Wednesday, April 13, 2011

Physics Question

Here's a case of where my knowledge of physics and related sciences has proven inadequate, so I'm asking for help in answering the following question: assuming a world much like Earth in terms of gravity and air density, what is the largest plausible size for a flying creature that's not some paper-thin jellyfish-like thing? Now, assuming you're free to adjust gravity, air density, and other factors, is it at all reasonable to imagine any world where a human-sized being could ride a flying creature?

These questions are pure speculation. I'm asking only because I was thinking first about flying dragons and then, inevitably, about dragon riding, both of which seem impossible without magic doing the heavy lifting. But then I thought maybe I was being too hasty and there was some way to justify such things. So, what's the considered opinion of the readership of this blog? Yea or nay?



  1. Well Quetzalcoatlus was (probably) around 10–11 meters (33–36 ft) in wingspan & (maybe) 90–120 kilograms (200–260 lb). Argentavis was like, 7 m (23 ft) in wingspan & 70 kg (150 lb) in weight.

    So...human-sized flying critters are plausible, is my first thought. That also means that there could totally be some of the smaller dragons-- heck, you could even claim that a "huge"-mini'd dragon was lighter than it looked & that wing-size was part of its spacing.

    As for riding? That...that strikes me as tough. So far I'm just looking at things that have existed in real life-- not scientifically plausible scenarios like gas-bladders (if that is plausible...) or atmospheric mixtures. Still, if the biggest fliers were around human sized, that makes me think that adding ANOTHER human sized piece of mass into the equation would sink it.

  2. Some things that come to mind are, Birds have hollow, porous bones to allow them to fly by reducing their weight. The largest birds do not even flap their wings, and utilize thermal rises to help them soar.

  3. is the largest flying bird ever discovered on earth. Wingspan ~23 ft., Weight ~150 lbs. It was built for eating prey whole, on the wing, and one assumes it could pick up pretty hefty creatures. and similar pterosaurs had wingspans of ~35 ft. and weighed an estimated 500+ lbs.

  4. A moon where humans could fly...


    The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms.

  5. Also, keep in mind modern fixed-wing aircraft. Speed easily makes up for size or just have to get the air moving over the wing fast enough to generate lift

  6. With aircraft, my understanding is you are still bound by certain limitations of mass and wingspan. At least until jets get involved, then all the rules get chucked and re-done. :-)

  7. You've also got to consider the maximum power output of muscle tissue, assuming you actually want these things to FLY rather than glide. That, I think, is where the top limit is likely to come in - how much flight muscle can a creature carry before the mass of said muscle becomes too heavy for it to lift itself plus the bone mass to which it's anchored?

    Now if you had insect musculature set on an avian skeleton, you might be getting somewhere. Where, I don't know, but somewhere!

  8. If you can adjust gravity/air density then I think the answer is definitely less. According to Poul Anderson, air density matters a bit more than gravity so a more massive world might do.

  9. When I worked for AMNH we did a bit on the evolution of dinosaurs into birds, and on that occasion I was told that according to our current models the muscle power required to take off from the ground fixed a practical limit of 15kg on birds. That larger pterosaurs and other prehistoric beasties seemed to break this limit, and that the current thinking was that these were gliders that routinely took off from high ledges and had to catch thermals to be able to land on the same ledge.

    Sounds a bit like a crock to me, but I'm not a biologist.

  10. From wikipedia:

    Studies have shown[citation needed] that it is physically possible for flying animals to reach 18 m (60 ft) wingspans, but there is no firm evidence that any flying animal, not even the azhdarchid pterosaurs, got that large.

    To bad the citation is missing.

    However, it would also have to have the strength to carry a rider. So assume that its a predator that preys on horses and such. Of course, such flyers don't have the greatest flying endurance.

  11. It would depend on how you classify human. There is a few real world fliers that could theoretically pick up a grown human if you go for the extreme side of the bell curve.

    As for fliers that could theoretically exist on earth, anything that would be the size of generic D&D dragons would probably not be able to but if you could get a big enough flier that the mass of a human would be quite small comparatively then yes. Anything dragon sized even if it could lift a human, because of the energy needs of such a lift the flight would be quite short.

  12. insect musculature set on an avian skeleton
    this is creepy cool. A lot of insects take advantage of weird viscosity properties of air at very small scales, though.

    So it's simple! Just make your characters and monsters what, 300x smaller than humans! There may be some problems with stuff like surface tension of water, but the advantages are probably huge.

