattern of the trajectory of the wing-tip in a two dimensional plane.

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Figure 3.1.1 Angles of primary and secondary wings during upstroke (left) and downstroke (right) [10]

Figure 3.1.2 The generation of wing-tip trajectory using five-bar mechanism [10]

The lengths of the links were directly taken according to the scaled model of the bird’s picture.And the required angle movement between the links were maintained depending on the data obtained from the picture analysis of the bird show in the previous figure. They didn’t derive the equation of the trajectory and hence the path produced by the linkages may or may not produce the required things.

3.2 FOUR BAR SINGLE POWER SOURCE MECHANISM

Figure 3.2.1 Four Bar linkage of slow hawk ornithopter

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The pilot throttle commands the speed of a Feigao DC motor, which through a gearbox and a four-bar linkage, flaps the wing to generate thrust and lift forces. The pilot elevator and aileron joysticks command two Hi-tech HS-56 servo motors that are aligned in a serial fashion to pitch and roll the tail relative to the fuselage, creating aerodynamic pitching and yawing control torques. This ornithopter flaps up to 8 Hz, flies up to 10 m/s i.e. 36kmph.

Comments

This four bar mechanism is widely used in ornithopters making for both high speed and slow speed flapping. The only problem here is the calculation of the linkages length, which can be calculated using bionic formula. This proportion can be used as length of bars which derive two wings in designing wing span mechanism.

3.3 DOUBLE PUSHROD FLAPPING MECHANISM

Figure 3.3.1 Chung Hua University MAV double pushrod mechanism (Double pushrod flapping mechanism) [10]

This mechanism uses a motor connected to a system of gears that increase flapping force while reducing flapping rate. Pushrods connect to each flapping spar, thus driving the wing motion up and down through pinned connections. Due to the pinned connections, the vertical translation is the only component of motion that is transferred from the drive gear. Since each wing spar has its fulcrum located at a fixed distance ‘x’ from the central axis of the mechanism, and the pushrods are of

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fixed length ‘l’, a problem arises. The two pushrods are never exactly in the same vertical location, except for the apex and the nadir of the flapping motion. This creates a phase lag between the two wings, resulting in slightly asymmetric flapping of the wings. At the miniature-size scale, this is an undesirable situation, where control is already difficult due to the low inertia of the fliers relative to their large wing and fin surface area.

Despite its inherent limitations, this configuration is popular due to its simple construction, light weight, and ease of part replacement. If the MAV is very small and has a sufficiently high flapping rate, it is possible that the asymmetry of the wings can be masked during the overall flap motion. If the throttle is reduced, however, the MAV will begin to exhibit noticeable oscillations and be more difficult to control.

3.4 SINGLE CRANK FUNCTIONAL SCHEMATIC

Figure 3.4.1 Single crank functional schematic [10]

This single crank functional schematic is very simple to develop and assemble and also produces symmetrical flapping which outcasts the limitation of vibration due to unsymmetrical flapping and hence gives better stability. It can be used wherever there is a need of high flapping rates and also the power can be supplied from single source or motor.

3.5 CRANK LEVER MECHANISM

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Figure 3.5.1 Crank and Linkages [11]

This mechanism was used with only one power source supplied to the crank and hence transferring motion to both the flapping wings. When the frequency of flapping is increased the stability decreases considerably although it can be masked at an optimized flapping rate. Wing loading ranges from 0.5 to 3 newton per square meter. Flapping frequency ranges from 5 Hz to 20 Hz. A pair of wings is flapped perpendicular to the body axis. Span of one wing is less than 100 mm. The Crank shaft was made of thin steel wire. One wing consists of two polystyrene frames and paper membrane. The body was made with balsa of which relative density ranges from 0.1 to 0.2 and hence makes the setup lighter.

4. CONCLUSION

After doing the thorough study of the anatomy of birds and wing structures we got to the conclusion that our aim will be to develop the bio-mimetic model with greater stress on lesser weight and high degree of freedom of its wings, so as to achieve higher maneuverability. Various mechanisms were analysed based on their advantages and disadvantages. Four bar single cranked mechanism seemed to be best suited for the purpose although; extensive work is required for generating more optimized mechanisms to fulfill our need. It was also observed that although the mechanisms can be generated from the data of the birds wing-tip trajectory using bionic formula but still mathematical relationship needs to be derived for the links to get the equation of the trajectory followed by the links. Since a major part of the literature survey has been completed our next aim will be to develop kinematic models and derive the mathematical relationships and hence analyse the motion of the virtual wing.

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REFERENCES

1. John R Hutchinson [1996], “Vertebrates Flight”, www.ucmp.berkeley.edu

2. Joseph M. Forshaw [1998], “Parrots of the World”, TFH Publication

3. Akira Azuma [2006], “The Biokinetics of Flying and Swimming”, Second edition, AIAA Education Series, pp. 33-46

4. Adaptation for flight, “Birds Flight”, www. en.wikipedia.org/wiki/Bird_flight

5. Cornell lab of ornithology, “Cornell University”, www.birds.cornell.edu/education/kids/books/wingshapes

6. Laurie E. Likoff [2007], “The Encyclopedia of Birds”, International Masters Publishers

7. K. Herzog [2012], “Development, Theory, Practice Great Ornithopters (translated)”, http://www.ornithopter.de/

8.Jemima Parry-Jones [2000], “Eyewitness Eagle & Birds of Prey”, DK. Publishing Inc.

9. Bingbing Liang and Jiankun Cui [2011], “Research on Birds Flapping-Wing Bionic Mechanism”, Canadian Center Of Science and Education.

10. John W. Gerdes and Satyandra K. Gupta [2012], “A Review of Bird-InspiredFlapping Wing Miniature Air Vehicle Designs”, Journal of Mechanisms and Robotics, ASME

11. Hiroto Tanaka, Kazunori Hoshino, Kiyoshi Matsumoto,andIsao Shimoyama [2010], “Flight Dynamics of a Butterfly-type Ornithopter”

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12. Dr. Ulla M. Norberg [1989], “Vertebrate Flight”, Springer-Verlag Berlin Heidelberg [1990], vol. 27, pp. 76

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