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I have been working on a world which has a very limited type of magic. This magic almost necessitates a bird likes creature that I think I might be able to convert into a non-traditional dragon. Currently, it is easy and convenient for the dragon to:

  • have magically durable rock-like scales (thin)
  • produce very large amounts of heat at will (by magic so without using food calories)

Because of it's niche and evolution it would currently need to:

  • frequently cover very large distances by flight
  • be rare
  • look like a wyvern
  • invoke no other types of magic
  • be quite small due to the square cube law

To make is more traditional, I had an idea to be make it bigger. If the dragon can heat up scales at will (by a built in magic/biological mechanism) can it use this ability to facilitate flight (and therefore be larger)? As both heat and scales are magic, allow the scales to be comfortably be heated and maintained upto 1500C. If needed, older dragons could have thicker scales so hotter max temp.

Realistic physics and biology allow it to be similar in size, wingspan, and flying ability to a shoebill without using magic. How big can I make it if it relies on your heating mechanism to fly?

I included the following clarification in the comments: "I do not want an organic jet engine that has air passing though internal organs or an oversized air bladder"

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  • $\begingroup$ There are several other "make my dragon fly questions". I think this is very different as it is asking whether its fire magic can be used to make it fly. I do not want an organic jet engine that has air passing though internal organs or an oversized air bladder (that creature is already speced out). $\endgroup$ – kaine Jan 12 '18 at 19:58
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    $\begingroup$ I don't see a good question here. Are you asking us to do the design of your heating mechanism for you? If so, this question would be POB and very broad. $\endgroup$ – Aify Jan 12 '18 at 20:37
  • $\begingroup$ @Ally. I'm asking if, in theory, magically heating the scales of a dragon could believably be used to help it fly. I specifically narrow it to keep it from being broad but it might be too narrow. While for a worldbuilding application, I'm actually wondering if I could phrase it to make it fit better on Physics.SE. $\endgroup$ – kaine Jan 12 '18 at 20:43
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    $\begingroup$ Gives the term "passing wind" a whole new meaning... $\endgroup$ – Samwise Jan 12 '18 at 21:26
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    $\begingroup$ Kaine, I suggest editing your initial comment into your actual question to prevent confusion and make things clearer. $\endgroup$ – Aify Jan 12 '18 at 21:50
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The answer is yes, but only barely.

Scales heat air, hot air goes up (relative to colder air), if the dragon could use his wings to trap said hot air, it would provide lift - however, unless the wingspan is absurdly large, it is unlikely that you'd be able to trap enough air to lift the dragon. Dragons are, after all, extremely heavy.

There's also one other problem; Every time the dragon flaps its wings, it loses all the air it had trapped under its wings. A hot air balloon has a really good system for keeping the hot air where it's supposed to be, whereas your dragon has no mechanism for keeping that air where it needs to be - right under the wings.

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  • $\begingroup$ Yeah... and that's providing that you don't have a mechanism of propulsion. Your Dragon would just float up like a balloon and hover as the wind carries him. The only option I can suggest is that your dragon is lifted up in this fashion and can release the hot air and use his wings to steer in a gliding fashion (similar to the titular Gargoyles from the Disney show. They do not fly but merely use their wings to glide and maneuver.) $\endgroup$ – hszmv Jan 12 '18 at 20:53
  • $\begingroup$ You are probably right. I thought it might do something in a gliding situation but the heat transfer is probably small enough to make that negligible (only small area in contact). Jet engines don't get much hotter but all the inlet air gets heated that hot quickly. The air balloon wings works as a really good analogy for that actually. You can only heat the air in that area, how much lift do you get? $\endgroup$ – kaine Jan 12 '18 at 21:18
  • $\begingroup$ Balloon dragons are absurd, too (this is also why the whole "hydrogen flight sack" theory is dumb). $\endgroup$ – Draco18s no longer trusts SE Jan 12 '18 at 21:28
  • $\begingroup$ @kaine Not enough lift. Hence why I said barely. $\endgroup$ – Aify Jan 12 '18 at 21:51
  • $\begingroup$ @Aify Sorry, I was just agreeing with your answer. The last question mark just rephrases the question into an obvious one based on your answer/analogy. $\endgroup$ – kaine Jan 12 '18 at 21:56
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YES

Your dragon needs a built-in jet engine. It takes air in (through its mouth or a specialized intake.), heats it in the inside, and expels it through a nozzle.Your temperature limit of 1500 K happens to be only a little bit lower than the operation temperature of jet engines: https://blog.klm.com/jet-engines-are-hot-in-at-least-4-ways/

Surely, there are problems: Combustion of kerosene creates somewhat more molecules than it consumes (39/50): https://en.wikipedia.org/wiki/Kerosene#Transportation so the combustion products would have a higher pressure even if they would not be heated, which means that an air-only dragon would have lower performance.

Also, rapidly rotating macroscopic structures are uncommon in living organisms, but your dragon could avoid axial-compressors and use pulsejet or scramjet (if it is fast enough) instead.

It will not have classic dragon style wings, it will be more like this: enter image description here

EDIT: If you don't like the jet engine concept, you can have an organic piston engine. Inhale air into pistons, heat it, and use the expanding pistons as substitutes to muscles to drive your wings, then repeat the cycle. It is still machine like, but if you reject both static floating and reaction engine, there is not much possibility remaining.

EDIT2: This poor dragon becomes in my imagination more robotic with every step, but if it can heat enough, it might helps him to create materials (plastics, alloys, etc.. like a biological smelter or chemical reactor) so it does not have to rely entirely on traditional organic materials for its 'airframe'

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  • $\begingroup$ Alexander and A. C. A. C had it before me. But it is so obvious answer, i have figured it out without seeing their comments. $\endgroup$ – b.Lorenz Jan 12 '18 at 21:39
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    $\begingroup$ Actually, i said "I do not want an organic jet engine" immediately after posting the question. I love the photo though. $\endgroup$ – kaine Jan 12 '18 at 21:49
  • $\begingroup$ Sorry. Didn't notice $\endgroup$ – b.Lorenz Jan 12 '18 at 21:57
  • $\begingroup$ It was my bad. I'm clearly terrible at communicating today. Sorry. (Its actually the main reason I write... to practice getting ideas across.) $\endgroup$ – kaine Jan 12 '18 at 22:00
  • $\begingroup$ "If it can heat enough, it could" become a nuclear fusion reactor that uses this excessive amount of energy to propel itself at unimaginable speeds. This is not really the goal x) $\endgroup$ – Alix Eisenhardt Jan 12 '18 at 22:41

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