# What is the size of the largest possible flying animal? Realistically speaking?

I’d like to create fantasy creatures that can fly using their own realistic physiology without magic or any kind of supernatural power. Many references on the internet mention that popular dragons like in GoT and LoTR are not realistically possible to fly given their anatomy and design. And in our earth history itself, flying dinosaurs or primordial birds is not big enough for my purpose of world building. So, how do I conceptualise a very large group of flying animals that are very plausible for them to fly on earth and being a mode of transportation for human race? Is there any good references for building a physically realistic creature?

• If the largest flying animal to have ever existed is not enough for you, I don't understand what you mean with realistic.
– L.Dutch
Dec 10, 2023 at 14:26
• "Is there any good references for building a physically realistic creature?" There are plenty of introductory articles on the internet allowing to calculate the required wing area for a given weight and airspeed. A typical bird wing has a wing load at take off of about 25 kg/m², whereas an airliner wing has a wing load many times higher, in the hundreds of kg/m². High wing load allows for smaller wings, but on the other hand airliner wings are made of metal and not thin bones and feathers, and aircraft take off at very much higher airspeeds. (Cont.) Dec 10, 2023 at 14:44
• (Continued) The other difference between birds and aircraft is that aircraft eat highly energetic fuel, and they do not have to hunt for their food, as humans arrange for it to be brought to them in large tankers. Aircraft engines are also much more efficient than muscles at converting the energy in fuel into mechanical power. A small Cessna 195 can carry four people, but to do that it consumes 61 liters of gasoline per hour; you would have to feed your four-passenger muscle-powered dragon one horse for every three hours of flight. Dec 10, 2023 at 14:52
• If you want fully realistic muscle-powered flying creatures significantly larger than the ones that have existed on Earth, you're going to have to change something about your setting. Raising atmospheric pressure and/or reducing gravity will make flight easier. However, it's generally much easier to give up on "fully realistic". Realistic is not a selling point in fantasy, almost by definition. For example, Anne McCaffrey's dragons were not realistic, but sold millions of books. Dec 10, 2023 at 14:52
• @AlexP - Aispeed? What is the airspeed of an unladen Swallow? Dec 10, 2023 at 19:11

Quetzalcoatlus may be the largest flying creature in Earth's history, with an 11m wingspan. If that's not big enough, take 10 of them and attach them together in a long train. You could have an animal like that, a sky-snake with many sets of wings.

You could have a gliding bird with a central body and wings like a 747. It would need to be very thin and light for its size. It would not be able to flap the wings much, and probably could not take off from the ground, but it could live its whole life in the air, catching thermals. It could have sets of smaller wings that it could flap for thrust. It might be able to land on very tall cliffs where it could use the drop to get airborne again.

You could have a giant winged blimp-bird filled with hydrogen, or with hot air heated by a special organ. Perhaps the top of it could be transparent so the interior can be heated by sunlight, like a greenhouse, though that alone would not be enough heat. There are few limits on the size of a blimp. They could be as big as a blimp. The only question is what blimpy would eat. Perhaps they could photosynthesize. Or their insides could serve as a roost for other birds, and when the other birds die naturally they fall to the bottom and the blimp digests them.

• I heard that the atmosphere had a higher oxygen percentage at that time, allowing for larger flying animals than today. Chances are, Quetzalcoatlus couldn't operate in today's atmosphere. Dec 11, 2023 at 16:08
• @HolyBlackCat Quetzalcoatlus dates to around 70 million years ago, when the atmosphere had a similar amount of oxygen as today. en.wikipedia.org/wiki/Geological_history_of_oxygen Oxygen levels peaked around 300 million years ago. Dec 11, 2023 at 17:25
• Thanks, I stand corrected. Wikipedia even explicitly confirms your point: en.wikipedia.org/wiki/… Dec 11, 2023 at 17:32
• The last idea is similar to Iain M. Banks' Look to Windward Dec 11, 2023 at 22:03
• @HolyBlackCat Scientists think one of the major factors that allowed Pterosaurs to get to larger sizes than flighted birds was their take off method/ground locomotion. Large birds eventually are limited by their ability to get off the ground, and are stuck on two legs if they need to move around on the ground. Pterosaurs walked on all fours, much like a bat, we know this from fossilized trackways we know came from them. They also took off the ground thrusting from all fours, again, much like a bat. Birds are limited when they need to get off the ground if they are too big. Dec 12, 2023 at 16:45

Very big (if you include lighter-than-air creatures)

Historically, airships were massive, dwarfing even the biggest flying machines we have today from rockets to aircraft to (the few) of today's airships. Interestingly, one of the major challenges in constructing these airships in the early 1900's was finding a material that's lightweight and gas proof. The answer? The engineers turned to nature and used the cow intestines for their gas-bags.

A notable example is the British R101: A 236 meter-long airship that used an alleged half-a-million cow intestines to create the (mostly) airtight gasbags.

Now, we don't have any real-life examples of lighter-than-air flying creatures, but I think clearly it is within nature's biological toolbox to create gas-tight membrane, just like generating a lifting gas is also possible (there are some microorganisms and algae types that produce hydrogen), I don't see a reason why you couldn't have "whales of the sky" that use enormous gas-bladders to float and filter-feed insects or smaller flying creatures like whales do in the ocean.

Without magic or physics bending, and staying within the bounds of biology, using gas-based lift like this basically means that there is no upper limit from a lift-perspective (it actually gets more efficient at scale for square-cube reasons), however there would need to be an ecosystem to support these creatures and an appropriate evolutionary pathway that created them in the first place.

• Flight of Dragons's dragons are basically dirigibles with hydrogen gas. Which they also use for fire. youtu.be/j0j0Bjy6hFc?si=J2Hvq9B45n7xKAem Dec 11, 2023 at 21:19
• We actually had a question here on exactly this topic. @kmh may want to look it over. Dec 12, 2023 at 16:58
• The square-cube tops out I think - a huge lift cavity runs into problems, as the lift-force has to be transmitted over its surface. Which results in the surface having to have unboundedly large tensile strength as the creature gets larger? You fix this to a certain extent with smaller cells, but said smaller cells then lose you the square-cube advantage you are talking about.
– Yakk
Dec 13, 2023 at 5:17

Is there any good references for building a physically realistic creature?

No, there is nothing you could scale up believably that would be reasonable for human transport unless it's very short flights. Powered flight is very expensive in terms of energy use. Gliding isn't but that doesn't seem to be what you're after.

So you could scale up a Haast Eagle or a Condor and get them to pick you up and take you a few km, but not fly around with you touring the countryside.

The biggest problem is the weight of the human and their gear. You're more than 4 times the weight of the heaviest flying birds (they're under 20kg) and evolution does not put that sort of redundancy into animals. They evolve for what they need, not what would be convenient for humans. Sometimes the two coincide, but not at that scale.

You might be able to train teams of large birds to pull a glider though.

• Does this change any with the example of the Quetzalcoatlus mentioned in causative's answer? From the linked Wikipedia article, it says the most common current estimates for the weight of the largest Quetzalcoatlus species are around 200-250kg. Dec 11, 2023 at 14:47
• ... for which a single 70kg human would be on the order of a 30% increase in overall weight, @JustinHilyard, before one even gets to the question of how that human is carried and whether that produces a significant additional impediment. Seems implausible to me. Dec 12, 2023 at 19:19