Flying vs. Gliding
Before analyzing different methods of flight, it is important to consider what different cultures and traditions may consider 'flight' or 'flying' in relation to modern conceptions. For many centuries, the term 'flying' could be applied to any animal that could be airborne and at least appear to safely navigate or land. Thus, many animals are attributed with misnomers, such as the "flying squirrel" and the "common flying lizard," both of which are gliding animals. Similarly, there are several species of "flying serpents," all of which are a part of the Chrysopelea family. These snakes flatten their bodies by extending their ribs, allowing them to glide through the air.
Gliding differs from flying in that a gliding animals navigate through the air but cannot propel into the air from the ground. Fliers, on the other hand, have the ability to propel into the air and traverse upward from a current position. Many gliding species use air foils to keep aloft, such as a simple flat of skin that can be stretched out, as in the case of Draco Volans. 
While some animals can glide but cannot fly, animals that can fly do glide.  So while many species are now known to be gliders, such as "flying squirrels" and "flying fish," the ability to fly has previously been attributed to them. It is likely that many dragon stories could be referring to a gliding animal.
Insects: First in Flight
Insects evolved the ability to fly long before any vertebrate species. Over four hundred million years ago, the first insects began to fly.  Unlike the vertebrate species that evolved flight later, flying insects became fully hybridized creatures, with the ability to crawl and move with legs while touched down as well as the ability to fly.  Pterosaurs, Avian species, and bats all became ungainly due to the evolution of the forelimbs into wings. While pterosaurs and birds certainly maintained their grounded mobility by becoming bipedal, the same cannot be said for bats, which must sprawl awkwardly on the ground.
Insects, on the other hand, retained the ability to crawl easily and fly. Their wings are thin, transparent membranes stretched between rigid veins, making a wing that is strong and flexible.  The wings are designed to stir up the air, and the muscles for insect wings are exclusively contained within the thorax, or midriff, of the insect, which means that their wings do not even have muscle tissues to weigh them down.  This is why insect can flap their wings far more quickly than other flying creatures, such as birds. The hummingbird has the fastest known flapping speed among birds, flapping its wings about fifty times per second.  Meanwhile, bumblebees flap their wings approximately 150 times per second  and some mosquitos can flap up to one thousand times per second. 
The largest known flying insects were the Meganeura, a genus of dragonfly-like insects that had a wingspan of 75 cm (2.5 ft), which are extinct. 
While insects evolved flight first, and retained their solid ground mobility, size is an important consideration. The fact that the largest known flying insects possessed a wingspan of roughly 75cm (2.5ft) is a strong indication that draconic creatures did not descend from this lineage.
However, the functional adaptations of flying insects, such as the concentration of the flight muscles in the thorax to reduce the wing's mass, make insects worth mentioning. All other species known to fly have evolved an extreme set of adaptations for the sake of being more aerodynamic.
From the time when fossils of pterosaurs were first discovered in 1784 , scientists had assumed that pterosaurs had a bat-like wing structure. This became perplexing, as investigations into the aerodynamics of the species revealed that a bat-like wing structure would make them particularly poor fliers. Kevin Padian is attributed with unraveling the mystery of the mistaken pterosaur flight design, while pushing his streamlined variation of the pterosaur,  revealing it to be a truly separate evolution of flight from bats and birds.
Like bats, pterosaurs had a skin wing that stretched out from "fingers" (aka digits and phalanges) and the forelimb to the hindlimb or torso/hip.  Unlike bats, however, pterosaurs did not evolve a skin attachment that required exact positioning of the hind limbs for flight (bats must be splayed). Much like birds, pterosaurs had a dual locomotion set, with bipedal walking and forelimb-driving flying. 
The largest known pterosaur, and the largest known flying animal to have ever existed, is Quetzalcoatlus northropi, with a wingspan of almost 12 m (39 ft.) and at a weight of approximately 127 kg (280 lbs.) Like the non-avian dinosaurs, the pterosaurs disappear from the fossil record around the time of the K-T extinction, approximately 65 million years ago.
Pterosaur flight is an important consideration in dragon flight because, unlike all other known fliers, pterosaurs came in enormous sizes. Of course, many of the species were far smaller than the Quetzalcoatlus northropi, but the ability to grow a wingspan of 12 m (39 ft.) would certain fit into the category of "huge" exhibited by many of the dragons in stories found around the world.
