Beta, theres a surveyor aircraft set to explore mars sometime in the next 5 years and it uses propellers. CO2 is the same as O2 where aerodynamics is concerned.
If i were actually to enable this, i would have research:
LVL0: Enables build of small, propeller-driven surveyor drones with one-shot movement; e.g. they cannot return and after one use (order), they fall to the ground. Very Cheap to build.
LVL1: Upgrades the surveyor drones chassis to something larger that can carry a conventional power system, Savant uplink and more sensor equipment. I havent decided if it should still use props or use jets. Cheap to build.
LVL2: Enables scout drones that are designed specifically for spying/recon. Uses jet propulsion to give almost doubled speed. Increased speed allows the wings to be smaller, narrower. Has simple ablative skin that disperses EMP effect. Expensive to build.
LVL3: Allows the launch of a modified LVL2 scout drone that can drop a single bomb the equivalent of a starflare. Due to the removal of equipment to allow the warhead payload, the craft has only one-shot movement. I dunno if only Plymouth should be able to build these, it depends really. Expensive to build.
And thats it. Thats all you can do with them. All you can do is spy with them and then only later after youve done research, gotten rare ore, fully researched explosives can you build a drone that can launch a single starflare bomb. This should keep things even since you couldnt research these immediately in the game and rush with them, but they would be great for harassment and thats about all they should be good for anyway.
Plus, Id allow research that lets most turrets fire at airborne units except for stickyfoam, EMP, ESG since their projectiles are basically packaged devices that deploy on impact. Only beam weapons and high-order mass drivers like the RPG would be able to hit them. Plus, the Meteor Defense would provide a massive area of defense from air units.
We wouldnt want air units to become a massive element of the game because it would change it way too much. If we have them at all, they would have to be something thats useful in high mark games and things like resource operation harassment and distractions are positions these aircraft would fill perfectly. They would be weak, but are enough of a concern to force a defending player pay attention and take care of them. And that would open up a huge amount of strategic possibilities. But again, we dont want to have waves of aircraft going back and forth in a Battle of Britain type thing. Just 3 or 4 roaming around the map irritating people. Plus the way the research would be setup, you wouldnt even see them until very high mark games at which point you would expect them anyway.