Introduction Flight morphology Flight physics Flight model Implementation notes Download and snapshots References

  Most gulls don't bother to learn more  than  the  simplest  facts  of flight - how to get from shore to food and back again. For most gulls,  it is not flying that matters, but eating. For this gull, though, it was  not eating that mattered,  but  flight.  More  than  anything  else.  Jonathan Livingston Seagull loved to fly.
 

The wings

  The wing is the lifting surface of the bird and as such must have the right shape and profile to provide the airflow to produce this lift. Much of the visible shape of the wing is produced by the covering of feathers, however, there is a narrow flap of skin that is called the propatagium. This is stretched tight along the leading edge between the shoulder and the wrist when the wing is held out. This is an important part of the wing, it is covered in feathers and forms the leading edge of the inner wing where it helps to provide much of the lift developed by the wing. 

  To provide it with the required rigidity against the airflow the bones of the wing tend to run nearer the leading edge, whilst the trailing edge is constructed solely of a rigid line of feathers. Covert feathers also cover the entire wing and at the trailing edge are found the much larger primary (remiges) and secondary feathers (secondary remiges) 

  The alula is formed by digit II, and lies almost at the wrist joint on the leading edge of the wing. generally it has 3-4 short vaned feathers attached to it. It acts in a similar fashion to the slots on an aircraft, naturally raising into the airstream when the wing approaches stall. The alula is prominent in birds such as the corvids and almost absent in some soaring seabirds.

The legs 

  Starlings and pigeons do not run to gain speed, instead they jump into the air. This jump is powerful enough to supply the initial inertia required to get them fully airborne. Once in the air they are able to use the full movement of their wings to continue their skyward motion. Albatross with their long wings and swans with their high wing loadings normally rely, like aircraft, on increasing their momentum on the ground until they reach a speed that allows them to become airborne, whilst some dabbling ducks, such as the mallard, are able to launch themselves vertically into the air straight from the water, an incredible feat of pure power. 

The tail 

  The wings of a bird generally lie slightly ahead of the centre of gravity, this means that when a bird flies its posterior trails in the airflow behind it. The tail provides not only the lift required to buoy up the weight of the body but it also helps in flight control, unfortunately it also adds to drag at higher speeds. The tail allows the wing design in birds to be tailored for efficient cruising and high speed flight and under these conditions it is furled to provide minimal drag. At lower speeds, however, or during maneuvers the tail can be quickly unfurled to reduce induced drag (see later) from the wings and provide a surface for enhanced steering, lift or braking. The tail is suspected to play an important part in maintaining balance and stability in flight and it would seem that it is required to generate lift at low speed when the interaction between the wings and the tail can also most effectively reduce drag. 

  The retrices, or tail feathers generally number 10-12, but are found to range between 8 and 24. These are normally straight and bilaterally paired and the bases covered by coverts to produce a smooth surface for airflow. When you look at a birds tail you see the retrices which are controlled by the tail muscles that allow for the various movements that are required for precision flying.

The feathers 

  Flight feathers are asymmetrical, with the leading edge vane being narrower, thicker and less flexible than the trailing edge. If the leading edge of the feather was to bend excessively in the airflow it would cause twisting of the feather, leading to a damaging loss of lift. This asymmetry, however, also ensures that the trailing web of the feather bends upward during the downstroke, providing forward momentum and lift. It has also been shown that the outer primary feathers, that is the ones closest to the wing tip, are mechanically stronger than those closer to the wing root (Ennos, Hickson & Roberts, 1995). This is probably to property that helps them withstand the larger aerodynamic forces that these feathers are subjected to. 

Sources for this section:

Principles of the aerofoil 

  The faster air moves across the wing the more lift the wing will produce, so moving it through the air by flapping increases this airflow and thus increases lift. The bird doesn’t paddle air underneath its wing, instead it cuts into the air with the leading edge to obtain the flow over the surface that it requires. 

