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I haven't really gotten into the size of my dragons, but I figured that a reasonable reference point for the size of my dragons would be Tyrannosaurus rex, since my dragons, as adults, look like miniature T. rexes with wings. The arms become less important the bigger the dragons get, so after some time, the arms will stop growing but every other part of the dragon's body keeps growing. This means that when juveniles become bipedal(yes, they start quadrapedal and lizard-like in appearance), their proportion of arm:body is more like that of a Velociraptor and then as they grow, the arms become tiny in comparison to the rest of the body, like how it is in T rex.

This actually makes sense, as these dragons are the apex predators in their environment. And, they would certainly be longer than a human is tall. Having large arms more like those of a human, or even the forelimbs of a dog, would make it hard for them to fly if they would even be able to fly. This doesn't affect juveniles as much because of how small they are, but it really affects the adults. Having small arms in comparison to the body helps with the aerodynamics by reducing drag. And drag reduction is important in a flying creature, avian or not. So, it would make sense that the arms would reach a maximum size, while the legs and the rest of the body keeps growing.

Now for the size of my dragons as adults. I figured that a reasonable length for them would be 15 feet, which is 3/8 the length of T. rex. To keep things proportional, I calculated the height to be 6.375 feet which is 6' 4.5", taller than your average human, but not that much taller. Since the mass, if I'm not mistaken, has roughly an inverse cube relationship with the length and height, this means that it wouldn't be 3/8 the mass but rather, more like 19x less massive than T. rex, which would make it have a mass of 1,637.6 lbs. But, I might be wrong on that so correct me if I am.

Anyway, given the size of my dragons as adults, how big would the wings have to be in order for them to fly?

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  • $\begingroup$ Huh! sorry for the previous, I misread your 6.375 feet as six thousand three hundred & seventy five feet ;D $\endgroup$ – Pelinore Nov 24 '19 at 20:24
  • $\begingroup$ I know it isn’t an exact duplicate text-wise, but it directly answers this question insofar as there is no wingspan that will make the dragons described able to fly. T-Rex is too big. $\endgroup$ – SRM Nov 25 '19 at 20:25
  • $\begingroup$ I think this idea would be more interesting if the wings stopped growing such that only younger dragons could fly, and adults are just like fire breathing T-rexes. It would actually make dragons make more since such that flight helps it survive infancy, and as it grows up and the whole square-cube law kicks in making it a ground apex predator. $\endgroup$ – Nosajimiki - Reinstate Monica Nov 25 '19 at 21:24
  • $\begingroup$ "the mass... has roughly an inverse cube relationship with the length and height" bzzt. Walking things and flying things are under very different pressures with regards to weight and things like bone density. Read up about Quetzalcoatlus northropi, for example. Your example dragon is too heavy. When you've made it lighter, the question linked by SRM should provide suitable answers for you. $\endgroup$ – Starfish Prime Nov 25 '19 at 22:01
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To calculate it, you need to choose a wing-loading factor:

Wing loading for birds is generally between 1 to 20 kg/m2. The maximum possible wing load before a bird is unable to fly is about 25 kg/m2. This is when the bird is too heavy for its wings (like the kiwi, which has tiny wings and a large body mass).

So 1638 lbs is c.a. 743kg. That means:

  • with a wing-loading of 1, the wings need to be $743m^2$
  • with a wing-loading of 20, the wings need to be $37m^2$
  • max. wing-loading of 25 results in $29m^2$

Regarding body size and weight, I would use the BMI index. According to wikipedia, the T-Rex was around 12.3m/40ft long and weighed around 8.4 metric tons. I think in this case we should use the length instead of the height, so it comes to a BMI of $55.52kg/m^2$. IF your dragon is 15ft/4.5m long and has the same BMI, it would weigh c.a. 1125kg/2480lbs.

With that in mind, the new wing sizes would be $1125m^2 / 12110ft^2$, $56.25m^2 / 605ft^2$ or $45m^2 / 484ft^2$.

This is a lot, compared to the body length. If the width of the wings is say 1/3 of the body length, the wing span would be with the highest wing-loading 30m, that is 6.6 times the body length. At least.

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