Birds evolved flight somewhere around one hundred and sixty million years ago  so there is a wide variety of flying avian species, ranging from the hummingbird to the albatross. How do birds fly? The short answer is "with difficulty."  Birds have many extreme adaptations to support their ability to fly, from having air sacs inside the bones that coordinate with the lungs to having fewer bodily organs, depending on the species. Avian species traded off strength and weight for aerodynamic leverage. 
The largest known wingspan for birds is 3.63 m (11 ft. 11 in), which is the maximum wingspan of the wandering albatross.  Most flying birds are much smaller in wingspan, since the larger the bird is, the harder it is to fly.
Unlike any other creatures, birds uniquely evolved and adapted feathers. No known living creature outside of the avian family has feathers, and even non-flying birds have feathers. Currently, there is much evidence to suggest that feathers evolved from reptilian scales, and feathers and scales are certainly related.  Therefore, there is a strong likelihood of the existence of many species of reptiles-with-budding-feathers and early avian species with reptilian scales.
Bats are the only known mammals that have evolved powered flight. Bats evolved flight fifty-five million years ago.  Like pterosaurs, bats have wings comprised of stretched membranes. The fifth finger (the 'thumb') of a bat is not elongated like the others; instead, it is a hook that can be used for climbing.  Despite the common expression "blind as a bat," no species of bat is blind, and most species have excellent eyesight. On top of sharp sight, bats also have echolocation, which enables them to fly at night to hunt for tiny insects. 
The largest known bat species is Giant golden-crowned flying fox (Acerodon jubatus), which has a wingspan of up to 1.5 m (5 ft.) and can weigh around 1.1 kg (2.5 lbs.) 
Novel Theory: Hydrogen Drifting
Although the origins of this theory are not entirely clear, it has been suggested that flight can be achieved in a dragon-like species by way of hydrogen. Hydrogen has been used in flying propulsion systems, most notably the Balloon, also referred to as the hot-air balloon. Hot air balloons rely on buoyancy - much like a ship floating in the water - to remain in the air, and hydrogen overcame the obstacle of keeping the fire lit constantly to keep the balloon aloft.
The trouble with this theory is that it relies on a propulsion system not found in any known species and that does not provide great control over mobility. An animal would have to have the ability to completely control their hydrogen production and retention, including expelling the hydrogen into a suitable bladder to promote buoyancy, and safely expelling the hydrogen out of that same bladder in order to descend. The animal would also have limited, if any, control over movement. Just as a strong current can move a buoyancy-driven boat with dangerous consequences, atmospheric conditions would affect the travel of an animal with hydrogen-powered-flight. Controlled ascent and descend would be the only sure abilities for such animals, so pockets of hot air and the wind would push the flier around considerably.
In order to be light enough to fly, the animal would also have to be fairly small, or sacrifice much of is muscle and fat mass at least, otherwise the bladder used to create buoyancy would be enormous. For example, consider a hot air balloon's balloon in relation to the basket contents it is carrying. Since modern science shows that avian-dinosaurs survived the K-T extinction and evolved into birds, it is safe to assume that from about four hundred million years ago, when insects first flew, some kind of predator inhabited the sky with powered flight. The pterosaurs would have likely been more potent predators, and modern birds remain formidable hunters. The ability to ascend into the sky, therefore, would not be nearly as useful to escape predators as one might think, especially if the creature ascends into the sky with no ability to steer once aloft.
In addition, hydrogen is a highly combustible element, and as a gas, the higher the concentration, the higher the change of combustion. Hydrogen gas also forms explosive mixtures with air at certain concentrations. The likelihood of a living organism developing and surviving such a dangerous, yet oddly useless, propulsion system is extremely unlikely, as the organism would have to avoid all sparks, flames, and even sunlight because high concentration of hydrogen and can spontaneously combust with sunlight. On top of that, the creature would have to diligently avoid being spotted by any airborne predators, as it could not steer to escape them.
While a hydrogen-filled bladder is unlikely to enable any living organism to fly, this novel theory does present an interesting concept for non-flying gliders. The ability to gain enough lift to remain aloft without expending a tremendous amount of muscle energy would be highly valuable for a glider or a flier that relies on soaring. However, as no known species has a biological structure that utilizes something like hydrogen to produce thrust (a force that counteracts the downward gravitational force), it is a difficult concept to integrate hypothetically into a flying organism.
