On Earth, unlike what's often claimed, the largest current flying birds don't approach the size limit for flight with Earthly biology. Quetzalcoatlus might be getting close to the limit, though. A big one might be 250kg with 12m wingspan, and the flight performance of pterosaurs is still debated, but some scientists suggest it was too big for flight powered by aerobic respiration and thus could only sprint short distances (without updrafts or tailwinds to help).

Now, take what I could term "Flyers' World" because it's easier to take flight. 10atm, 0.5g (That seems to be about the lowest gravity that could maintain an Earth-like atmosphere -- I justify pushing to the limit because the denser atmosphere gives a stronger greenhouse effect, meaning a slightly lower upper-atmosphere temperature for the same surface temperature, and that's where atmospheric escape happens. But back to the main point...) But how much easier? Assume biology close to that of Earth vertebrates (birds, bats, pterosaurs). How big can the biggest fliers be?

Yes, I've seen Size cap for alien sky-whales on a high gravity dense atmosphere planet Not nearly enough information, and it's about buoyant animals anyway. More on target, I've seen Will my bird likely be able to fly in this atmosphere on this planet? but I need to go beyond just the thrust-weight-lift-drag relation. I'm asking... At what size will its muscles no longer generate enough force to flap its wings, or its wing bones break under the force of that flapping? What other things do I need to consider? I'm looking for consideration of multiple parameters, as I don't know which will be the limiter.

I recognize there won't be a published answer to this exact question, but I'm looking for calculations based on properties of Earth animals and their tissues.

  • $\begingroup$ Klasson Surface gravity does not maintan air presure, escape velocity maintains air pressure. $\endgroup$ Jun 29, 2022 at 18:58
  • $\begingroup$ Yes, I know it's escape velocity, but given the limited range of densities feasible for solid planets... I'm not talking about making somewhere like Saturn. $\endgroup$ Jun 29, 2022 at 21:39

1 Answer 1


The wing size of the animal does not need to be as large due to the higher pressure of the atmosphere. As a wing would still push the same amount of mass downwards in a smaller area. Look at the flippers of a penguin.

Our flyer would also be subject to the square cube law. This states that the ability of an object to keep itself standing is proportional to the area. Whereas the force pulling it downward is proportional to the volume. The bones of a Quetzalcoatlus probably are near their limit of how much force they can take. The reduced gravity helps a bunch on this part as it reduces the force downwards per volume of animal.

The gravitational force on a Quetzalcoatlus is 250*9.81=2452.5N That is then the maximum amount of gravitational force a flyer can cope with. So on a world with half the gravity such an animal can weigh 2452.5/4.905=500kg

the wing area would be 1/10th of the Quetzalcoatlus meaning that the wingspan would decrease by the root of 10 or 12/3.16=3.8

So i predict the flyer to weigh 500kg and have a wingspan of 3.8m.

This definitely feels more like sky whale territory.

  • $\begingroup$ "That is then the maximum amount of gravitational force a flyer can cope with." That makes no sense to me. I don't think it's nearly as simple as a limit on gross weight. "the ability of an object to keep itself standing is proportional to the area." But even a walker isn't standing, it's walking, it experiences dynamic loads which are greater than static loads. And I'm talking about flying, and wondering at what size the dynamic loads will break it. $\endgroup$ Jun 29, 2022 at 21:35
  • $\begingroup$ short of modelling it professionally. i think that this is the only tool we have available. note that i am just scaling the relevant parts of the Quetzalcoatlus. that way all the dynamic forces are being taken into account. $\endgroup$ Jun 29, 2022 at 23:59
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    $\begingroup$ I think your reasoning is, even in very simple static terms, dead wrong. Let's look at scaling a walker. If you double linear dimensions, mass goes up 8x. Cross-sectional areas of bones, etc. go up only 4x, so pressure (remember, pressure is a per-area quantity) on them doubles. Now, halve gravity. You halve the pressure, and you're back to the level of structural stress the original walker experiences. Thus, in half gravity, the walker can be 2x as tall, 8x the mass, still 4x the weight. I don't see how maximum total force of gravity (IOW, weight) has anything to do with it. $\endgroup$ Jun 30, 2022 at 13:53

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