4

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.

Alright itsCRO
  • 49
  • 1
  • 3
  • Welcome to Worldbuilding! If you have a moment, please take the [tour] and visit the [help] to learn more about the site. You may also find [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! – Gryphon Jan 25 '19 at 17:06
  • 5
    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. – Tim B Jan 25 '19 at 17:13
  • 1
    @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. – John Jan 25 '19 at 17:33
  • There is a rather useful calculator here - https://www.omnicalculator.com/everyday-life/helium-balloons – chasly - supports Monica Jan 25 '19 at 20:45
  • 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. – Ilmari Karonen Jan 25 '19 at 21:11
  • It's worth noting that as a noble gas, (and the least reactive of them, to boot,) helium is not something that interacts easily with biological metabolic systems. Living creatures are going to have a very difficult time gathering it. – Mason Wheeler Jan 25 '19 at 22:46
  • The more gas you add to give the creature lift, or buoyancy, the larger and more bloated it becomes. May need to stick with aerodynamic lift. – Sonvar Jan 25 '19 at 23:27
  • @IlmariKaronen: Yes, but with this backstory they'd punt it here. If posted with no explanation as to why they'd probably accept it if not for too short. – Joshua Jan 25 '19 at 23:31
  • It sounds like you're planning to launch yourself in a lawn chair. When you do, please make sure you film it for posterity. – Richard Jan 26 '19 at 00:31
  • You should consider using Hydrogen instead of Helium. In addition to adding buoyancy it's highly flammable, allowing the dragons to breathe fire! It can be easily gathered from water or other organic materials. The downside is said flammability: it's what brought the Hindenburg down! Maybe they have adaptations to prevent that too. – stux Jan 26 '19 at 00:35

3 Answers3

12

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.

AlexP
  • 88,883
  • 16
  • 191
  • 325
5

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.

The Square-Cube Law
  • 141,440
  • 29
  • 264
  • 586
-1

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.

Rose
  • 836
  • 1
  • 6
  • 16