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I have dragons in mind for my world, but I'm trying to find logic in how would a medium-sized anything fly off and still be a danger to humans.

So far, I have a lizard-like cheetah with wings, hollow bones and very strong legs but I still doubt it would be able to fly if it had over 60 kg. So anyway I devised a plan for that, so if anyone has knowledge of physics and such that could answer the question I would be very grateful, thanks.

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  • $\begingroup$ Welcome to Worldbuilding! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Gryphon
    Commented Jan 25, 2019 at 17:06
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    $\begingroup$ We know that there were flying dinosaurs weighting 145kg or more so there's no reason your dinosaur at 60 or 80kgs could not fly without using buoyancy. $\endgroup$
    – Tim B
    Commented Jan 25, 2019 at 17:13
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    $\begingroup$ @TimB pterosaurs are not dinosaurs, but that is a good low end mass estimate for Quetzalcoatlus. the largest flying dinosaur aka bird was around 70kg. $\endgroup$
    – John
    Commented Jan 25, 2019 at 17:33
  • $\begingroup$ There is a rather useful calculator here - omnicalculator.com/everyday-life/helium-balloons $\endgroup$ Commented Jan 25, 2019 at 20:45
  • $\begingroup$ This seems like a pure real-world physics question to me. Since we already have a site for those, I feel like this question should be migrated there. $\endgroup$ Commented Jan 25, 2019 at 21:11

3 Answers 3

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For a back-of-the-envelope calculation the rule of thumb is that you need one cubic meter (1000 liters) of hydrogen or helium to lift one kilogram; so for 80 kilograms you need about 80 cubic meters (80,000 liters) of lifting gas.

For a more detailed calculation:

  • The average density of air at sea level is about 1.2 kg/m³.

  • The density of helium at 1 atmosphere pressure and 0°C temperature (that's called "standard temperature and pressure", STP) is about 0.18 kg/m³.

  • The lifting force is the difference between the weight of a volume of helium and the same volume of air, or 1.02 kg/m³.

  • For 80 kg you need 80 / 1.02 = 78.4 m³ of helium.

For a quick comparison, that's quite comparable with the capacity of a large railroad tank wagon, and about 15% more than the internal volume of a standard 40-foot (12.2 meters) intermodal container:

A stack of 40-foot intermodal containers

A stack of standard 40-foot (12.2 meters) intermodal containers. Each container has an internal volume of about 68 cubic meters. Photograph by Martini171, available on Wikimedia under the CC BY-SA 3.0 Unported license.

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I still doubt it would be able to fly if it had over 60kgs

Helium has a lifting force of one gram per liter. For a 60kg person or animal, you will therefore 60,000 liters. That is 15,850.323 gallons for americans.

The first thing that comes to my mind when I think about 60,000 liters is that it's the usual volume capacity for yhese trucks:

60,000 liters

Also notice that flying this way may be very hard. Larry Walters comes to mind.

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    $\begingroup$ Thatsalottahelium.... $\endgroup$
    – JBH
    Commented Jan 25, 2019 at 17:22
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it would theoretically take 115920 cubic feet of helium to lig=ft 80 kilograms. Because the difference in the up and downforce is 0.069 pounds. each cubic foot of helium could lift 0.069 pounds. In order to lift a single kilogram, you would need 1449 cubic feet of helium. Now we have to transfer the cubic feet to liters. 1 cubic foot= 28 litres 115920x28 Theoretically, it would take 3245760 liters to lift 80 kilograms.

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