https://www.space.com/amp/new-class-habitable-exoplanets-hycean-worlds Hycean worlds are planets up to 2.5 the size of Earth with huge oceans and lots of hydrogen in their atmosphere. They're probably really common in the galaxy so thinking about life being there is important. But my question is if all that extra hydrogen would make flying more likely or less. Would it make gas bags meant to lighten the body less effective?
Flying animals swim in air the same way you swim in water, by floating and flapping a lil bit....some flying animals are in air like hippos in water, no floating but pure muscular strength keeps them going.
To make flying easier, make air heavier by using different elements .
To make flying harder, make air lighter.
The ease of aerodynamic flight depends on the density, but not the molecular weight, of the atmosphere. Due to its low molecular weight, hydrogen must be under higher pressure than air to achieve higher density, but as long as the pressure is high enough, yes, even more hydrogen will make active flight even easier.
Aerostatic flight with gas bags is a whole different deal. Higher density helps a bit, because it reduces the density differential between incompressible air, and higher pressure helps because it increases the absolute differential between different types of gasses. But, if you can't find a gas lighter than the atmosphere, that latter effect doesn't matter much.
So, if you want aerostatic flight, your creatures will need to exploit two facts: 1) that the atmosphere is not pure hydrogen, and 2) that organisms can control their temperatures. If there is lots of hydrogen, there is probably lots of helium as well, and other trace gasses. Organisms can get some lifting capacity just by actively concentrating hydrogen in their gas bags. Furthermore, at high pressures, large amounts of lift can be produced just by keeping the gas bags a little bit warmer than the surrounding air.
Whether aerostatic flight ends up on average easier or harder than on Earth depends on more than just how much hydrogen there is. But there does come a point where adding more hydrogen just makes hot-air flight easier and easier, even if concentrating chemically different gasses is no longer worth it.
On Earth (sea level, 15 celcius) Hydrogen has density of about 0.08 kilos per cubic meter. This is much lighter than the typical atmosphere which weighs about 1.2 kilos per cubic metre. That's why a hydrogen balloon flies.
But if your planet has a super thick atmosphere the hydrogen at the bottom is compressed by the hydrogen at the top, and becomes more dense. Dense fluids are easier to fly through. That's why it's easier to swim in water than air.
Your Homework: Find out how compressible is Hydrogen, and how much pressure is needed to get it to the same density as Earth's air.