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So a friend of mine came up with an interesting question. Considering the fairly universal following facts about western-style fantasy dragons:

a) dragons can breath fire

b) dragons themselves are fireproof

c) dragons have large wings, allowing them to fly (though the actual science of this is well-known to be fishy, let's for now pretend a dragon such as Toothless or Smaug can actually fly. Super light bones or whatever.)

d) hot air rises and creates lift

If the dragon can produce a large quantity/sizable blast of extremely high heat, could it be useful to the dragon for it napalm/torch the ground beneath it as it was taking off, in order to create additional lift for itself? Or would the effect of doing this be so negligible that it wouldn't be worth it to bother?

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    $\begingroup$ This isn't quite what you're asking, so I'm leaving it as a comment rather than an answer, but there was a pseudo-documentary (possibly this one: en.wikipedia.org/wiki/The_Last_Dragon_(2004_film) ) about dragons (the framing device being that it was a real documentary made in a world where they existed) that discussed something similar. Its dragons didn't fly by using their fire breath to heat air; rather, they had a swim bladder-like organ filled with hydrogen that was used to both maintain buoyancy for flight and to fuel the fire breath. $\endgroup$ – Ray Apr 2 at 4:53
  • $\begingroup$ "in order to create additional lift for itself?" quite the contrary. As the air temperature increases the amount of lift wings can produce is reduced due to the lower pressure. $\endgroup$ – DeepSpace Apr 2 at 8:38
  • $\begingroup$ @DeepSpace: very much depends on the mode of flight and nature of the hot air. Birds use thermals to gain height all the time. $\endgroup$ – Joe Bloggs Apr 2 at 12:07
  • $\begingroup$ @JoeBloggs Indeed, but that does not mean that their wings produce more lift than in cold air $\endgroup$ – DeepSpace Apr 2 at 12:48
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    $\begingroup$ @DeepSpace Guess it depends on how technical you are with language. If by ‘lift’ you mean the casual ‘upward force applied to the wings’ then you have got more in an updraft caused by hot air. If you mean the more technically correct ‘upward force caused by fluid flow over and under the wing in air of homogeneous temperature’ then you have less. $\endgroup$ – Joe Bloggs Apr 2 at 13:05
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Unlikely if not impossible. The lift created by heat (pressure differences) is different than the lift created by the wings (mechanical). The "help" given by one is unlikely to help the other - and the difference would likely be negligible.

Mechanical Lift

Lift as a mechanical force is created by motion. Here is a good explanation, and Wikipedia has a decent article on lift. You can read more about how birds create lift here.

Pressure Lift

Lift by pressure differences is how hot balloons work - but this is because of the density of the balloon (here, dragon) in relation to the surrounding air. Unless the dragon could contain the heat entirely beneath it, the heat would dissipate very rapidly - especially if it is significantly hotter than the air around it. This would make for a very bad way to create lift.

Cannons

I suppose, in theory, if a dragon was stuck upside-down inside a pipe, then a good heave might push it out. But this would be more like how cannons and firearms work than any concept of actual lift. In such a scenario, the dragon would be "pushed" out, and could then flap it's wings - assuming that it could create enough of an explosion (fireball) to force itself loose.

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    $\begingroup$ Fascinating, thanks. I just got a wonderful image of loading tiny dragons upside down into launch tubes and watching them blast themselves backwards out again like little rockets. That mental image in going to feed my happy for a while :D $\endgroup$ – MarielS Apr 2 at 2:48
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    $\begingroup$ In fact, if you ask any pilot who's had to take-off from McCarran International Airport in Las Vegas during high summer, they'll tell you that hot air is horrible for lift-off because it causes the molecules to move further apart, meaning there's fewer molecules under the wings to encourage lift. I've seen some planes leaving McCarran in August that I wondered would get off the ground before the end of the runway. $\endgroup$ – JBH Apr 2 at 5:13
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Thermal Soaring

I think its possible:

  • Birds like raptors (eagles/hawks etc), vultures, and storks can gain altitude without flapping by hopping a ride on a rising column of warm air. This is called Thermal Soaring.
  • We assume (according to your point c) that dragons can fly reasonably well, but can they soar? Soaring ability in dragons seams likely, as dragons are typically thought of as carnivores which means they have to fly around looking for prey to catch (like eagles/hawks) or fly around looking for dead prey (vultures/condors). All that flying around means you need to be energy efficient and be able to soar. If dragons are capable of soaring in general, then they should be capable of thermal soaring as well.
  • According to my googling, wildfires can cause thermal columns.

Thus, the dragon starts a wildfire, takes off for flight (likely by jumping up like giant pterosaurs) and flaps a few times to get in the air over the wildfire, and then can thermal soar up high.

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    $\begingroup$ When Nurmberg was bombed during WWII the resulting tempest of fire lifted up in the air wooden beams, as reported by some bombers crew. And wooden beams have no wings. So definitely a dragon can use the mechanism you suggest. $\endgroup$ – L.Dutch Apr 2 at 6:06
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    $\begingroup$ Yes. When you see a vulture circling, it is typically doing that to stay close to a rising thermal. Glider pilots use the same technique to gain altitude even though they have no power. A good pilot will feel a slight lift on one wing and then move in that direction to get into the updraft (e.g. rising from a large parking lot on a sunny day). This technique allows them to go to great heights and to stay aloft for hours. Dragons usually live near volcanoes, so they should have no problem generating altitude before departing. $\endgroup$ – Ray Butterworth Apr 2 at 13:44

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