The wing 

Drag

  The first thing to note about drag during flight is that for experimental purposes it can be subdivided into three main categories and so we find parasite, profile and induced drag being produced. 

  Parasite drag is the drag produced by the flow of air over the body of the bird and like a car this is influenced by any uneven surfaces or jutting protrusions such as legs. Remember that drag increases with speed and as herons fly much more slowly than ducks or geese this aerodynamic requirement comes secondary to the functional requirement of food gathering. It isn’t just the front of a bird that has to be aerodynamic though , its at the rear end where the greatest energy savings are to be had. It is here that air bleeds off the body and the potential for a great deal of energy sapping turbulence is produced. Most birds have a tail that helps the flow of air leave the body and can be used to increase lift if required. When a bird is flying quickly it furls its tail and tries to emulate the water droplet, which you’ll remember narrows to a fine point at the back and is the shape that is imposed on the fluid by the forces induced by the passage of the air over it. That is why swallows, frigatebirds and an number of other fliers have sharply forked tails and why falcons and nightjars have very narrow ones. 

  The production of lift comes at the expense of an increase in drag, this is termed induced drag. When a wing producing lift passes through the air it leaves circulating air in its wake that represents lost kinetic energy, thus induced drag is the product of lift production and does not include profile drag which we will come to shortly. When calculations of induced drag are made they assume that air has no viscosity, which we all know to be incorrect. Profile drag takes into account the viscosity of the air so that the addition of profile drag values to those of induced give the total drag value of the wing. Profile drag is in effect what it sounds like, that is the drag produced by the profile of the bird as it moves through the air. So if we add the profile drag, the parasite drag and the induced drag together we find the figure for the total drag. Its not only the wings that add to induced and profile drag though, the tail can also act as a lifting body thus causing induced drag. At the same time, because it is unfurled, it will also add to the profile drag too. 

Angle of attack 

  A bird can obviously adjust this angle of attack, not just simply by rotating its wing but also by changing the attitude of its entire body with respect to its forward motion. So during slow flight birds and aircraft tend to fly nose up with a fairly high angle of attack, whilst traveling at speed they tend towards nose down, producing a much lower angle of attack. During take off, when there is very little airflow over the wings, birds such as pigeons increase the angle of attack to give the wing greater purchase on the air so that a larger amount of force can be generated. During a stoop on the other hand a falcon will minimize the angle of attack to allow it to slip through the air with a minimum of drag. 

  The shape of wings have also evolved to help produce lift from lower angles of attack. This is accomplished by the use of a cambered wing. Thus the birds wing is not flat in section, but instead is concave. Because of this the leading edge attacks the air with a lower aspect (it faces straight on into the airstream) than the trailing part and thus minimizes drag, bringing with it the bonuses of better stall and lift characteristics. 

Aspect ratio 

  Drag is produced when high pressure air passing under the wing swirls upward into the low pressure area above and behind the wing. As it does the air creates a sheet of eddies that disrupts the movement of air across the wings trailing edge. This phenomenon reduces lift and leads to drag creating turbulence which is most pronounced at the wingtip, where it is called the tip vortex . This phenomena can often be seen in aircraft where vapour trails can be seen emanating from the wingtips, especially during hard maneuvers. This effect can be diminished by increasing the length of the wing and so decreasing the tip to wing length ratio. Incidentally it is this tip vortex that is used by gulls, geese etc when they fly in the V type formations that you often see. The air directly behind the wingtip is the rising part of the vortex and it is this that the formation flying birds exploit. To get the best out of it they must be tucked in closely behind the leading bird or they will fly into the downwash. In this manner they can minimize their energy usage. 