  13. Glider Aircraft are of a reasonable size and weight to something like a dragon.

    That one weighs in at around 800 pounds, can carry two humans comfortably:

    Schleicher ASK 21
    * Capacity: 1 pilot, 1 passenger
    * Length: 8.35 m (27 ft 5 in)
    * Wingspan: 17.00 m (55 ft 9 in)
    * Height: 1.55 m (5 ft 1 in)
    * Empty weight: ca. 360 kg (793 lb)
    * Gross weight: 600 kg (1,323 lb)
    * Maximum speed: 280 km/h (174 mph)

  14. Richard,

    That made me think of a talk I heard once from a physicist and mathematician. He seemed to have no problem with evolution in the overall sense, but questioned the nuts and bolts of many things we just can't explain - like the evolution of flight. He pointed out that flight is more than the existence of wings. Glue wings to a car and it doesn't automatically fly. Everything in any object has to be made solely for the purpose of flight, or flight doesn't exist. And yet, if everything in an object is geared for flight, with flight not yet existing in that object, that object becomes a sitting duck - literally, questioning how then flight, with all of its myriad interconnected necessities, could have evolved at all. It made me think. Which might suggest that we could always say that dragons fly because that's how they were made, and leave it at that.

  15. One thing to think about - if you increase the oxygen content of the atmosphere then muscles have more power and thus flapping wings can lift more. Couple that with a bit less gravity and people-sized fliers are probably quite possible with only a small amount of environmental tweaks.

    - Ark

  16. Dave G. said: "Everything in any object has to be made solely for the purpose of flight, or flight doesn't exist. And yet, if everything in an object is geared for flight, with flight not yet existing in that object, that object becomes a sitting duck ..."

    Neither of those statements are true.

    The current top theories for the structures which were re-purposed evolutionarily for flying are: (1) long arms used to capture prey, (2) bipedal leaping action, (3) gliding behavior.

    You can see a presentation on the subject at UCMP Berekeley. If you following the links, you'll see that the fossil record shows flying evolved separately in three different animal groups (pterosaurs, birds, bats), in each different case winding up with the same basic flapping behavior.

  17. I presume you're aware of Peter Dickenson's
    "The Flight of Dragons" which often gets brought up in these discussions, and so I'll throw that one into the ring.

    Long story short, dragons are biological zepplins who's fire breathing is a result of the chemical process' that fill their internal chambers with flammable gas.

    It's a fascinating read.

  18. Possibly interesting, plausible angle, semiscientific:

    human-sized beings can ride pterosaur-sized beings however, it requires the humans make some existing-tech modifications to themselves. The humans carefully design hang-glider like fins so that they catch the air properly and so are only actually bearing down with 100% of their weight during sudden direction shifts.

    possibly the humans also employ helium balloons or other lighter-than-air gasses to make themselves lighter.

    This is -so- not what you were asking, right?

  19. You could take a freaky Lovecraftian approach where the reason they can fly is because the laws of physics experienced on the planet are only part of the entire spectrum.

    The dragons/whatever came from... outside... and as such can not only do certain 'impossible' things but also creates terror in most people because of their nature.

    The people who would ride or work with them would necessarily become a bit... other... in order to work with them without suffering the terror attacks.

  20. Anything is possible!!! Look at the BEEZZZZ...

    Turns out bee flight mechanisms are more exotic than thought.

    "The honeybees have a rapid wing beat," Altshuler told LiveScience. "In contrast to the fruit fly that has one eightieth the body size and flaps its wings 200 times each second, the much larger honeybee flaps its wings 230 times every second."

    This was a surprise because as insects get smaller, their aerodynamic performance decreases and to compensate, they tend to flap their wings faster.

    "And this was just for hovering," Altshuler said of the bees. "They also have to transfer pollen and nectar and carry large loads, sometimes as much as their body mass, for the rest of the colony."

    Try this!

    In order to understand how bees carry such heavy cargo, the researchers forced the bees to fly in a small chamber filled with a mixture of oxygen and helium that is less dense than regular air. This required the bees to work harder to stay aloft and gave the scientists a chance to observe their compensation mechanisms for the additional toil.

    The bees made up for the extra work by stretching out their wing stroke amplitude but did not adjust wingbeat frequency.

    "They work like racing cars," Altshuler said. "Racing cars can reach higher revolutions per minute but enable the driver to go faster in higher gear. But like honeybees, they are inefficient."