Dragon Flight: Evolving the Ability to Fly
Insects, pterosaurs, birds, and bats, oh my! These known creatures can give us a stronger understanding of the drastic requirements of flight on a living organism. Many animals have sacrificed size in order to fly, and bats have lost grounded mobility for better flight mobility.
It is important to remember that while many of these novel developments for flight seem elegant and even intelligent, evolution is not chosen. Pre-avian dinosaurs did not "choose" to adapt scales into feathers, and bats did not "choose" to give up mobility on the ground for the ability to fly. Changing environments, from the weather to local predator and prey populations, fueled survival rates of certain biological adaptations that resulted in the ability to have powered flight. However, the ability to fly does not mean that the animal can fly well, or that there are not severe restrictions as a result of the development.
It is also important to keep in mind that true flying and gliding have been separate on the basis of mechanics. A glider keeps aloft by slowing its momentum towards the ground due to gravity, while a true flier has the ability to power its own flight with muscles and movement. That being said, for many thousands of years animals that could glide well were considered flying animals, so when considering theoretical dragon flight, it is important to consider the possibility of a dragon being a gliding animal.
How do animals evolve the ability to fly? Unfortunately, there is not much fossil evidence that revels the exact method of evolution for birds and pterosaurs, although there are several theories out there, the two basic ones being "Ground Up" and "Trees Down."
In short, the "Ground Up" theory posits that before an animal becomes a flying animal, it must become bipedal. Generally, the animal would probably be a quick, bipedal runner, which would allow it to have its forelimbs ready to grab prey as it runs.  Incidentally, this same adaptation would allow the forelimbs to be free to develop novel adaptations that would enable the animal to gain lift during high leaps (pre-flight techniques) as well as developing into full-fledged wings.  Eventually, the ability to fly by flapping would develop in some species.
Meanwhile, the "Trees Down" theory has evolutionary traits that start with plummeting, usually for arboreal species. The next development would be parachuting for safe landing, then gliding, then powered flight. 
Both theories have generated due controversy, but it has been widely acknowledged that different forms of flight (such as bat flight vs. avian flight) may well have evolved differently. Another key consideration is penguins, which have developed wings that help propel them through the water instead of the air. Clearly, in the case of avian species, bipedalism came first, and the environment influenced the evolutionary adaptations from there. Some birds lived in areas where traversing the air become beneficial, so those birds that developed the ability of powered flight survived. Other birds lived in areas where the ability to swim promoted survival, so those that developed fin-like swimming wings survived.
The progression of biological adaptations for bats is better understood, so it is easier to describe the evolutionary steps from a mammalian gliding ancestor to the powered-flight flapping flier known today as the bat , which fits the "Trees Down" theory of the evolution of flight. However, since flight has evolved independently for insects, pterosaurs, bats, and birds, it is possible that the "Trees Down" theory does not apply to birds or to pterosaurs.
Dragon Flight: Thermoregulation
Luckily, the abundance of birds today presents a fuller picture of the necessities of flight. Pterosaur fossils cannot reveal if these animals were ectothermic (as modern reptiles, which acquire their bodily temperature through external sources) or endothermic (as modern birds and mammals, which generate and maintain their own bodily temperature). Birds are exclusively endothermic, maintaining their own bodily heat, making it likely that pterosaurs would also be endothermic.
Thermoregulation is necessary because it promotes the use and fluctuation of energy within the animal; an animal that generates its own heat can have more efficient muscle movements, for example. Warmer muscles can provide more energy than muscles are a cooler temperature, which is why birds and mammals are endothermic, maintaining their own internal heat. However, ectothermic animals, which obtain heat from the environment, save energy by absorbing heat from outside sources, meaning that their metabolisms would not demand additional food for body heat maintenance.
While there is a substantial amount of controversy over the evolution of endothermic species from ectothermic species, science has revealed the positive aspects of both forms of thermoregulation. Insulation, which happens to be provided by feathers in spades, is necessary for an endothermic creature living in a cool, cold, or seasonal climate. However, ectothermic creatures cannot live in severely cold climates, as there must be some source of heat by which to regulate their own bodily temperature.