  Many birds have a requirement for shorter wings and so another method of dissipating tip vortex has evolved. In aircraft, vertical winglets are often placed at the end of the wing to dissipate these vortices in a vertical plane. Birds use a similar method, although in this case their winglets are formed by the primary feathers of the wing. This design of wing is often termed, slotted, meaning that there are gaps between the feathers so that each feather acts independently. As I say, this is most pronounced at the wing tip, where, if you watch a bird in flight you will see these feathers bending upward as the air from below the wing gushes up into the low pressure above. This bending of the feathers leads to the displacement of the tip vortices to the vertical dimension, thus spreading it across a greater area of the wing, leading to a decrease in induced drag. It also leads to the production of forward momentum. 

  To understand a little more the important implications of wingspan and aspect ratio on flight, imagine a bird soaring in level flight through the air at a given speed. Now take a snapshot of 1 second of that birds flight. Looking from directly above the bird you could measure the area that it has passed over by measuring the span of the bird by the distance covered in that second. If the bird has a long wingspan the area will be larger than that of a bird with a lesser wingspan. The measurement that you get can be correlated to the amount of air that that bird has passed through in that given time frame. From this we can see that a bird with a longer wingspan is able to move through more air than a bird of shorter wingspan. Now if that bird is gaining its lift from the air that it has passed through we can also see that a bird of the same weight, with a smaller wingspan, will have to gain more lift from that air than the one with a longer wingspan. In other words the bird with the shorter wingspan has to move more of the air (produce more lift from a given amount of air) that it passes through to obtain the lift that it requires to keep it airborne. 

  It is not only the length of the wings that affect the flight characteristics of the bird, so does the chord length or width of the wing. The longer the chord length of the wing, the greater the separation of the air over its surface can be. So birds with wide wings are generally able to derive more lift from the air they pass through than birds with narrow wings. 

  It has been found in the Procellariiforms (petrel and albatrosses) and it is probably a general phenomenon with a number of bird families, that there is a tendancy for larger birds to have the higher aspect ratio wings.

Wing loading 

  Wing loading is a very important factor in the flight performance of birds. In general it is found that animals with lower wing loadings tend to fly more slowly and are more maneuverable than do those with higher values. 

  Wing loadings will also be affected by other phenomenon, such as the long legs stork and herons. These birds must land carefully to ensure that they do not damage their legs and so they need low wing loadings to allow them to come down very gently. Compare this to the swan that requires a strip of water to land on, here the bird removes the remainder of its airspeed by sticking its feet out and ploughing through the water. 

The alula 

Moment of inertia 

  It is vitally important for a bird to keep the moment of inertia of its wing as low as possible and a close look at the wing highlights that adaptation. The moment of inertia depends upon the distribution of mass along the wing, the more distal the mass, the higher its influence on inertia. Most of the, muscles, blood supply, tendons etc in the wing are fairly proximal and so have less influence on inertia. On the other hand, looking past the wrist it can be seen that the mass is made up predominantly of purely weight bearing material such as bone and feather. 

  At the same time as keeping the moment of inertia low, the wing must also maintain a reasonable resistance against bending or failure and so wing design must reflect a compromise between strength and weight. Long wings will generally have a higher inertial value than shorter wings and so will be more difficult to flap. Gulls with their relatively long wings tend toward a lazy flight gait with a slow rate of flapping whilst sparrows and other small passerines, with their relatively short wings, are considerably more frenetic in their flight. 

Wing types 

• Elliptical
• High Speed 
• High aspect-ratio
• Slotted high-lift 

  Elliptical wings tend to be found on birds adapted to forested, wooded and shrubby habitats where birds require good maneuverability and are generally short with a low aspect ratio. These wings often have a high degree of slotting which is associated with the requirement of slow speed flight. Elliptical wings are found in the passerines, the gallinaceous species as well as woodpeckers and doves.

  High speed wings are possessed by birds that feed in the air or have to make long migratory hauls. They are seen in the swifts, swallows and also the falcons and some shorebirds. These wings have a relatively high aspect ratio and are only slightly cambered, having an almost flat profile and are often swept back. 