  21. Zak's idea-- maybe without the balloons-- has a lot of stick with me. Halflings with hollow bones who have been modified (or evolved or whatever) to be dragonriders? Boom, that is some...that is some hot shit right there.

  22. Delta,

    Well, he was the physicist with an emphasis in aerodynamics, so I would probably mutilate his talk, me being but a humble historian. But I think the link more or less demonstrates his point. On one hand, you have much about long arms and wings and gliding and cabbages and kings. On the other hand, you have examining birds, bats, and pterosaurs already capable of flight, in full possession of the thousand interconnected necessities needed for flying. He merely mentioned that saying you have animals that could be ancestors of flying creatures already in full possession of the mechanics needed for flight simply because a pair of arms may have grown long is skipping a pretty big gap. That's like saying 'How to change an automatic transmission in four easy steps. Step 1. Remove the engine.' There's a lot in between those two steps not accounted for.

    Again, he wasn't trashing evolutionary theory per se. He was simply pointing out the limits we have, and the sometimes leaps of faith we take in explaining and applying things that the overarching evidence doesn't quite support. Beyond that, I'm sure he said it better than I.

    Not to drag Jame's post down with the eternal debate or anything. In terms of the post, I'm fine with 'because they were made to fly and have dragon riders, that's why!' It is, after all, a world in which we concede dragons in the first place.

  23. Dave G.: "But I think the link more or less demonstrates his point."

    I think the link demonstrates exactly the opposite of his (your) point. So now I've given a link to a specific exhibit on the subject at UCMP Berekeley, and you've given some hearsay from a speaker whose name you can't remember. So I'm happy to leave it at that.

  24. I'll echo Bigfella's comment. You MUST read "Flight Of Dragons" because it contains a reasoned argument that the many mythic qualities of dragons are all consequences of their mode of flight, it has a lot of nice historical overviews of dragons in legend in many cultures, and finally because the artwork is spectacular.

  25. Delta,

    Sorry about the lack of a link. It was more or less an off the cuff thought. I didn't think James would want a prolonged discussion. In another forum I would be happy to roll up my sleeves and post away, but not here. I was responding to Richard's post that simply made my mind wander back to a time a few years ago when I sadly didn't have a camcorder in hand to post on a web page. I didn't mean to strike a nerve in anyone or step on any sacred cows. Thought it would be an interesting addition to Richard's thoughts, that's all. No big deal.

  26. Well, I did have this awesome dream once where a black snake unfurled Black Feather Wings and as they flexed, they revealed sacks in the skin containing Lighter than air gasses which when heated by the sun provided some lift... So other than some freakish critter that retains its ability to fly by working those muscles and burning fuel...A bag of Hydrogen gas to provide lift seems plausible to keep a huge Zeppelin sized beast aloft. Considering Dragons like flamable gasses to warm their marshmallows, They must store Hydrogen somewhere and simply breath in oxygen as needed (for respiration and blowtorching Knights).

  27. Quetzalcoatlus Northamericanus was probably one of the largest airborne species. Although I believe there was one larger, recently discovered.

    Biologically it is a mix between energy expenditure and energy intake, which is why most large flyers are actually soarers that rely on thermals to gain altitude (just like a glider pilot, although far more efficiently). This dynamics of thermals also serves to cap the effective wing-span, since with larger wing-spans the efficiency and availability of useful thermals declines (more so than is governed by the structural strength of biological materials).

    As to carrying a rider, most of the large flyers are already at the limit of the muscular power to weight ratio for flight. That's before you start accounting for the effects of drag and loss of lift due to the disruption of the airflow (for most large birds the entire bird is a lifting body, not just it's wings). So it's a matter of reducing the flyer's weight, or increasing their muscle power, and reducing the weight and disruption caused by the flyer (perhaps by slinging them underneath).

    If you want to look at changing the environment you have to examine the attributes of aerodynamic lift provided by Bournelli's Principle [v^2/2 + gz + p/ρ = constant], which measures the conservation of energy in a fluid (air in this case being the fluid).

    Increasing velocity (v) has the greatest effect, which is why the faster a plane flies the more lift the wings produce. One idea to replace the gz term is to assume that gravity obeys an inverse-cube** (rather than inverse-square) law (g is acceleration due to gravity, which varies with height, z is height). Similarly increasing the air pressure and reducing it's density (say by replacing the nitrogen in the atmosphere with helium) will increase lift.

