Weight vs wingspan

I researched this a few months ago. With Earth gravity, Earth atmospheric density, and realistic physics it's essentially impossible to have a flying animal bigger than about 100lbs and 20ft wingspan. The biggest birds and pterosaurs have all been vulture or albatross-like gliders which have great difficulty getting in the air unless they can run down a hill or jump off a cliff. So my opinion is, it's fantasy, make the creatures weigh what what a mammal would for their size and give them wings that look proportional and aren't so big they'd be hopelessly inconvenient. No they couldn't fly on Earth, but who cares.

http://en.wikipedia.org/wiki/Quetzalcoatlus


Estimated weight 200 - 250 kilograms.

Estimated wingspan 10 - 11 metres.

So... Quite possible to have a flying animal larger than 20ft wingspan/100 lbs.

And birds tend to be hollow-boned so can have a bigger body cos it weighs less. But you do have to take into account whether they can take off from the floor or need height.

Have you ever seen a swan try and take off in a hurry? That will give you some idea of the problems.