Birds must deal with not only the energy requirement of flight, but also the incredible amount of heat produced by their muscles during flight. Many birds have specialized air sacs throughout their bones, enabling a respiratory-related cool-down to prevent hyperthermia. All flying animals would have to deal with both challenges of thermoregulation to survive.
Since both bats and birds are endotherms, many have assumed that other flying species would also be endotherms. Due to the heavy energy costs of flight, it is unlikely that an ectothermic animal would be able to fly. However, another possibility for thermoregulation can be found in white sharks.
White sharks, and other mackerel sharks, are the only known sharks to have a partially endothermic thermoregulation system.  Most sharks are ectothermic entirely, and since they live in the ocean and have no insolation, the body heat generated by their powerful trunk and tail muscles as they swim dissipates quickly into the surrounding water.  Unlike true endotherms, white sharks do not maintain a constant body temperature, but because of their modified circulatory systems, select organs have elevated temperatures to improve efficiency. [5,6]
The mechanism for this heat exchange is simple. Arteries supplying cold blood to the muscles run parallel to veins draining warm blood from the muscles. Since the muscles of the shark generate heat, the outgoing blood warms the incoming, cold blood, resulting in retention of heat by way of movement.  The main heat exchangers of the shark lie in the trunk muscles, or the large, powerful muscles, of the shark. It is important to mention, however, that white sharks are also subject to constant movement due to their oxygen intake method, dubbed ram ventilation, where a constant move forward pushes water over gills so that oxygen can be absorbed.  If the shark stops moving for too long, the heat produced by its movement will dissipate, but it will also soon drown because its gills must have water flowing across them in order to breath. Near-constant motion keeps white sharks alive both by oxygenation and by enabling better heat retention.
Drawing on the white shark's modified respiratory and circulatory system, it is possible to consider a flying species that could also be partially endothermic, keeping key muscles and organs at an elevated temperature. This would also reduce the need for a sophisticated cooling system, which avian species possess. However, the animal would have to be in some form of constant motion, or otherwise be producing substantial body heat. Since weather often affects flying ability, it would be unlikely that such an animal would need to constantly fly. So if there was to be a partially endothermic species, perhaps represented in the mysterious pterosaurs, they would have to have another method of generating high body temperature in case of poor weather or even in times of heavy rain.
Dragon Flight: How would dragons fly?
Much like birds, it is likely that dragons would fly with difficulty. The wide variety of dragon types makes exploring the possibility of flight even more arduous, since a wyvern and an amphiptere have a wide deviation in body type and size.
However, there are some general rules that can be taken away from creatures that are known to fly or to glide.
- Gliding and flying are separated by the animal's ability to power its own flight.
- Most gliding animals are small.
- Most flying animals are also small, especially when it comes to weight.
- Thermoregulation can add additional strain on an animal, and all known flying animals are endothermic.
- Powered flight requires severe adaptations.
Depending on the type of dragon, its ability to fly would be restricted, above all, by its size. By and large, both gliding animals and those with true, powered flight tend to be small. The largest wingspan among the known, living bats is 1.5 m (5 ft.) and the largest wingspan among the known, living birds is 3.63 m (11 ft. 11 in.). While neither of these wingspans can be considered "tiny," they certainly do not reflect the idea of an enormous beast that is of unthinkable size. The only known flier to be larger than this is the largest known pterosaur, with a huge wingspan of 12 m (39 ft.). This would certainly qualify it as an enormous monster, especially compared to average sized humans (1.47 m to 1.9 m tall or 4ft. 10in to 6ft 3in tall).
There have been large, flying animals on this earth, but the majority of fliers are small. It is therefore likely that dragons would fall into a small range of sizes (approximately 1 m to 3.5 m, or 3.3 ft. to 11 ft. for wing span) and be very light animals.
- Alexander 2
- Alexander 6
- Bird 70
- Coupe 47
- Coupe 50
- Coupe 51
- Nuridsany 65
- Nuridsany 66
- Shedd 65
- Shipman 55
- Shipman 178
- Shipman 179
- Shipman 182
- Shipman 204
- Shipman 205
- Shipman 224
- Shipman 224-226
- Shipman 225
- Shipman 237
- Shipman 238
- Shipman 248
- Giant Golden Crowned Flying Fox
For more information on footnotes and references, please see the bibliography.