How can the lift produced by animal wings be calculated?

[Necronox]

that's what i thought, or at least that is my logic,

but anyway, back to the original topic, how light could such a creature become? assuming a wingspan of roughly of 15 meters? (out of curiosity, what about 30 meters?)

 

[sophiecentaur]

As big as you want it to be.

Any large creature in sci-fi is going to be criticized. People will point to square-cubed problems, say their muscles can't be strong enough or they would weigh too much, etc. But that's overlooking the fact that creatures that evolve to large sizes also, by necessity, evolve ways of overcoming those problems. For example, large dinosaurs evolved lightweight bones filled with air sacs to reduce their actual weight, extraordinarily growth-optimized metabolisms, etc. Biological history has shown in the evolution of organisms all the way up from the microscopic scale that if there's an evolutionary mechanism that favors larger size, life finds a way.

You are right here. of course but, when someone posts a question about the likely performance of animal-like animals then we can only use established data. It's a thin line between Sci Fi and Fantasy and the OP didn't want Fantastical creatures if they could be avoided. There is a lot of potentially valid discussion about the actual viability of a 'super eagle'.
Your suggestion about an alternative skeletal material is interesting and I spent a while, investigating that aspect. Needless to say, most information about these matters relates to domestic animals, raised for food; not any kind of predator. In birds (wild, I assume and not Turkeys, and Foi Gras Geese), the proportion of Fat, bone, muscle is, according to one reference, pretty much the same over all sizes (a surprise to me). This link talks around the subject.
The reason this interested me was that the advantage in making the skeleton lighter for the same strength (or stronger for the same weight) is not obviously pro-rata. The muscle is equally important here, too (very relevant to power/weight ratio).
There is no definitive answer to this question and it depends upon how deep you want to delve into probable viability of any very large bird. It would only be really big if there was an advantage (food or defence) and it would be the result of an 'arms race' with some other prey organism - possibly a very large species of goat that fed of some high altitude super plant. Nothing ever evolves without some pressure / advantage in a change.
I know "life finds a way" but only the survivors / winners get to write the history and most deviations turn out to have been failures.
P.S. I am not really 'hurt' by the "smartypants" comment because most people on PF do their share of smartypantsing. (And I realize that I am guilty as charged here)

 

[Necronox]

Interesting that the fat, bone and muscle is the same, surprising really, but this particular bird would almost require a high power to rate ration wouldn't it? especially for take-off.
In terms of your second last phrase, this bird is an apex, obviously predatory bird. they'd be pressure from other predators, but not much considering it's apex. I suppose, biologically speaking, they wouldn't be an advantage for a bird to become so radically big and different.

 

[Artribution]

I suppose, biologically speaking, they wouldn't be an advantage for a bird to become so radically big and different.

Sexual selection? That's a big thing with birds, after all. Maybe the female only mates with the biggest male. Or the bigger males simply kill the smaller ones. Or it could be there were other predators/prey driving an arms race, but they've died off since.

The reason this interested me was that the advantage in making the skeleton lighter for the same strength (or stronger for the same weight) is not obviously pro-rata. The muscle is equally important here, too (very relevant to power/weight ratio).

Well, skeletal pneumaticity varies considerably in birds. The Wiki article I linked to notes that diving birds have very little, and loons have none at all. You make an important point though. There are many different possible avenues it could take. Muscles, bone, cellular chemistry, etc.

There is no definitive answer to this question and it depends upon how deep you want to delve into probable viability of any very large bird. It would only be really big if there was an advantage (food or defence) and it would be the result of an 'arms race' with some other prey organism - possibly a very large species of goat that fed of some high altitude super plant. Nothing ever evolves without some pressure / advantage in a change.

That's what's thought to have happened with sauropods. The young are defenseless and have to grow fast and end up large enough to be unassailable as adults. Therefore, an arms race ensues between them and theropod predators. The result after a while is that a sauropod chick that starts out at 10 or eleven pounds, comparable to a human baby, will grow to 100 metric tons by the time the human reaches adolescence, or 2000 times human growth rate.

 

[sophiecentaur]

Maybe the female only mates with the biggest male.

Yes. That always strikes me as odd. They choose the guy who has the most time and effort to waste on a fancy and sometimes very impractical appearance. That's a modern trend. HAHA

an arms race ensues between them and theropod predators.

The sort of arms race I was meaning is the one between predator and prey, though. A predator needs to run fast enough and be strong enough to catch enough large / fast prey.These super birds would have evolved in order to get at enough of their prey species.

 

[Artribution]

Or maybe the male birds have to grow continuously larger because the female birds simply don't want to mate and have to be overpowered, and there's an arms race between them. Birds are messed up.

The sort of arms race I was meaning is the one between predator and prey, though. A predator needs to run fast enough and be strong enough to catch enough large / fast prey.These super birds would have evolved in order to get at enough of their prey species.

That's what I meant. Sauropod young were preyed on by theropods and their size was their only defense, so they had to grow up faster, and end up larger, all the time. But the theropods did too, to stay competitive. So there was an arms race. Theropods and sauropods both kept getting larger until the end of the Cretaceous.

edit:

but anyway, back to the original topic, how light could such a creature become? assuming a wingspan of roughly of 15 meters? (out of curiosity, what about 30 meters?)

Sorry, I don't really have any guesses there, and it depends on what avenue(s) for weight reduction you want to take.

 

[newjerseyrunner]

I don't see any limitation to size. You have to define what "flying" is. If you think about it, there is really no difference between flying and swimming. As long as an object is neutrally buoyant, it can "fly." The largest animal on Earth "flies" through water. A planet with extremely low gravity and an extremely dense atmosphere would probably have almost exclusively fly.

There is no reason that a very small object can have an extremely dense atmosphere, Titan's atmosphere is significantly denser than Earths. Venus' is so dense that at the surface, it acts more like water than air. The landers that actually landed on Venus, didn't have parachutes, they just sort of settled to the bottom.

I'm surprised I didn't see any reference to this yet: you also need energy. Birds and insects got much larger in the past. This was because the oxygen content was much higher so there was more potential chemical energy in the air.

 

[sophiecentaur]

there is really no difference between flying and swimming.

That's right in principle but the OP wants to have humans (or near as possible to humans) and they would need to be able to walk around and behave near to how we do Within those constraints, the exciting possibilities that you suggest are not really open to us.
PF gets this sort of question on a regular basis and it's impossible to answer definitively. All we can do is throw up ideas and opinions and a would-be author can take his or her pick. If the story is good enough, the reader can suspend disbelief enough to enjoy the tale.