  The high aspect wing is found on soaring sea birds such as albatross and can also be seen on gliders. They, like the slotted high-lift wing can provide plenty of lift in the windswept environment in which they live. 

  The slotted high-lift wing is a characteristic of many terrestrial soaring birds, such as the eagles and the vultures. These wings have a moderate aspect ratio with marked slotting and are heavily cambered. This wing design not only allows them to carry heavy loads but also enables them to minimise energy expenditure whilst foraging for food. 

  So why do we have this dichotomy in wing design between the slotted high-lift and the high aspect wing. Well this can probably be answered by looking at their foraging technique and the environment on which they hunt for food. Vultures tend to loiter in the air, gaining lift from thermals that can be small and require tight turning to remain inside. To them the most important factor is remaining aloft so they can continue to scan for carrion. Speed is not particularly important as they cross back and forth across the plains. This is all in marked contrast to the albatross, this is a large bird, with a fast cruising speed. It must cover many miles of ocean in search of food, whilst at the same time gaining its lift from the winds that are constantly blowing. This foraging technique calls for a whole new set of equipment and so to a completely different wing type. 

The Tail 

  On the ground, unless being used for display purposes, the tail is furled away and plays no part in movement, although some of the tail muscles are active. It is during take off and landing when the action of the tail can be seen most clearly, next time you are on the sea front watch the pigeons take off. It all happens very fast, so you will need to watch very carefully. As the wings are initially raised prior to the first downstroke the tail is unfurled and as the bird launches itself into the air its retrices (tail feathers) are rapidly spread producing a concave fan that is tucked under its body and flared out further during the first downstroke. The tail is now in an almost vertical position. As the bird makes further progress, its airspeed increases, the tail lifts progressively and the flaring becomes less pronounced until finally as the tail reaches a more horizontal attitude it is furled once more to minimize drag. During landing the whole procedure is re-enacted except this time the tail is used to help with deceleration. 

  It has been shown in studies with swifts that the use of the tail in flight helps the bird initiate much harder turns. However, during non-maneuvering flight the bird keeps its tail furled to minimise drag. 

Soaring Flight 

Thermal soaring 

  Thermals are generated by rising columns of warm air that develop over differentially heated surfaces during the day and are the domain of birds with high lift slotted wings. The shorter wings of these birds, give them better turning characteristics that make them better able to stay within the confines of small thermals. This form of soaring is used by a number of larger species during migration where they climb within a thermal to gain altitude before gliding down to the next thermal and climbing once more. As thermals are generally not developed over large water masses birds tend to collect in huge numbers in places like Gibraltar where they try to obtain as much altitude as they can to help them negotiate the straights into Africa beyond. 

  Thermals are commonly used by vultures who do not take to the wing until the sun has heated the ground long enough to allow the production of rising air columns. 

Slope soaring 

  Slope soaring occurs where winds meet an obstruction and are forced to rise over them. This rising air provides the lift for a large number of different bird species and is particularly noticeable when watching Kestrels hunting along the sea cliffs where large amounts of uplift are developed. Normally they have to hover when hunting, but in these conditions they can often remain motionless bouyed up by the rising air.

Dynamic soaring 

  Dynamic soaring is carried out by sea birds over water where no thermals or slopes are available to provide free lift. This is a more complicated affair than the other two forms of soaring and relies on a birds ability to convert momentum into altitude. For this reason birds with high aspect wings are by far the most proficient at this soaring technique and the albatross with its huge wingspan and high weight is the perfect practitioner. 

  Dynamic soaring relies on a gradient of wind speed being produced over the water. This occurs as energy is lost from wind in the production of waves close to the water surface, so close to the water the wind is moving more slowly. 

  The bird starts by gaining altitude, flying against the wind. Once it is high enough it starts to glide downward. With the wind behind it it rapidly picks up speed and when it gets close to the water it turns into the wind. The air near the water is slower than above and it quickly passes through this with the decrease in drag produced by the slower moving air. The bird then rises quickly on the wind which is now head on. The wind increases the airspeed over its wings and so provides lift that the bird exploits until its airspeed comes close to a stall. It then turns smartly and heads down wind once more gaining speed as it goes. Using this technique sea birds are able to cover huge tracts of the ocean.