    [* Having been in a thermal with a wedge-tail eagle, and having sensed it's disdain for the amateur flyer sharing the thermal with it, I feel qualified to comment on this. <grin>]

    [** There was a an old White Dwarf article (well, pre-100) that investigated the consequences of doing this in a fantasy campaign.]

  28. One thing about upscaling size is that legs (and wings) are not in general designed to support huge mass. The problem in part is that legs (and wings) tend to break or not based on their cross-sectional area (a squared function) while mass tend to grow as its volume (a cubed function).

    In other words, if an ant was the size of an elephant it wouldn't look like an ant anymore; it would look more like an elephant so that its own weight doesn't break its own legs. I assume that wing size/mass would have a similar relationship.

  29. The thing is that while it might be possible to build a creature that could fly with a rider's weight, one is not likely to evolve unless there is some reason for it to have that excess of lift power -- maybe a predator that carried human-sized prey back to its nest or something, only it would be more practical to eat the prey and regurgitate bits for young, as many birds do.

  30. Realistically, I would say no.

    Even if you had a saddle or some sort of aerodynamic howdah, the speeds needed to overcome the air resistance would be something fierce.

    But this is fantasy, so selection, breeding and training of mounts can explain that away: if yr dragons have a pronounced notch between thoracic and cervical vertebrae, you could possibly put a saddle there, why not? Train them to carry increasingly heavier loads until they can bear a human-sized rider.

    * * * * *

    To the commenters:

    Ack! Where are the paleobiologists when you need them...

    First: Birds are descended from dinosaurs, so reverse engineering doesn't work well here. The pterosaurs have massive forelimb bones and very slender bones in the hindlimbs. Not like modern birds at all...

    This is Julia Molnar's animation of Michael Habib's theoretical launch, note the use of all four limbs.

    These are some of the major muscles that power that lift. I think 'weight' is going to come down more on the differing muscle and tissue densities, rather than that of bone.

  31. Don't worry about size and weight of human riders.
    These depend essentially on nutrition. Think about African pygmies - nowadays young pygmy people grow as tall as other people from central Africa, while their parents, who grew up with access to root and berry based food only, are still REALLY small (halfling-sized).
    A culture riding Quetzalcoatlus or Argentavis-like creatures could exist, eating vegan or seafood only.

  32. @Dave G:

    This sounds like a common, wrong objection to evolution.

    It's not limited to flight. For example a polar bear has to have both reflexes and claws to catch fish, and has to be able to digest fish to gain any benefit. One without the others has no use. So how would either evolve? You might say that they developed gradually, but what use is almost being able to catch a fish, or almost being able to digest it?

    The answer, as I understand it, is that there is indeed an advantage to almost being able to catch a fish, or almost being able to digest it. If you can't quite catch a fish, you might be able to catch something a bit slower. Or your claws might have some completely different useful purpose. Perhaps your claws and reflexes can't catch fish, but they might help you defend yourself.

    In terms of flight, almost being able to fly is another way of saying 'able to glide'. And almost being able to glide is another way of saying 'doesn't fall quite as hard', the survival benefits of which should be obvious.

  33. James, your setting sounds like it's based on the kind of science fiction that's closest to fantasy, like the Barsoom series.

    So I wouldn't look for a scientifically-plausible justification, but just come up with something that sounds right. Maybe dragons chew liftwood (and the fire in their stomachs helps them digest it).

  34. They must store Hydrogen somewhere

    Or generate it and have to vent it periodically. Maybe they're nuclear-powered. I rather like the idea of the Hindenburg-dragon, which you can surprise and turn into a fireball, or fail to surprise and be incinerated by a venting of its flight apparatus.

  35. Apparently the oxygen content of the air was higher at certain points in the past, which allowed the giant pterosaurs to reach such a size.

  36. Large flyers like to use thermals. Dragons breath fire*. I would suggest that a classic dragon hunting technique would be to set fire to something**, eat whatever comes out of cover from the fire, then use the thermals from the fires to fly to a new hunting ground.

    In this ecosystem, dragon riders would probably have to be nomads, burning out areas, then moving on. They spend most of their time squabbling with one another, until one day arises a leader known as Dracis Khan...

