Horacio said:
Bumblebees?
If a normal fly grew to let's say horse size, and it weight grew up in the same proportion, it couldn't even stand on its legs... Insect can fly because they are really tiny, it they were bigger the weight of their chitinous exoskeleton would pin them on the ground...
That's one of the reasons why you don't see giant insects...
You seem to have missed the point.
Giant insects are neither here nor there. For many years people said that (regular-sized) bumblebee flight should be impossible, considering the bees' anatomy. Well, this was faulty reasoning at best for a few reasons and the thinking regarding this, amongst people in the know, has changed.
Quoting from
http://rotor.fb12.tu-berlin.de/technik/bumblebee.html
The key to understanding bumblebee (and dragonfly and butterfly and mosquito) aerodynamics is in understanding bumblebee airfoils. If you imagine a bee's wing, with its veins and undulations in cross section, the first question that comes to mind is, "why would anyone pick such a horrible airfoil and try to fly with it?" Of course, individual bumblebees have little choice in the matter, but mother nature *could* have chosen another design.
So why isn't there a bee-sized insect out there with a NACA 64-series airfoil?
...
The answer is scaling. At the sizes of bee wings, the bumpy-looking wing cross section is actually a very efficient airfoil. The key is to recognize that at bee dimensions, Reynolds numbers are low! Many of the fluid dynamic assumptions students use in the study of airfoils do not hold up (since few airliners are built to bee dimensions, the assumptions are still fine for most of us aerocritters). Once you have a higher Cl airfoil, the L/D goes up and power requirements go down.
And, voila! the bee flies!
Perhaps it is the same with dragons, they are simply misunderstood. Birds fly, planes fly, blimps fly, and yes, even bumblebees fly, though not all of them do it in
exactly the same way. Sometimes you have to look beyond wings and weight, sometimes there are other factors.
Having said ALL of that though, quite frankly, I think applying science to Dragons is, more often than not, pure drudgery and a waste of time.

Dragons, as far as I know, are a flight of fancy, and that's good enough for me.
Finally, For some interesting theories on dragon flight, find the book The Flight of Dragons by Peter Dickenson.