Wing geometry and fligth stability (by R. Dryden)

  A clue from hang gliders: The hang glider wing forms two 'billows', one on each side of the midline. The underside of each wing forms a conical surface, the centreline of which is angled towards the nose of the hang glider. This arrangement, together with the low centre of gravity provided by the weight of the pilot, gives the hang glider stability around all three axes (pitch, roll, and yaw).

  In the early 80s I began to wonder whether the wings of birds such as seagulls conformed to a conical geometry like that of hang gliders. A series of models with wings built over conical jigs confirmed that this arrangement imparts stability during gliding flight - no additional surfaces such as a tailplane are required. Furthermore, it became clear that if the joint axes of the skeleton supporting the bird's wing were set perpendicular to the conical form, the wing is able to extend and flex whilst still retaining the conical geometry required for stability. 

Flapping flight 

  The outer wing is the powerplant of the wing, it produces lift, but more crucially it produces forward momentum. It is less cambered than the inner wing and more flexible and it is this flexibility that leads to the momentum. As the wing is flapped downward the outer wing tends to twist slightly forwards, this is due to a number of reasons, one being that air passing under the wing tends to well up toward the tip and as it does so it forces its way out under the back of the wingtip, tilting the wing forward.

  To better understand flapping flight, we will make our observations on a bird in stabilized flight, without taking into account takeoffs and landings (which are more complex to analyze). One can consider that during the flight, the bird carries out three essential motions with its wings: 

• a motion of vertical flap (up and down).
• a horizontal motion (forward and backward).
• a torsion motion (twist of the wings).

  Generally, the bird does not use its tail during a straight flight phase, but remains retracted most of the time. During a cycle of flaps, the wings exert, alternatively, a propulsion (while going down), and a deceleration (while going up). According to whether they take place below the average plan or not, these forces create an alternative couple on the trim. The bird compensates this by a continuous search for balance, moving its aerodynamic center with respect to its center of gravity (i.e. by moving its wings back and forth alternatively). Both vertical and horizontal motions are combined together to describe eight-shaped kinematics of the wing. 

  In flight, on one hand, the horizontal speed due to the displacement is the same all along the wingspan, but on the other hand, the vertical speed due to the beat is null at the base and increases gradually to the end of the wing. Both speeds combine together and make the angle of attack of the airflows vary according to the position of the wing : horizontal at the base and more vertical near the extremity. But a profile only tolerates a small variation in its angle of incidence. Consequently, the bird adapts its wing twist to keep a nearly constant angle of incidence. This results in a twist of the wings ends, twisting negative while going down, and positive while going up. As the lifting force has a direction perpendicular to the plane of the wing, it will swing the wing forwards during the descent (by the same way that a helicopter rotor swings forwards and propels the aircraft horizontally). Thus it is the flapping, associated with a judicious twist of the wings, that propels the bird and allows him to fly.

Another description:

  There are three important motions in addition to the bird's forward motion: 

    
1. Flapping                              2. Twisting                                3. Folding 

  By flapping its wings down, together with the forward motion of the body, a bird can tilt the lift of its wings forward for propulsion. Why don't birds simply move their wings up and down, without twisting and folding? Notice that the outer part of the wing moves down much farther than the inner part close to the body. Twisting allows each part of the wing to keep the necessary angle relative to the airflow. If part of the wing is angled lower than the airflow, there might not be enough lift. If part of the wing is pointed too high, there could be a lot of drag. The wings are flexible, so they twist automatically. 

  Wing folding isn't essential - ornithopters fly without it - but it helps birds fly with less effort. To see why it is helpful, think about what happens during the upstroke. Because the wing is going up, the lift vector points backward, especially in the outer portion of the wing. The upstroke actually slows the bird down! By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.