    * Other dragon breaths are left for more thought
    ** Forest, grassland, city

  37. This is slightly off topic, but has anyone else noticed that this is a very silver-age discussion?

  38. Birds have hollow, porous bones to allow them to fly by reducing their weight

    At the risk of sounding pedantic, the hollow bones are an integral part of the avian respiratory system, which is vastly superior to the mammalian one.

    I'll also put in a recommendation for Dickinson's Flight of Dragons, it's one of the greatest examples of sustained whimsical lunacy I've ever encountered.

  39. Yeah. Dragons man... DRAGONS!!

    (and flying dinosaurs with 30-40' wingspans)

  40. anarchist,

    Well, heh, there's a reason that's a common objection. Because it's a big one. There is no real actual verifiable proof demonstrating the objection is baseless. There are tons of theories of course. And if you assume certain things as true, then the theories work. Sort of like if you go ahead and assume the defendant is guilty due to clear motive, lack of actual evidence is less problematic.

    As I understand it, the problem with flight, as well as a few other complex, interdependent biological phenomena, is its nature. Everything needed to fly has to be in place at once, or you don't fly, or glide for that matter. Get into a plane next time that says 'almost able to fly.' Everything in the Wright Brother's plane had to be in place to fly. Everything in a 747 or a helicopter has to be in place for it to fly. Same with a bat, sparrow, fly, bumblebee, humming bird, or whatever. We simply have no records of 'almost able to fly or glide' leading to 'there he goes!'.

    A polar bear almost able to catch a fish is almost able to catch a fish. That's one thing. That's saying almost able to walk is almost able to run. Flying compared to catching a fish is like lasers compared to rocks and clubs.

    As for 'almost able to fly is able to glide', not really. Gliding is its own phenomenon. The mechanisms needed to glide are different than those needed to fly. There are a lot of steps needed between those two. For that matter, there is a gaping hole between organisms not able to glide and those able to glide. A huge, big, giant gaping hole. Going from 'animal unable to get off the ground to animal able to glide through the air' is like trying to make the case that almost able to walk is almost able to play Mozart's piano concerto No. 21. A whole lot of steps in between, all traits needed to be in place at once or you don't glide - you fall.

    Plus, and this is a biggie, there is no evidence that all of the various physical and interconnected traits needed for flight in the legions of different organisms capable of flying are advantageous in and of themselves. More than that, there is reason to argue some would be disadvantageous to an organism's survival without the other traits that, working together, allow gliding or flying (that was his main point). I've found that looms large in many cases in which flight is brought up.

    Sadly, as I said, I didn't have a link to the fellow since I merely was there some years ago listening because of our friendship, sans camera. I just brought it up because of what Richard wrote, and given the topic of the post, it clicked in my mind and I thought it would be a neat addition. But you don't have to look long on the Internet to see hundreds of pages dedicated to explaining why flight really isn't a problem. Because, in the end, flight is really a problem. As are other things. FWIW, the point of the talk wasn't to poke or attack evolution per se. It was to demonstrate the differences between science and faith in science. But thanks for the info. It's the stuff that makes science fun.

  41. As to the original question ...

    You want a multi-ton flyer, you just need something like 3x air density and 3/4 gravity. Four times the lift vs weight for the same structure allows through the cube-square limits on muscle power a dragon-size flyer with enough extra lift for a man (4 x as long, 16 times as strong, and 64 times as heavy as IRL limits, or a touch less than that for various biological reasons).

    The lower gravity also helps support upright giants and more epic mountain ranges, and makes dungeons more stable with all that volume removed.

    High air pressure explains falls that max out speed after 200 feet, fast giant insects, and leaves the flyers fairly slow. A relatively low O2 content could explain the difficulty of burning down your adventure modules.

    With smaller 1st edition dragons and giants, you don't need much change at all. Helps you ignore things like narrow day/night temperature ranges on high-pressure worlds.

    Technical problems introduced would be the ease of condor-sized birds doing clever acrobatics, and the odd funny look when noticing how fast a Monk can still run.

  42. Part I (off topic): I listened to an interesting science show recently that said that a number of dinosaurs and pterosaurs were nocturnal, and others straddled both nocturnal/diurnal (whereas we used to assume that dinosaurs/pterosaurs were diurnal kritters). A lot of the raptors were apparently nocturnal predators.

    Part II (on topic): flight for dragon-size things would also be easier if their bones were constructed of a stronger & lighter compound (which doesn't seem that unreasonable), and perhaps if their muscles were some combination of stronger/lighter/more efficient too.


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