What about the dragonflies with those 3 to 4 feet wingspans? Those membranous wings would probably break even if the air was replaced by vacuum. If the medium were thicker than present-day air, even then they have too long a span owing to which they most likely wouldn’t be able to handle the inertial stresses developed during flapping.
Please regard my responses on this post as “thinking out loud by keyboard”. I have no knowledge of aerodynamics etc. Just throwing thoughts out there to think about. I am curious tho, did these ancient dragonflies have wings similar to today’s dragonflies?
Megafauna and the attenuated gravity of the antique system:
http://www.bearfabrique.org/Catastrophism/sauropods/biganims.html