  In addition to the three basic movements described here, birds can do a lot of other things with their wings to allow them to maneuver in the air. Instead of using their tails for flight control, they move their wings forward and backward for balance. To make a turn, they can twist the wings or apply more power on one side. For slow flight, birds can flap their wings almost forward and backward instead of vertically; the upstroke and downstroke produce lift without forward body motion.

Varying flight speed 

  Flying, like any other mode of locomotion can be done at different speeds. Unlike humans that either walk or run though, birds do not have such clear cut gaits in their flight. Generally though it is thought that there are two overall flight strategies used by birds. These have been designated minimum power and maximum range and are derived from power curve produced during flight. These power curves have been made by measuring the power requirements of birds in air tunnels flying at different speeds. The maximum range speed would be the one that a bird would be expected to use if it was migrating or was in a hurry to get somewhere. The minimum power speed on the other hand would be best used when a bird was foraging or had no time requirement to its flight. Measurements taken of migrating birds do suggest that these figures might represent a real phenomenon. The strength of the flight muscles then will obviously set a limit to the birds power output and the bird is able to fly at high speeds for a short bursts or at lower speeds for a longer periods, like a human running. 

Different flight gaits 

  There are a number of different ways that a bird can flap its wings and this is influenced by the musculature, the type of wing and the speed at which the bird wishes to fly. It is also found that some birds also have the ability to extract lift from the wing flip, or upstroke whilst others can only produce power on the downstroke. By flying in this fashion the birds gait is changed, so that if it flies through helium filled bubbles, the wake produced by using the upstroke to generate lift produces what is known as a continuous-vortex gait whilst when only the downstroke is used a vortex-ring gait is produced.

  Small passerines and some larger birds have another way of flying, instead of continuously flapping their wings they change rhythmically from flapping to gliding (when the wing is extended) or flapping to bounding (when the wing is flexed). In this way they undergo an undulating flight where first using flapping they propel themselves forward on a slightly rising vector, then at the peak of the rise they cease flapping and continue forward by either gliding or closing their wings and bounding. This type of flying is stereotypic for some bird species, with for instance, the woodpecker that undergoes pronounced flap-bounding and the flap-gliding seen in the falcons and be used as a characteristic for field identification. 

  The reasons for this type of flying are still being examined and there are a number of different aspects under discussion. It is thought that this intermittent flying gait, used mainly in small birds such as the passerines may simply be due to the birds flight muscles being unable to perform continuous flapping, so they are rested during the bounding or gliding phase. Another theory, the ‘fixed gear hypothesis,’ suggests that flap-bounding is used to vary the power output of the flight muscles which are otherwise constrained by a fixed muscle contraction and wing motion. The ‘body-lift’ hypothesis suggests that some gliding during flap-bounding could be advantageous in comparison with continuous flapping over most flight speeds (Tobalske, Peacock & Dial, 1999). Small birds are very light and so drag will often have a stronger influence on their flight than gravity. For this reason it has been suggested that birds may bound between flapping to minimise drag.
 

Sources for this section:

• University of Wales, Aberystwyth: http://users.aber.ac.uk/jrj/
• School of Biology, University of Leeds, Prof. Jeremy Rayner: http://www.biology.leeds.ac.uk/staff/jmvr/Flight/modelling.htm
• True Fly: http://www.chez.com/truefly/Images/ctrflya2.htm
• Nathan Chronister. Flapping flight. http://www.ornithopter.org/flapflight/home.html
• Richard Dryden. Flapping wing flight. http://www.nurseminerva.co.uk/adapt/flap1.htm

Wings

  Therefore we can simplify it as follows:  Flapping wing hand creates Thrust, which builds up Airspeed, which flows around wing arm and generates Lift.

  Gulls maintain lift all along the flapping cycle. However, most birds cannot generate lift during the flap-up.

