So, a quick rundown:
Giant pterosaurs lived during the Cretaceous period before some a-hole wiped the server clean. These pterosaurs ranged in appearance and lifestyle from sorta-scavengers, like the long-necked Arambourgiania and the Quetzalcoatlus, to the Hateg islands' monster, the Hatzegopteryx Thambena with a short but T H I C C neck and a more proactive predatory lifestyle.
Most paleontologists, like Mark Witton, Michael Habib, etc... attribute their large size to quad-launch.
You see, since launching is the hardest part of flying, it takes some power to do. Birds use mostly their legs to initiate the launch. The problem with that is that those beefy (and rather heavy) legs become a dead weight during flight.
So, what bats and pterosaurs did was to use their flight muscles to take off, basically pole-vaulting into the air. This means that increasing the power of the flight muscles increases the power available to launch. These muscles were most likely fast-glycolytic, very powerful but tire quickly.
So, if you see "realistic" dragons (read: wyverns) that don't quad-launch, it's probably the Dunning-Kruger effect.
Rant aside, it IS a neat launch strategy, I'm just not sure if it can be scaled further "by default" to achieve flying creatures heavier than even giant pterosaurs.
What do I mean by "default"? Simple, really:
- Let's assume we have lightweight materials with the right mechanical properties to be made into wings, bones, etc, and they aren't going to fail under very heavy loads.
- Handwave metabolism. To be fair, at this size, our fliers will probably be soaring for most of the time. They'd only flap their wings when they need to go really fast, or when they're climbing out to cruising (?) altitude.
So, in terms of variables, we're left with the shape of the muscles and the wings. We're gonna assume the wings won't break, no matter what, but their shape and aerodynamics will still affect how much muscle-power is needed to flap with them.
Given these circumstances, how scalable would this flight strategy be?
Papers We'll be referencing:
From damselflies to pterosaurs: How burst and sustainable flight performance scale with size:
Why we think giant pterosaurs could fly:
Constraining the Air Giants: Limits on Size in Flying Animals as an Example of Constraint-Based Biomechanical Theories of Form:
It seems like clearance is a big problem for wings larger than 12-ish meters. The question then becomes whether we can increase the wing loading or change the wing's shape (and lower the aspect ratio) without negatively impacting the creature's soaring capability.