  Air density decreases with altitude.

  The flight will be modeled in two modes:

1. Soaring/gliding mode: It will be basically a glider model. The 3D animation will show how the wings are used to roll and pitch.
2. Flapping mode: The player will be able to control the power of each flap, each one producing an added thrust.

  The precise movements of the wings will not be included into the flight model. However, they will hopefully show accurately in the 3D animation

Tail

Bird weight and aerodynamism

Landing and take-off

Landscape-dependend wind field

1. Differential wind speed with altitude: a lower wind speed close to the sea surface will allow the player to reproduce albatross' infinite gliding technique.
2. Above land, wind will be modelled in a more elaborate way, with upwelling on the windward side of a hill, acceleration at its top, and turbulences on this leeward side.
3. Thermal effects will generate vertical air movement.

• *** -. Flight Lectures, University of Wales, Aberystwyth. http://users.aber.ac.uk/jrj/
• Aviary.org. Wings to fly. http://www.aviary.org/curric/wings.htm
• *** Richard Bach: Jonathan Livingstone Seagull. http://suchit.net/inspiration/JonathanLivingStoneSeagull.html.
• Bird Site. Flight mechanics. http://www.nhm.org/birds/guide/pg018.html
• Burgers, P., and L. M. Chiappe. 1999. The wing of Archaeopteryx as a primary thrust generator. Nature 399:60-62.
• J. A. Cheyne. The Discovery of Flight: Models, Commitment, and Flexibility in Creative Problem Solving. http://www.arts.uwaterloo.ca/~acheyne/Misc/12WRIGHT.html
• *** Nathan Chronister. Flapping flight. http://www.ornithopter.org/flapflight/home.html
• Richard Dryden. Flapping wing flight. http://www.nurseminerva.co.uk/adapt/flap1.htm
• *** Gull, Vulture & Pelikan. 2003. Santa-Barbara Waterfront and backcountry. I learned a lot just by observing them soaring and gliding.
• *** Hedenstrom, A. 2002. Aerodynamics, evolution and ecology of avian flight. Trends in Ecology & Evolution 17:415-422.
• *** Tony Mander.1999. Introduction to flight. http://www.airbp.com/Downloads/introtoflight.pdf
• L. P. Mouillard. Gliding Flight. http://invention.psychology.msstate.edu/library/Magazines/Mouillard_Cosmo.html
• Matthias F. Neef. Numerical Computation of Flapping Flight. http://www.ism.tu-bs.de/WRLD/mneef/neef_e.html
• C.J. Pennycuick. 2001. Speeds and wingbeat frequencies of migrating birds compared with calculated benchmarks. J.Exp. Biol. 204, 3283-3294. http://jeb.biologists.org/cgi/reprint/204/19/3283.pdf
• C.J. Pennycuick, A Hedenstrom, and M Rosen. 2000. Horizontal flight of a swallow (Hirundo rustica) observed in a wind tunnel, with a new method for directly measuring mechanical power. J. Exp. Biol. 203(11): 1755 - 1765. http://jeb.biologists.org/cgi/reprint/203/11/1755.pdf
• Jeremy Rayner. Modelling Flapping Flight. http://www.biology.leeds.ac.uk/staff/jmvr/Flight/modelling.htm
• *** Seattle AirGear. The engineering of stunt kites. http://www.seattleairgear.com/pd.htm
• Jeff Smith. A Method for Rigging an Eagle's Wings in Maya. http://www-viz.tamu.edu/students/jsmith/617/rigging/riggin.htm
• Tobalske, B. W., T. L. Hedrick, K. P. Dial, and A. A. Biewener. 2003. Comparative power curves in bird flight. Nature 421:363-366.
• "True Fly". Theory of flapping flight. http://www.chez.com/truefly/Images/ctrflya2.htm 
• Mary Taylor Young. Airborne. http://wildlife.state.co.us/Colo_Wild_Co/sum2000/airborne.htm